Danielle
Danielle, after a period of rapid intensification yesterday afternoon and evening, has weakened just as quickly today, dropping below hurricane status as of the 5:00 PM EDT advisory from the National Hurricane Center. Speaking of the 5:00 PM advisory, here is the information they put out during that time:
Wind: 70 mph, with higher gusts
Pressure: 993 mb
Movement: WNW at 18 mph
Location: 17.5°N 48.2°W
Category: Tropical storm
The last of the visible satellite imagery along with animation of shortwave infrared imagery, suggest that the cyclone is once again becoming better organized, with the center now pretty much fully underneath the steadily increasing deep convection.
Additionally, a recent SSMI/S microwave satellite overpass, along with CIMSS vorticity analysis indicates that the low- and mid-level centers are fairly well aligned in the vertical, with only perhaps a very slight eastward decoupling of the centers. More importantly, the aforementioned SSMI/S overpass suggests that Danielle is in the process of reforming an eye and eyewall, though the former is not currently visible on satellite imagery.
Danielle is currently experiencing about 10 knots of westerly vertical wind shear. It appears that over the last 18 hours, the well-defined 200 mb anticyclone, observed on the link above, has detached from the circulation of Danielle, and, rather than aiding it, it has been shearing it. Additionally, wind shear has decreased over the system the last six hours.
Based on all of this, Danielle's weakening throughout the day should be relatively short-lived, and I expect she will reattain hurricane status by the time of the next advisory. And if not at that time, definitely at the 5:00 AM one. Based on the 18z GFS, the anticyclone should become reestablished over the storm in about 18-24 hours, so I expect some more steady intensification to begin tomorrow evening, and into Thursday morning. Shear could increase again during the 72-96 hour time frame, as the cyclone comes underneath the influences of upper southwesterlies associated with a longwave trough currently moving across the central and southern United States. However, this shear may not be quite as detrimental as one would expect if Danielle moves more NE with acceleration, as it will be moving in the same direction as the shear vector.
As far as track is concerned, I expect that, based on water vapor animations, CIMSS steering data, and 0z steering data from PSU's e-wall, Danielle will continue to track more westerly (specifically, WNW, though a slight W motion cannot be ruled out at this time, either) for the next 12 hours or so, as the weakness in the ridge evident on water vapor imagery and steering data from CIMSS to the north of her, along 25N from 50 to 45W, doesn't appear to be appreciably affecting the cyclone just yet.
My forecast track is a blend of the 18z GFS and 12z CMC, which calls for a continued WNW motion for the next 12 hours or so, followed by a NW turn and an associated decrease in forward speed as the system approaches the aforementioned weakness. This general motion, albeit with some possible oscillations to the left or right, should continue through at least day three. Thereafter, models diverge, with some turning Danielle N quicker, and some slower. The GFS, in a sharp contrast from what it has been doing the past several days, foresees a sharp westward turn near the end of the forecast period, which could potentially mean a landfall in the mid-Atlantic or New England in about seven days. However, I will not go that far west at this time, since the models have the luxury of flipflopping at random, while I am forced to remain consistent.
All in all, I expect a NW to begin in about 12 hours, and this should persist until around 108 hours, at which point a turn to the N is likely. However, given that the usual uncertainty in long-range forecasts, as well as the fact that the GFS trended westward at 18z, and the 12z NOGAPS tracks the center directly over Bermuda at day five, interests in Bermuda should closely follow the progress of Danielle over the next several days.
The tropical storm force wind probability map from the National Hurricane Center indicates that there is a 30% chance of Bermuda receiving tropical storm force winds from Danielle over the next five days, and that island is not out of the woods by any means. I still suspect Danielle will pass east of Bermuda, but this is not certain, and it could come closer to the island than I was thinking last night.
Interests along the mid-Atlantic and New England coasts should also be monitoring Danielle, as should Atlantic Canada.
Invest 96L
A strong tropical wave, "Invest 96L", is located near 13N (nearing 14N) 26W. This system continues to display signs of organization on satellite imagery, though deep convection is currently minimal, and a bit to the west of the 850 mb vorticity maximum.
CIMSS shear data indicates that this system is experiencing 20 knots of southwesterly shear. However, there are no apparent signs of this shear on satellite imagery. The likely catalyst for the recent wane in convection is some slight dry entrainment noted on water vapor imagery. This dry air is associated with a pocket of Saharan Air Layer (SAL) located to the west of 96L.
The dry air notwithstanding, conditions appear favorable for continued slow development over the next couple of days, and this system could become a tropical depression during the next couple of days. The GFS suggests that this will develop in just about 12 hours. I suspect it will take a little more time than that, based on current trends and forecast shear from the GFS. Instead, I expect this will undergo tropical cyclogenesis in about 18 to 24 hours. If dry air continues to hinder this system, however, it could take even longer. It should be noted that the models have shifted considerably southward today, and now foresee 96L being a long-range threat to the southeast United States.
The weaker it stays, the more southerly it should go. Given that the system will be passing over the cold water wake generated by Danielle, it should remain below hurricane status until around day four. None of the models, save the GFS, show this system becoming a hurricane during the next five days. I tend to agree with the models on the track, and the United States may not be so lucky with this one.
Elsewhere
Elsewhere, showers and thunderstorms across the central Gulf of Mexico are associated with a surface trough. This system is currently very poorly organized, and any development should be slow to occur as the system moves W at 5 to 10 mph over the next several days. A turn to the NW could occur on day three as the ridge weakens somewhat. Residents from northeast Mexico to the upper Texas coast should closely monitor the progress of this system over the next several days. I am not really expecting development here, and none of the models show it either.
The 18z GFS is forecasting that a third tropical cyclone will develop behind Danielle and future Earl late on day five. This is a sign that the tropics are heating up, and residents in hurricane prone areas should review their preparedness plans. We're still likely to churn out at least 15 storms this season, and it's not over by a longshot.
Tuesday, August 24, 2010
Monday, August 23, 2010
Tropical weather analysis - August 23, 2010
Danielle
Hurricane Danielle continues to churn in the central Atlantic Ocean, and is located between Africa and the Lesser Antilles. Here is the latest information put out by the National Hurricane Center at the 11:00 PM EDT advisory:
Wind: 85 mph, with higher gusts
Pressure: 982 mb
Movement: WNW at 20 mph
Location: 15.6°N 43.2°W
Category: 1 (Saffir-Simpson Hurricane Wind Scale)
The hurricane appears to be steadily organizing, with infrared satellite loops indicating a well-defined central dense overcast (CDO), alongside a developing eye.
However, Danielle appears to be battling some slight dry air on its western periphery, as evidenced by water vapor loops. However, recent convective trends seem to indicate that perhaps the hurricane is managing to gradually mix out the aforementioned dry air. We'll see if this continues. Either way, only a gradual intensification is to be expected tonight, and Danielle should not reach Category 2 status until tomorrow morning, depending on how well she does against the dry air. This dry air will not be enough to weaken her, however, so that should definitely be remembered.
