Tropical weather analysis - October 24, 2010

Richard

Hurricane Richard put on a burst of intensification this morning, which brought it to the 10th hurricane of this very active season. The last time 10 or more hurricanes occurred in a season was in 2005, and before that, 1998. Even more remarkable is the 17 named storms we've had. Historical records indicate that the last time this occurred was in 1995, when 19 named storms developed.

As of the latest (4:00 PM CDT) NHC advisory on Richard, here is the latest information:

Wind: 90 mph, with higher gusts
Movement: WNW at 13 mph
Pressure: 981 mb
Location: 17.2°N 88.0°W
Category: 1 (Saffir-Simpson Hurricane Wind Scale)

Visible satellite animations depict a small but well-organized hurricane, with recent images indicating a more symmetrical eye and eyewall. In addition, spiral banding has increased and become more organized as well, particularly to the north.

Doppler radar out of Belize also shows that Richard is a well-organized hurricane.

I suspect that Richard will make landfall between Rockville and Belmopan in about 3-5 hours as an intensifying Category 1 hurricane. It is not out of the question, however, for Richard to attain Category 2 status prior to landfall, and residents of Belize should be vigilant for that possibility.

If this forecast track verifies, Rockville will be on the worst side of the hurricane, and may be close enough to catch Richard's eastern eyewall. Sustained Category 1 force winds (74-95 mph) are possible in this area, with higher gusts.

Sustained tropical storm force winds are already impacting a portion of mainland Belize, and sustained hurricane force winds are not far off, and should begin impacting central Belize in about 2 and a half to 3 hours.

Given that Richard is intensifying, sustained tropical storm force winds will be possible all the way to the western end of the Yucatan Peninsula through Monday night or Tuesday morning. Sustained hurricane force winds may penetrate as far inland as western Belize.

In addition to the wind, Richard is expected to produce a storm surge in excess of 3 to 5 feet near and to the north of where the center makes landfall. This surge will be accompanied by large, destructive waves.

The National Hurricane Center forecasts that 3-6 inches of rain will fall across the Bay Islands, Belize, and the southern Yucatan Peninsula, with lesser amounts (1-2 inches) possible across northern Honduras. They predict that localized amounts could be in excess of 10 inches across the Bay Islands, Belize, and the Yucatan Peninsula.

This pretty much falls in line with my own forecast, and interests in the aforementioned areas should be on alert for the possibility of dangerous, life-threatening flash floods and mudslides, especially in mountainous areas.

As is always the case with landfalling tropical systems, Richard could produce isolated tornadoes, especially in the right-front quadrant.

After landfall, Richard will steadily weaken, and given its small size, it may not survive the trek across Belize and the Yucatan Peninsula.

However, given current organizational trends observed on satellite imagery, along with the fact that these areas aren't terribly mountainous, I expect that Richard will be able to survive the crossing intact (albeit in a severely weakened state, obviously), and will emerge into the southeastern Bay of Campeche in about 36 hours as a severely weakened, vertically tilted tropical depression.

It's worth noting that the 12z GFDL forecasts that Richard will move much slower across Belize, Guatemala, and Mexico. As a result, Richard doesn't enter the Bay of Campeche until much later -- at day three or four. That would coincide with the GFS' forecast of somewhat lower (but still prohibitive) westerly to west-southwesterly vertical wind shear across that portion of the Gulf of Mexico.

Consequently, that model foresees Richard reintensifying to a hurricane before making a second and final landfall near Veracruz, Mexico. However, both the GFDL and the HWRF aren't at all notorious for recognizing obvious shearing conditions, so it's quite likely that this model is failing to appreciate just how fragile Richard will be after crossing the Yucatan, and just how prohibitive zonal shear is to tropical cyclogenesis and intensification.

My own forecast track agrees quite well with the NHC's current one, albeit slightly farther east, as previously noted, for the first 6-12 hours.

The HWRF is an eastern outlier, and forecasts that Richard will accelerate toward the NE toward the Big Bend area of Florida after entering the Gulf of Mexico north of the Yucatan Peninsula. However, the rest of the models are not in agreement with this, and 12z steering forecasts from PSU's e-wall indicate that the low-level flow will be southeasterly, and since Richard is expected to be a very weak tropical cyclone after emerging into the Bay of Campeche, he should move generally NW at that point. Richard would have to maintain some vertical integrity in order for it to follow the track of the HWRF, and this will not be the case.

Sorry JFV, looks like you'll have to wait until next year to get your much fantasized hurricane hit.

Watches and warnings

A HURRICANE WARNING IS IN EFFECT FOR...
* BELIZE

A HURRICANE WATCH IS IN EFFECT FOR...
* EAST COAST OF YUCATAN MEXICO FROM PUNTA GRUESA TO CHETUMAL

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* EAST COAST OF YUCATAN MEXICO FROM PUNTA GRUESA TO CHETUMAL

Invest 90L

A tropical wave in the eastern Atlantic located a few hundred miles west-northwest of the westernmost Cape Verde Islands, dubbed "Invest 90L" by the National Hurricane Center, continues to produce disorganized showers and thunderstorms.

Vertical shear is prohibitively strong, but the GFS predicts that shear will fall in about three to four days as the system moves generally NW, which may allow for some slow development during that time. Indeed, several of the global models eventually foresee this becoming at least a tropical depression. It is not out of the question we could see Tropical Storm Shary from this.

90L is no threat to land areas, nor will it ever be.

Elsewhere

Elsewhere in the tropics, several of the global models, most notably the GFS and ECMWF, are forecasting the development of a tropical cyclone in the eastern Caribbean Sea in about 7-10 days. The latter turns this system into a powerful, westward-moving hurricane, which is a little unnerving. The GFS forecasts that vertical shear will start to lessen in this part of the Caribbean by day seven as well, so this certainly bears watching.

Tropical weather analysis - October 22, 2010

Richard

Tropical Storm Richard is slowly intensifying. As of the most recent NHC advisory, here is the latest information on Richard:

Wind: 45 mph, with higher gusts
Movement: W at 5 mph
Pressure: 1005 mb
Location: 16.2°N 81.7°W
Category: Tropical storm

Visible satellite animations suggest that the center is located underneath the mass of deep convection, but the imagery also suggests some vertical shearing over the system. However, there are some indications that this shear is lessening. Additionally, Richard's cyclonic circulation appears to be wrapping some stratocumulus clouds into the western semicircle. Indeed, the latest run of the SHIPS model places 700-500 mb relative humidity values at only 59%, suggesting a very dry mid-level airmass. Finally, Richard's overall circulation envelope appears to be a bit elongated, as evidenced by low-cloud motions observed on satellite imagery. These factors should prevent any rapid intensification of Richard for now.

The dry air and shear notwithstanding, Richard certainly has the potential to become a formidable hurricane over the next several days, as upper-level winds are forecast to be quite favorable for intensification. There are also some indications on water vapor imagery that the very dry airmass is gradually moistening, and high cloud motions derived from satellite imagery imply a lessening of the aforementioned vertical shear.

I expect continued slow intensification of Richard over the next 18 hours, with a more steady rate of strengthening thereafter. However, the potential certainly exists for Richard to rapidly intensify beyond that time, given the very high oceanic heat content located across the western Caribbean Sea, and the light vertical shear profiles forecast.

I expect that by 24 hours, Richard will have become a hurricane.

Richard is currently located in a col region between a developing anticyclone across the Gulf of Mexico and a very potent longwave trough over the western and central Atlantic. This synoptic scale setup indicates that