Danielle also has a well-organized structure aloft, with a recent SSMI/S microwave pass indicating a closed eyewall and gradually clearing eye aloft.
Upper-tropospheric outflow is well-defined both poleward and equatorial, but restricted along the western side, owing to the dry air. I suspect that dry air will continue to limit outflow in that direction for at least the next 12 hours.
All in all, based on everything I've analyzed, I believe that Danielle will continue to steadily intensify over the next 24-30 hours, as an an upper-level anticyclone moves more or less in tandem with the system. Thereafter, southwesterly shear is forecast to increase as a secondary trough, a weaker one currently in the vicinity of the east coast/western Atlantic moves eastward and impinges upon the hurricane. This should slow intensification beyond that time, but will probably not be enough to effectively prevent it, as I still feel the GFS is overdoing the shear, based on the current strength of the aforementioned trough. The 18z GFDL generally agrees with my thinking, and foresees that Danielle will be a steady state hurricane from 36-60 hours, after which point it will have a brief, 12 hour window of opportunity to strength to a major hurricane. The GFDL brings this up to 107 knots, and this is certainly possible.
I'll go slightly higher, however, and forecast a 110 knot hurricane, which is a hurricane with winds of 125 mph.
The track forecast for Danielle has, fortunately, become much simpler this evening. The models now unanimously agree upon a highly amplified upper air pattern fully recurving the hurricane out to sea. Indeed, even the NOGAPS, which has had an equatorial bias with this storm, has joined the recurvature crew.
Current model trends suggest that Danielle will pass well to the east of Bermuda, by about 150 to 200 miles, on day five. However, given some uncertainties in the evolution of a well-defined upper-level trough currently moving across the central plains and midwestern states, interests in Bermuda should closely monitor the progress of this system. Interests in eastern Atlantic Canada should also follow the progress of Danielle over the next several days, even though at this moment it appears that a landfall will not occur there (same with Bermuda). My track forecast is in best agreement with the 18z GFDL.
Invest 96L
A well-defined, convectively active tropical wave, is located in the far eastern Atlantic Ocean, about 150 miles south of the Cape Verde Islands. This wave was dubbed "Invest 96L" by the National Hurricane Center this afternoon. This system certainly has the potential to become the season's next named storm over the next couple of days.
Infrared satellite loops suggest that, while convection had waned in recent hours, it is making something of a comeback now. However, CIMSS 850 mb vorticity data suggests that the system still needs to tighten up its low-level circulation, which will take some time. However, conditions appear favorable for some slow additional development of this system as it moves W to WNW over the next couple of days, at around 15 to 20 mph.
Most the models, save the GFDL and NOGAPS, which does not develop it, make this system a tropical cyclone anywhere between tomorrow morning and Wednesday morning. Shear is forecast to be favorable for intensification of the system throughout much of the forecast period, with an upper-level anticyclone forecast to move roughly in tandem with the system.
The 18z HWRF makes this system a hurricane in four days, and more intensification should be expected during the three to four day time frame, so this possibility cannot be discounted. At this time, I do see this system being named, probably by Wednesday morning.
The track of 96L is much more uncertain than it is with Danielle. Some models forecast recurvature, while others forecast a more westward motion, toward the southeast United States coast. I'll split the difference out of respect for the models, but I still think they are overdoing the weakness in the ridge, and the amplification of the upper pattern. This is one to watch, if the equatorial shift we observed in the models earlier continues to occur.
Hurricane Danielle continues to churn in the central Atlantic Ocean, and is located between Africa and the Lesser Antilles. Here is the latest information put out by the National Hurricane Center at the 11:00 PM EDT advisory:
Wind: 85 mph, with higher gusts
Pressure: 982 mb
Movement: WNW at 20 mph
Location: 15.6°N 43.2°W
Category: 1 (Saffir-Simpson Hurricane Wind Scale)
The hurricane appears to be steadily organizing, with infrared satellite loops indicating a well-defined central dense overcast (CDO), alongside a developing eye.
However, Danielle appears to be battling some slight dry air on its western periphery, as evidenced by water vapor loops. However, recent convective trends seem to indicate that perhaps the hurricane is managing to gradually mix out the aforementioned dry air. We'll see if this continues. Either way, only a gradual intensification is to be expected tonight, and Danielle should not reach Category 2 status until tomorrow morning, depending on how well she does against the dry air. This dry air will not be enough to weaken her, however, so that should definitely be remembered.
Danielle also has a well-organized structure aloft, with a recent SSMI/S microwave pass indicating a closed eyewall and gradually clearing eye aloft.
Upper-tropospheric outflow is well-defined both poleward and equatorial, but restricted along the western side, owing to the dry air. I suspect that dry air will continue to limit outflow in that direction for at least the next 12 hours.
All in all, based on everything I've analyzed, I believe that Danielle will continue to steadily intensify over the next 24-30 hours, as an an upper-level anticyclone moves more or less in tandem with the system. Thereafter, southwesterly shear is forecast to increase as a secondary trough, a weaker one currently in the vicinity of the east coast/western Atlantic moves eastward and impinges upon the hurricane. This should slow intensification beyond that time, but will probably not be enough to effectively prevent it, as I still feel the GFS is overdoing the shear, based on the current strength of the aforementioned trough. The 18z GFDL generally agrees with my thinking, and foresees that Danielle will be a steady state hurricane from 36-60 hours, after which point it will have a brief, 12 hour window of opportunity to strength to a major hurricane. The GFDL brings this up to 107 knots, and this is certainly possible.
I'll go slightly higher, however, and forecast a 110 knot hurricane, which is a hurricane with winds of 125 mph.
The track forecast for Danielle has, fortunately, become much simpler this evening. The models now unanimously agree upon a highly amplified upper air pattern fully recurving the hurricane out to sea. Indeed, even the NOGAPS, which has had an equatorial bias with this storm, has joined the recurvature crew.
Current model trends suggest that Danielle will pass well to the east of Bermuda, by about 150 to 200 miles, on day five. However, given some uncertainties in the evolution of a well-defined upper-level trough currently moving across the central plains and midwestern states, interests in Bermuda should closely monitor the progress of this system. Interests in eastern Atlantic Canada should also follow the progress of Danielle over the next several days, even though at this moment it appears that a landfall will not occur there (same with Bermuda). My track forecast is in best agreement with the 18z GFDL.
Invest 96L
A well-defined, convectively active tropical wave, is located in the far eastern Atlantic Ocean, about 150 miles south of the Cape Verde Islands. This wave was dubbed "Invest 96L" by the National Hurricane Center this afternoon. This system certainly has the potential to become the season's next named storm over the next couple of days.
Infrared satellite loops suggest that, while convection had waned in recent hours, it is making something of a comeback now. However, CIMSS 850 mb vorticity data suggests that the system still needs to tighten up its low-level circulation, which will take some time. However, conditions appear favorable for some slow additional development of this system as it moves W to WNW over the next couple of days, at around 15 to 20 mph.
Most the models, save the GFDL and NOGAPS, which does not develop it, make this system a tropical cyclone anywhere between tomorrow morning and Wednesday morning. Shear is forecast to be favorable for intensification of the system throughout much of the forecast period, with an upper-level anticyclone forecast to move roughly in tandem with the system.
The 18z HWRF makes this system a hurricane in four days, and more intensification should be expected during the three to four day time frame, so this possibility cannot be discounted. At this time, I do see this system being named, probably by Wednesday morning.
The track of 96L is much more uncertain than it is with Danielle. Some models forecast recurvature, while others forecast a more westward motion, toward the southeast United States coast. I'll split the difference out of respect for the models, but I still think they are overdoing the weakness in the ridge, and the amplification of the upper pattern. This is one to watch, if the equatorial shift we observed in the models earlier continues to occur.
Sunday, August 22, 2010
Tropical weather analysis - August 22, 2010
Danielle
Tropical Storm Danielle has formed today in the eastern Atlantic Ocean several hundred miles west of the Cape Verde Islands. Here is the latest information from the National Hurricane Center, released at 5:00 PM EDT:
Maximum sustained winds: 50 mph, with higher gusts
Minimum central pressure: 1000 mb
Movement: NW at 13 mph
Location: 14.2°N 35.9°W
Shortwave infrared satellite images suggest that the circulation center is almost completely tucked underneath the large mass of very deep convection, but is very slightly exposed to the east. In fact, cloud tops are as cold as -90°C near the middle of the large convective ball.
There is still no evidence of upper-level outflow in the eastern semicircle, due to the continuance of strong easterly shear, analyzed as high as 30 knots by data from the University of Wisconsin's CIMSS. However, given current organizational trends on satellite imagery, I do not believe that the shear is as strong as is being analyzed by CIMSS. This shear is being induced in part by a large and complex area of disturbed weather several hundred miles to the east of Danielle, which some models forecast will become the season's next tropical depression over the next several days, and also partly by an intensifying upper-level anticyclone centered to the north of Danielle.
The cyclone currently appears to be moving NW, based on low cloud motions observed on satellite imagery. However, water vapor animations and 500 mb steering data from CIMSS suggest that this is merely a temporary motion, albeit pronounced, and Danielle should resume a WNW track over the next 6-12 hours. And based on water vapor imagery, a due W motion is even possible as Danielle nears 45W. But this will depend upon how the subtropical ridge evolves over the coming days. But this is a possibility that needs to be carefully assessed.
Based on water vapor animations and various other steering data, I strongly believe that the models are overdoing the trough and the consequent weakness in the Atlantic subtropical ridge. The culprit for potential recurvature that the models are seeing is a vigorous mid- to upper-level trough currently centered across the west-central United States. Though it cannot be said with absolute certainty at this juncture, it is quite possible that this trough will emerge too far north to completely recurve Danielle.
All of the models unanimously agree on Danielle moving generally WNW for the next 72 hours or so with a gradual increase in forward speed, after which point the subtropical ridge is forecast to reveal a weakness from 60 to 70W as the aforementioned trough emerges from the mid-Atlantic or New England. At that point, a gradual deceleration can be expected, and Danielle will likely turn NW after 72 hours, and could turn N by day four or five. However, this is becoming less likely. Instead, a slow NW motion should be expected by that time.
Beyond 120 hours, most of the models show a reestablishment of the subtropical ridge to the north of the system, which would tend to favor a more W track at the end of the forecast period. So for now, I agree with the NHC's current forecast track, given the strong uncertainty, greater than usual, in the forecast track.
The intensity forecast, thankfully, is much easier. The currently easterly shear should abate by tomorrow as the aforementioned anticyclone becomes more collocated with Danielle. It should then move in tandem with the cyclone for the next three days, providing a low shear environment favorable for strengthening. After 72 hours, southwesterly shear is forecast to increase as Danielle encounters an upper low currently centered in the central Atlantic Ocean. However, I feel the GFS may be overdoing this shear, as the upper low is also forecast to fill and gradually dissipate by that time. And the trough appears to weak at 300-200 mb to produce appreciable shear over Danielle during this time.
Based on everything I've observed tonight, I have opted to go slightly higher than the NHC intensity scheme, and forecast an intensity of 105 mph (90 kt) at day five. However, there is still very much a chance that Danielle could strengthen further and become a major hurricane. The 18z GFDL reflects this possibility, forecasting a 101 knot hurricane at the end of the forecast period. This is certainly possible, given that vertical shear could decrease by late day four and into day five.
Interests in Bermuda, Atlantic Canada, and the East Coast of the United States should closely follow the progress of Danielle over the next several days.
Elsewhere
Elsewhere, some of the models are suggesting that the area of disturbed weather behind Danielle will develop into a tropical depression over the next few days. This is certainly possible, and will have to be watched closely over the coming days.
Tropical Storm Danielle has formed today in the eastern Atlantic Ocean several hundred miles west of the Cape Verde Islands. Here is the latest information from the National Hurricane Center, released at 5:00 PM EDT:
Maximum sustained winds: 50 mph, with higher gusts
Minimum central pressure: 1000 mb
Movement: NW at 13 mph
Location: 14.2°N 35.9°W
Shortwave infrared satellite images suggest that the circulation center is almost completely tucked underneath the large mass of very deep convection, but is very slightly exposed to the east. In fact, cloud tops are as cold as -90°C near the middle of the large convective ball.
There is still no evidence of upper-level outflow in the eastern semicircle, due to the continuance of strong easterly shear, analyzed as high as 30 knots by data from the University of Wisconsin's CIMSS. However, given current organizational trends on satellite imagery, I do not believe that the shear is as strong as is being analyzed by CIMSS. This shear is being induced in part by a large and complex area of disturbed weather several hundred miles to the east of Danielle, which some models forecast will become the season's next tropical depression over the next several days, and also partly by an intensifying upper-level anticyclone centered to the north of Danielle.
The cyclone currently appears to be moving NW, based on low cloud motions observed on satellite imagery. However, water vapor animations and 500 mb steering data from CIMSS suggest that this is merely a temporary motion, albeit pronounced, and Danielle should resume a WNW track over the next 6-12 hours. And based on water vapor imagery, a due W motion is even possible as Danielle nears 45W. But this will depend upon how the subtropical ridge evolves over the coming days. But this is a possibility that needs to be carefully assessed.
Based on water vapor animations and various other steering data, I strongly believe that the models are overdoing the trough and the consequent weakness in the Atlantic subtropical ridge. The culprit for potential recurvature that the models are seeing is a vigorous mid- to upper-level trough currently centered across the west-central United States. Though it cannot be said with absolute certainty at this juncture, it is quite possible that this trough will emerge too far north to completely recurve Danielle.
All of the models unanimously agree on Danielle moving generally WNW for the next 72 hours or so with a gradual increase in forward speed, after which point the subtropical ridge is forecast to reveal a weakness from 60 to 70W as the aforementioned trough emerges from the mid-Atlantic or New England. At that point, a gradual deceleration can be expected, and Danielle will likely turn NW after 72 hours, and could turn N by day four or five. However, this is becoming less likely. Instead, a slow NW motion should be expected by that time.
Beyond 120 hours, most of the models show a reestablishment of the subtropical ridge to the north of the system, which would tend to favor a more W track at the end of the forecast period. So for now, I agree with the NHC's current forecast track, given the strong uncertainty, greater than usual, in the forecast track.
The intensity forecast, thankfully, is much easier. The currently easterly shear should abate by tomorrow as the aforementioned anticyclone becomes more collocated with Danielle. It should then move in tandem with the cyclone for the next three days, providing a low shear environment favorable for strengthening. After 72 hours, southwesterly shear is forecast to increase as Danielle encounters an upper low currently centered in the central Atlantic Ocean. However, I feel the GFS may be overdoing this shear, as the upper low is also forecast to fill and gradually dissipate by that time. And the trough appears to weak at 300-200 mb to produce appreciable shear over Danielle during this time.
Based on everything I've observed tonight, I have opted to go slightly higher than the NHC intensity scheme, and forecast an intensity of 105 mph (90 kt) at day five. However, there is still very much a chance that Danielle could strengthen further and become a major hurricane. The 18z GFDL reflects this possibility, forecasting a 101 knot hurricane at the end of the forecast period. This is certainly possible, given that vertical shear could decrease by late day four and into day five.
Interests in Bermuda, Atlantic Canada, and the East Coast of the United States should closely follow the progress of Danielle over the next several days.
Elsewhere
Elsewhere, some of the models are suggesting that the area of disturbed weather behind Danielle will develop into a tropical depression over the next few days. This is certainly possible, and will have to be watched closely over the coming days.
Friday, August 20, 2010
Tropical weather analysis - August 20, 2010
Invest 95L
A broad area of low pressure, associated with a large area of disturbed weather within the ITCZ, is centered a couple hundred miles south-southwest of the Cape Verde Islands. This area was designated "Invest 95L" by the National Hurricane Center early this morning. Satellite animations suggest that the system is gradually becoming better organized, but shower activity is not persistent yet, and in fact, has waned markedly in recent hours. Additionally, vorticity data from CIMSS indicates that, although the low- and mid-level circulations are gradually becoming better organized, this system still has quite a bit of work to do, namely detaching itself from the active ITCZ in which it is currently embedded. The aforementioned vorticity data reveals that 95L is embedded within a large scale cyclonic circulation associated with an active ITCZ.
95L is currently moving W under the influence of a well-defined low- to mid-tropospheric ridge. This general motion should continue until a more WNW component becomes established in about two days. By about four days, the models are in unanimous agreement on a highly amplified mid- to upper-level pattern, as an unseasonably powerful trough erodes the subtropical ridge. At this point, 95L (by then a powerful hurricane) is forecast to approach this weakness and begin turning more poleward with a decrease in forward speed.
However, this is where the forecast gets tricky. Not all of the models agree on there being enough troughing to completely recurve the system, at least insofar as Bermuda is concerned. The CMC depicts the strongest trough, allowing 95L to completely recurve well east of Bermuda, and the GFDL and GFS follow suit, though depict a somewhat weaker trough. The HWRF, NOGAPS, and ECMWF depict a less amplified trough, and keep a more zonal flow in place, maintaining the Atlantic subtropical ridge enough to prevent complete recurvature prior to reaching the longitude of Bermuda. In fact, the 12z ECMWF is indicating that 95L will approach the southeast United States coast in approximately ten days, though this may be related more to the fact that it keeps the system quite weak until this time, and therefore it follows the low-level flow.
Regardless, there are still a number of possible scenarios for this system, though eventual recurvature seems probable, it's just the degree and timing of such that's uncertain at this juncture.
95L is currently experiencing about 15 kt of easterly shear. The 18z GFS keeps the shear quite weak throughout the forecast period, and even starts to develop an 200 mb anticyclone over the system in as little as 24 hours. The anticyclone becomes particularly well-established by 48 hours, and continues to strengthen throughout much of the forecast period. It should also move in tandem with 95L, particularly beyond 48 hours. These conditions should allow 95L to ultimately become the season's second hurricane, and quite possibly, the first major hurricane, as well.
In the short-term though, 95L has work to do. I feel the models are developing this a little too quickly, with some doing it as early as tomorrow. I will forecast continued slow development, and I expect this system to become a tropical depression on Monday afternoon. It should be noted that the weaker the system stays, the more westward it goes.
Residents along the Leeward Islands and Puerto Rico, as well as Bermuda and the east coast from North Carolina to Atlantic Canada should monitor the progress of this system very carefully over the coming days, given the inherent uncertainties ever present in long-range forecasts.
Elsewhere
There really isn't much going on in the tropics at the moment, other than Invest 95L. However, most of the models agree on a piece of 850 mb vorticity detaching from the aforementioned trough in about four days, the one forecast to recurve 95L. This trough split is forecast to occur along the warm waters of the Gulf Stream offshore the east coast of the United States, so this bears watching.
There are also a couple of decent looking entities over Africa, at around 8 to 11N, which may be identified as tropical waves later on as they emerge into the eastern Atlantic. These features have some rather decent cyclonic circulations, so they certainly bear watching as they move into the Atlantic over the next several days.
A broad area of low pressure, associated with a large area of disturbed weather within the ITCZ, is centered a couple hundred miles south-southwest of the Cape Verde Islands. This area was designated "Invest 95L" by the National Hurricane Center early this morning. Satellite animations suggest that the system is gradually becoming better organized, but shower activity is not persistent yet, and in fact, has waned markedly in recent hours. Additionally, vorticity data from CIMSS indicates that, although the low- and mid-level circulations are gradually becoming better organized, this system still has quite a bit of work to do, namely detaching itself from the active ITCZ in which it is currently embedded. The aforementioned vorticity data reveals that 95L is embedded within a large scale cyclonic circulation associated with an active ITCZ.
95L is currently moving W under the influence of a well-defined low- to mid-tropospheric ridge. This general motion should continue until a more WNW component becomes established in about two days. By about four days, the models are in unanimous agreement on a highly amplified mid- to upper-level pattern, as an unseasonably powerful trough erodes the subtropical ridge. At this point, 95L (by then a powerful hurricane) is forecast to approach this weakness and begin turning more poleward with a decrease in forward speed.
However, this is where the forecast gets tricky. Not all of the models agree on there being enough troughing to completely recurve the system, at least insofar as Bermuda is concerned. The CMC depicts the strongest trough, allowing 95L to completely recurve well east of Bermuda, and the GFDL and GFS follow suit, though depict a somewhat weaker trough. The HWRF, NOGAPS, and ECMWF depict a less amplified trough, and keep a more zonal flow in place, maintaining the Atlantic subtropical ridge enough to prevent complete recurvature prior to reaching the longitude of Bermuda. In fact, the 12z ECMWF is indicating that 95L will approach the southeast United States coast in approximately ten days, though this may be related more to the fact that it keeps the system quite weak until this time, and therefore it follows the low-level flow.
Regardless, there are still a number of possible scenarios for this system, though eventual recurvature seems probable, it's just the degree and timing of such that's uncertain at this juncture.
95L is currently experiencing about 15 kt of easterly shear. The 18z GFS keeps the shear quite weak throughout the forecast period, and even starts to develop an 200 mb anticyclone over the system in as little as 24 hours. The anticyclone becomes particularly well-established by 48 hours, and continues to strengthen throughout much of the forecast period. It should also move in tandem with 95L, particularly beyond 48 hours. These conditions should allow 95L to ultimately become the season's second hurricane, and quite possibly, the first major hurricane, as well.
In the short-term though, 95L has work to do. I feel the models are developing this a little too quickly, with some doing it as early as tomorrow. I will forecast continued slow development, and I expect this system to become a tropical depression on Monday afternoon. It should be noted that the weaker the system stays, the more westward it goes.
Residents along the Leeward Islands and Puerto Rico, as well as Bermuda and the east coast from North Carolina to Atlantic Canada should monitor the progress of this system very carefully over the coming days, given the inherent uncertainties ever present in long-range forecasts.
Elsewhere
There really isn't much going on in the tropics at the moment, other than Invest 95L. However, most of the models agree on a piece of 850 mb vorticity detaching from the aforementioned trough in about four days, the one forecast to recurve 95L. This trough split is forecast to occur along the warm waters of the Gulf Stream offshore the east coast of the United States, so this bears watching.
There are also a couple of decent looking entities over Africa, at around 8 to 11N, which may be identified as tropical waves later on as they emerge into the eastern Atlantic. These features have some rather decent cyclonic circulations, so they certainly bear watching as they move into the Atlantic over the next several days.
Tuesday, August 10, 2010
Tropical weather analysis - August 10, 2010
Tropical Depression Five
Invest 94L developed into Tropical Depression Five this afternoon in the eastern Gulf of Mexico. As of the 7:30 PM EDT advisory from the National Hurricane Center, the center was estimated to be near 26.0°N 84.1°W, moving NW at 6 mph. The minimum central pressure is estimated at 1007 mb. Visible satellite animations suggest that though the overall circulation is quite vigorous, large, and well-defined, the cyclone lacks deep convection near the assumed center.
The environment across the Gulf of Mexico is characterized by light easterly shear and some minor subsidence (dry air), as noted on water vapor imagery. Water vapor imagery also depicts a weak upper low centered about 100 miles to the north of the depression. This low appears to be moving rapidly toward the west, however, and this feature is forecast by the 18z GFS to quickly lift out and deamplify while moving westward toward the upper Texas coast. This upper low is currently having a slight negative effect on TD5, with a small dry slot seen to the northwest of the center on water vapor imagery. As the low lifts out, however, the environment will become more favorable, characterized by less subsidence and more uplift. This should allow the cyclone to begin intensifying at a more steady rate in around 12 hours.
Further supporting the GFS' forecast of a rapidly weakening and deamplifying upper low is 200 mb vorticity data from CIMSS, which strongly implies that said upper low has significantly weakened over the last six hours, while having moved westward. There are also indications within the mid- upper level steering that the upper low will move away from the depression at a very rapid rate. I am expecting a general W motion of the upper low throughout the next two days until it reaches the upper Texas coast and dissipates, although a slightly S of due W component of motion cannot be ruled out in the near-term.
Regardless, it appears that the upper low will have a negative influence of TD5 for about the next 12 hours. My own forecast track is in best agreement with the 12z UKMET, which moves the system WNW within the next 6-12 hours, followed by a steady WNW motion until landfall. My track takes the system inland near Cocodrie in two days. I feel the UKMET is a little slow in bringing this system inland, however (60 hours). Based on the current and forecast steering, I would expect to see a bit of a westward shift of the NHC's track over the next day or so, along with the models. At this time, a landfall between Intracoastal City and Buras cannot be ruled out, but the greatest threat appears to be central Louisiana at this time.
Nevertheless, given the uncertainty, residents from Intracoastal City, Louisiana eastward to Pensacola, Florida should closely monitor the progress of the depression over the next couple of days. Should the depression take a day or so longer to move ashore, as predicted by some models, it would likely become stronger than is currently being forecast by the NHC or myself.
All in all, the NHC's current track looks good, but I'm just a bit to the west of that. Some moderate easterly should could develop over the system in about 24-30 as hours as the high pressure ridge to the north strengthens. However, given that the cyclone will be moving roughly in tandem with the upper flow, I am not expecting this to slow intensification significantly. In fact, I think the current NHC forecast of only 45 mph is much too low. I'm going with 60 mph at the moment.
Watches and warnings
A tropical storm warning has been issued from:
Destin, Florida to Intracoastal City, Louisiana. This includes the city of New Orleans and Lake Pontchartrain.
Invest 94L developed into Tropical Depression Five this afternoon in the eastern Gulf of Mexico. As of the 7:30 PM EDT advisory from the National Hurricane Center, the center was estimated to be near 26.0°N 84.1°W, moving NW at 6 mph. The minimum central pressure is estimated at 1007 mb. Visible satellite animations suggest that though the overall circulation is quite vigorous, large, and well-defined, the cyclone lacks deep convection near the assumed center.
The environment across the Gulf of Mexico is characterized by light easterly shear and some minor subsidence (dry air), as noted on water vapor imagery. Water vapor imagery also depicts a weak upper low centered about 100 miles to the north of the depression. This low appears to be moving rapidly toward the west, however, and this feature is forecast by the 18z GFS to quickly lift out and deamplify while moving westward toward the upper Texas coast. This upper low is currently having a slight negative effect on TD5, with a small dry slot seen to the northwest of the center on water vapor imagery. As the low lifts out, however, the environment will become more favorable, characterized by less subsidence and more uplift. This should allow the cyclone to begin intensifying at a more steady rate in around 12 hours.
Further supporting the GFS' forecast of a rapidly weakening and deamplifying upper low is 200 mb vorticity data from CIMSS, which strongly implies that said upper low has significantly weakened over the last six hours, while having moved westward. There are also indications within the mid- upper level steering that the upper low will move away from the depression at a very rapid rate. I am expecting a general W motion of the upper low throughout the next two days until it reaches the upper Texas coast and dissipates, although a slightly S of due W component of motion cannot be ruled out in the near-term.
Regardless, it appears that the upper low will have a negative influence of TD5 for about the next 12 hours. My own forecast track is in best agreement with the 12z UKMET, which moves the system WNW within the next 6-12 hours, followed by a steady WNW motion until landfall. My track takes the system inland near Cocodrie in two days. I feel the UKMET is a little slow in bringing this system inland, however (60 hours). Based on the current and forecast steering, I would expect to see a bit of a westward shift of the NHC's track over the next day or so, along with the models. At this time, a landfall between Intracoastal City and Buras cannot be ruled out, but the greatest threat appears to be central Louisiana at this time.
Nevertheless, given the uncertainty, residents from Intracoastal City, Louisiana eastward to Pensacola, Florida should closely monitor the progress of the depression over the next couple of days. Should the depression take a day or so longer to move ashore, as predicted by some models, it would likely become stronger than is currently being forecast by the NHC or myself.
All in all, the NHC's current track looks good, but I'm just a bit to the west of that. Some moderate easterly should could develop over the system in about 24-30 as hours as the high pressure ridge to the north strengthens. However, given that the cyclone will be moving roughly in tandem with the upper flow, I am not expecting this to slow intensification significantly. In fact, I think the current NHC forecast of only 45 mph is much too low. I'm going with 60 mph at the moment.
Watches and warnings
A tropical storm warning has been issued from:
Destin, Florida to Intracoastal City, Louisiana. This includes the city of New Orleans and Lake Pontchartrain.
Monday, August 9, 2010
Tropical weather analysis - August 9, 2010
Invest 93L
Invest 93L continues to churn out in the central Atlantic Ocean, about 875 miles east-northeast of the Leeward Islands. Latest shortwave infrared imagery reveals that this system is still disorganized, with the well-defined surface circulation clearly evident at 23N 50W, moving WNW at 10 to 15 mph. This center is completely exposed from the pulsating deep convection, as has been the case over the last couple days. However, it looks even less organized today than 24 hours ago, with any deep convection displaced over 100 miles to the east and northeast of the system.
Additionally, CIMSS vorticity data suggests a vertical decoupling of the 850 and 500 mb circulations. CIMSS vertical shear analysis indicates only 5 kt of southwesterly shear over the center of the system, with 20 kt on the western side. Certainly, satellite-observed low-cloud motions seem to indicate a relaxation of the shear, confirming the analysis from CIMSS.
93L is being steered by a low- to mid-level tropospheric ridge to the north. A weakness is evident in the ridge near 60W westward to nearly 75W. The models suggest that this general motion should continue until around 24 hour, at which point a turn to the NW with some deceleration is expected as 93L rounds the aforementioned weakness in the Atlantic subtropical ridge. By 36 hours or so, a turn to the N should occur, followed by a turn to the NE by 48-54 hours. This is a rather fortunate track scenario, taking the system completely out to sea. The models differ drastically as to how strong 93L will get, and when.
The GFDL and HWRF models take the system to hurricane status on day four as the system interacts more strongly with the trough. We have seen trough interactions fuel intensification in the past, so this possibility cannot be ruled out. And given that sea surface temperatures (SSTs) in this area, 35 to 40N 45 to 50W, where the aforementioned models foresee hurricane status, are running at 26 to 28C, this is probably not baroclinic strengthening being foreseen in these models.
The remainder of the models are less bullish, and peak the system about a day earlier. I will compromise between these extremes, and still, for the third consecutive day, forecast this system to peak at 65 mph in about 84 hours. I tend to lean toward the latter camp (GFS, ECMWF, UKMET, CMC, and NOGAPS) more strongly, based on previous and current convective trends, along with shear forecasts from the GFS. Nevertheless, environmental conditions are slowly becoming more favorable for development, with a lowering of the shear today (with further lowering expected), along with a gradual dissipation of the dry air, as noted on water vapor imagery.
I still expect this system to eventually become a tropical cyclone.
Invest 94L
The basin's biggest threat is still Invest 94L, a broad area of low pressure located in the extreme eastern Gulf of Mexico off the southwestern coast of Florida. My best estimate of the broad surface center, based on the remaining visible satellite images, doppler radar animations, and CIMSS 850 mb vorticity data is 25N 81W. Recent convective trends suggest that 94L is attempting to organize, with the development of a small area of deep convection over its estimated center.
Some modest pressure falls have been noted across the area this afternoon, and buoy observations across the eastern Gulf suggest that the system maintains its closed surface circulation, and that it might have become even better defined today.
The key player in the genesis, or lack thereof, of Invest 94L will be a newly-developed upper low just to the north of the northern Bahamas. Currently, this low is moving west-northwest, while 94L is moving west-southwest. Steering data from CIMSS suggests a WSW motion should continue for the next 6-12 hours, after which point a turn to the WNW should occur, followed by a turn to the NW in about 72 hours as the system nears the northern Gulf Coast. The majority of the models still bring this system into southeast or eastern Louisiana in about 72 hours, and the 18z GFDL makes this system a hurricane and brings it west of New Orleans. I will not go that high just yet, because I want to see the evolution of the aforementioned upper low first, as if it gets close enough, it could shear 94L, and inhibit significant development.
The 18z GFS suggests that the upper low will move generally westward and gradually weaken and deamplify as it does so. Additionally, steering data from PSU's e-wall suggests that the upper low could well outrun Invest 94L, leaving behind a very favorable upper environment as upper ridging builds overhead. In addition, the shear is forecast to be very light, and the currently northerly shear is forecast to abate soon, and veer to easterly or southeasterly by 24 hours. The shear may switch back around to northeasterly and become stronger again before the system moves inland.
There are three possibilities right now with 94L:
1. It ultimately becomes sheared and dissipated due to the upper low.
2. It becomes a tropical cyclone.
3. It becomes a subtropical cyclone.
Based on what I've seen, I'm expecting tropical cyclogenesis to occur with this wave by tomorrow night or Wednesday morning. However, this will be highly dependent upon the evolution and strength of the upper low. Subtropical development could also occur if the two become vertically aligned, or near so. A reconnaissance aircraft is scheduled to investigate this system tomorrow, if necessary.
Residents along the northern Gulf Coast from Biloxi to Morgan City should closely follow the evolution of this system over the next several days. Regardless of development, locally heavy rainfall, above normal tides, and strong gusty winds will overspread portions of the aforementioned areas during the next few days.
Invest 93L continues to churn out in the central Atlantic Ocean, about 875 miles east-northeast of the Leeward Islands. Latest shortwave infrared imagery reveals that this system is still disorganized, with the well-defined surface circulation clearly evident at 23N 50W, moving WNW at 10 to 15 mph. This center is completely exposed from the pulsating deep convection, as has been the case over the last couple days. However, it looks even less organized today than 24 hours ago, with any deep convection displaced over 100 miles to the east and northeast of the system.
Additionally, CIMSS vorticity data suggests a vertical decoupling of the 850 and 500 mb circulations. CIMSS vertical shear analysis indicates only 5 kt of southwesterly shear over the center of the system, with 20 kt on the western side. Certainly, satellite-observed low-cloud motions seem to indicate a relaxation of the shear, confirming the analysis from CIMSS.
93L is being steered by a low- to mid-level tropospheric ridge to the north. A weakness is evident in the ridge near 60W westward to nearly 75W. The models suggest that this general motion should continue until around 24 hour, at which point a turn to the NW with some deceleration is expected as 93L rounds the aforementioned weakness in the Atlantic subtropical ridge. By 36 hours or so, a turn to the N should occur, followed by a turn to the NE by 48-54 hours. This is a rather fortunate track scenario, taking the system completely out to sea. The models differ drastically as to how strong 93L will get, and when.
The GFDL and HWRF models take the system to hurricane status on day four as the system interacts more strongly with the trough. We have seen trough interactions fuel intensification in the past, so this possibility cannot be ruled out. And given that sea surface temperatures (SSTs) in this area, 35 to 40N 45 to 50W, where the aforementioned models foresee hurricane status, are running at 26 to 28C, this is probably not baroclinic strengthening being foreseen in these models.
The remainder of the models are less bullish, and peak the system about a day earlier. I will compromise between these extremes, and still, for the third consecutive day, forecast this system to peak at 65 mph in about 84 hours. I tend to lean toward the latter camp (GFS, ECMWF, UKMET, CMC, and NOGAPS) more strongly, based on previous and current convective trends, along with shear forecasts from the GFS. Nevertheless, environmental conditions are slowly becoming more favorable for development, with a lowering of the shear today (with further lowering expected), along with a gradual dissipation of the dry air, as noted on water vapor imagery.
I still expect this system to eventually become a tropical cyclone.
Invest 94L
The basin's biggest threat is still Invest 94L, a broad area of low pressure located in the extreme eastern Gulf of Mexico off the southwestern coast of Florida. My best estimate of the broad surface center, based on the remaining visible satellite images, doppler radar animations, and CIMSS 850 mb vorticity data is 25N 81W. Recent convective trends suggest that 94L is attempting to organize, with the development of a small area of deep convection over its estimated center.
Some modest pressure falls have been noted across the area this afternoon, and buoy observations across the eastern Gulf suggest that the system maintains its closed surface circulation, and that it might have become even better defined today.
The key player in the genesis, or lack thereof, of Invest 94L will be a newly-developed upper low just to the north of the northern Bahamas. Currently, this low is moving west-northwest, while 94L is moving west-southwest. Steering data from CIMSS suggests a WSW motion should continue for the next 6-12 hours, after which point a turn to the WNW should occur, followed by a turn to the NW in about 72 hours as the system nears the northern Gulf Coast. The majority of the models still bring this system into southeast or eastern Louisiana in about 72 hours, and the 18z GFDL makes this system a hurricane and brings it west of New Orleans. I will not go that high just yet, because I want to see the evolution of the aforementioned upper low first, as if it gets close enough, it could shear 94L, and inhibit significant development.
The 18z GFS suggests that the upper low will move generally westward and gradually weaken and deamplify as it does so. Additionally, steering data from PSU's e-wall suggests that the upper low could well outrun Invest 94L, leaving behind a very favorable upper environment as upper ridging builds overhead. In addition, the shear is forecast to be very light, and the currently northerly shear is forecast to abate soon, and veer to easterly or southeasterly by 24 hours. The shear may switch back around to northeasterly and become stronger again before the system moves inland.
There are three possibilities right now with 94L:
1. It ultimately becomes sheared and dissipated due to the upper low.
2. It becomes a tropical cyclone.
3. It becomes a subtropical cyclone.
Based on what I've seen, I'm expecting tropical cyclogenesis to occur with this wave by tomorrow night or Wednesday morning. However, this will be highly dependent upon the evolution and strength of the upper low. Subtropical development could also occur if the two become vertically aligned, or near so. A reconnaissance aircraft is scheduled to investigate this system tomorrow, if necessary.
Residents along the northern Gulf Coast from Biloxi to Morgan City should closely follow the evolution of this system over the next several days. Regardless of development, locally heavy rainfall, above normal tides, and strong gusty winds will overspread portions of the aforementioned areas during the next few days.
Sunday, August 8, 2010
Tropical weather analysis - August 8, 2010
Colin
Tropical Depression Colin degenerated into an open trough of low pressure earlier this afternoon. Regeneration is not expected as the system moves NNE to NE within the southwesterly flow associated with a deep-layer trough exiting the east coast of the United States. Colin, or rather, what's left of it, will ultimately make extratropical transition and subsequently become absorbed into said trough over the next several days. Some rain showers and gusty winds are still possible across Bermuda, but nothing significant. However, some isolated tropical storm force wind gusts, specifically along higher elevations, have been reported on the island.
Invest 93L
Invest 93L, located several hundred miles west-northwest of the Cape Verde Islands, continues to display signs of organization. Shortwave infrared satellite animations suggest that the center is located at 23N 46W, moving WNW. Earlier this afternoon, visible satellite animations clearly depicted a closed surface circulation, albeit exposed from the deep convection due to strong vertical shear. This is still the case this evening, with the well-defined surface center (which is closed) displaced well to the southwest of the pulsating deep convection. 93L continues to be embedded within a relatively dry environment, evidenced on water vapor loops. This, in conjunction with the aforementioned vertical shear, analyzed at 10-20 kt by data from the University of Wisconsin CIMSS, should preclude any rapid development over the next day or so.
This system has been slow to develop, which wasn't really unexpected given the meager environment in which it is currently embedded. But I must admit, this system has certainly developed slower than I thought it would. I thought for sure that by tonight or early tomorrow morning, it would have been labeled a tropical depression.
Additionally, CIMSS vorticity data suggests a vertical decoupling of the low- and mid-level circulations. This is undoubtedly due to the aforementioned shear, which is being induced by an upper-level low several hundred miles west of Invest 93L. All of these negative factors notwithstanding, however, vertical shear has decreased over the system during the last 12 hours. Vertical shear is forecast to ease somewhat over the next several days as the upper low deamplifies and retreats slightly northward. In fact, the 18z GFS suggests that the system will move in tandem with a weak upper-level anticyclone currently centered just to the south of the disturbance. The GFS foresees 93L losing the anticyclone after about 60 or 66 hours. Though shear will increase over the system subsequent to this, it will still be weak enough to allow slow intensification, because the system will probably be moving in tandem with the upper flow beginning in about 84-90 hours.
I expect a continued WNW motion, followed by a gentle bend to the NW with a deceleration in the forward speed beginning in about 48-54 hours as the subtropical ridge weakens with the approach of the powerful longwave trough that recurved Colin. Thereafter, a gradual turn to the N is expected late on day two, followed by a swing to the NE with a gradual increase in forward speed expected late on day three or very early day four.
None of the models significantly intensify the system, and most barely depict it as a perturbation. Though I will still forecast gradual intensification, peaking the system at 65 mph, the models could certainly be onto something given how hesitant this system has been to develop since its designation as an invest. I will continue to carefully assess this possibility.
Invest 94L
An area of disturbed weather located just offshore the Florida east coast was designated "Invest 94L" this afternoon by the National Hurricane Center. The poorly defined center is estimated to be about 75 miles east of Vero Beach, moving slowly to the SW. Surface, buoy, and ASCAT observations suggest that a broad but closed circulation may exist at the surface. Surface pressures are somewhat low in the area, but not overly so, and are not significantly falling at this time.
94L is currently experiencing 20 kt of northwesterly shear induced by the longwave trough currently recurving Colin. This has resulted in a vertical decoupling of the low and mid-level circulation centers, with the latter being displaced about 35-50 miles southeast of the low-level center. In addition to the shear, dry air is being entrained into the circulation due to the aforementioned northwesterly shear. These factors should inhibit any significant development over the next day or two.
Current steering/model data suggests that 94L will be slow to move inland, and most of the models do not bring it inland over central or eastern Florida until around 36 hours. This is certainly possible, and this will be a difficult storm to forecast in the near-term due to the weak steering regime in the region. It is entirely possible that this system could perform an anticyclonic loop over the next day or so. However, I will instead forecast a general, but quite slow (5 mph or less at times) motion to the SW, in agreement with the models and the overall synoptic scale steering pattern.
Thereafter, the system is forecast to pick up speed and emerge into the Gulf of Mexico. This is where the forecast begins to get easier. Most of the models take this system inland over eastern or southeastern Louisiana sometime on Thursday, though many of them do not show much of a surface reflection. This is likely due to the fact that it doesn't appear to have more than a couple days over water after emerging into the Gulf.
After 36 hours, the upper-level winds will relax significantly as upper ridging replaces the current upper troughing. Furthermore, because 94L will be moving WNW, in tandem with the forecast upper-level winds at day two and beyond, it is a good possibility that this system will try and undergo tropical cyclogenesis while moving across the eastern and northern Gulf of Mexico.
I give this system a 40% chance of eventually becoming a tropical cyclone, and residents from Destin to Beaumont should carefully monitor the progress of this system over the next several days. Though at this time, based on the models, it appears that southeast Louisiana is the most likely target for an eventual landfall.
Regardless of development, heavy rainfall will continue spreading over portions of central and south Florida over the next day or two, potentially causing some localized flooding.
Tropical Depression Colin degenerated into an open trough of low pressure earlier this afternoon. Regeneration is not expected as the system moves NNE to NE within the southwesterly flow associated with a deep-layer trough exiting the east coast of the United States. Colin, or rather, what's left of it, will ultimately make extratropical transition and subsequently become absorbed into said trough over the next several days. Some rain showers and gusty winds are still possible across Bermuda, but nothing significant. However, some isolated tropical storm force wind gusts, specifically along higher elevations, have been reported on the island.
Invest 93L
Invest 93L, located several hundred miles west-northwest of the Cape Verde Islands, continues to display signs of organization. Shortwave infrared satellite animations suggest that the center is located at 23N 46W, moving WNW. Earlier this afternoon, visible satellite animations clearly depicted a closed surface circulation, albeit exposed from the deep convection due to strong vertical shear. This is still the case this evening, with the well-defined surface center (which is closed) displaced well to the southwest of the pulsating deep convection. 93L continues to be embedded within a relatively dry environment, evidenced on water vapor loops. This, in conjunction with the aforementioned vertical shear, analyzed at 10-20 kt by data from the University of Wisconsin CIMSS, should preclude any rapid development over the next day or so.
This system has been slow to develop, which wasn't really unexpected given the meager environment in which it is currently embedded. But I must admit, this system has certainly developed slower than I thought it would. I thought for sure that by tonight or early tomorrow morning, it would have been labeled a tropical depression.
Additionally, CIMSS vorticity data suggests a vertical decoupling of the low- and mid-level circulations. This is undoubtedly due to the aforementioned shear, which is being induced by an upper-level low several hundred miles west of Invest 93L. All of these negative factors notwithstanding, however, vertical shear has decreased over the system during the last 12 hours. Vertical shear is forecast to ease somewhat over the next several days as the upper low deamplifies and retreats slightly northward. In fact, the 18z GFS suggests that the system will move in tandem with a weak upper-level anticyclone currently centered just to the south of the disturbance. The GFS foresees 93L losing the anticyclone after about 60 or 66 hours. Though shear will increase over the system subsequent to this, it will still be weak enough to allow slow intensification, because the system will probably be moving in tandem with the upper flow beginning in about 84-90 hours.
I expect a continued WNW motion, followed by a gentle bend to the NW with a deceleration in the forward speed beginning in about 48-54 hours as the subtropical ridge weakens with the approach of the powerful longwave trough that recurved Colin. Thereafter, a gradual turn to the N is expected late on day two, followed by a swing to the NE with a gradual increase in forward speed expected late on day three or very early day four.
None of the models significantly intensify the system, and most barely depict it as a perturbation. Though I will still forecast gradual intensification, peaking the system at 65 mph, the models could certainly be onto something given how hesitant this system has been to develop since its designation as an invest. I will continue to carefully assess this possibility.
Invest 94L
An area of disturbed weather located just offshore the Florida east coast was designated "Invest 94L" this afternoon by the National Hurricane Center. The poorly defined center is estimated to be about 75 miles east of Vero Beach, moving slowly to the SW. Surface, buoy, and ASCAT observations suggest that a broad but closed circulation may exist at the surface. Surface pressures are somewhat low in the area, but not overly so, and are not significantly falling at this time.
94L is currently experiencing 20 kt of northwesterly shear induced by the longwave trough currently recurving Colin. This has resulted in a vertical decoupling of the low and mid-level circulation centers, with the latter being displaced about 35-50 miles southeast of the low-level center. In addition to the shear, dry air is being entrained into the circulation due to the aforementioned northwesterly shear. These factors should inhibit any significant development over the next day or two.
Current steering/model data suggests that 94L will be slow to move inland, and most of the models do not bring it inland over central or eastern Florida until around 36 hours. This is certainly possible, and this will be a difficult storm to forecast in the near-term due to the weak steering regime in the region. It is entirely possible that this system could perform an anticyclonic loop over the next day or so. However, I will instead forecast a general, but quite slow (5 mph or less at times) motion to the SW, in agreement with the models and the overall synoptic scale steering pattern.
Thereafter, the system is forecast to pick up speed and emerge into the Gulf of Mexico. This is where the forecast begins to get easier. Most of the models take this system inland over eastern or southeastern Louisiana sometime on Thursday, though many of them do not show much of a surface reflection. This is likely due to the fact that it doesn't appear to have more than a couple days over water after emerging into the Gulf.
After 36 hours, the upper-level winds will relax significantly as upper ridging replaces the current upper troughing. Furthermore, because 94L will be moving WNW, in tandem with the forecast upper-level winds at day two and beyond, it is a good possibility that this system will try and undergo tropical cyclogenesis while moving across the eastern and northern Gulf of Mexico.
I give this system a 40% chance of eventually becoming a tropical cyclone, and residents from Destin to Beaumont should carefully monitor the progress of this system over the next several days. Though at this time, based on the models, it appears that southeast Louisiana is the most likely target for an eventual landfall.
Regardless of development, heavy rainfall will continue spreading over portions of central and south Florida over the next day or two, potentially causing some localized flooding.
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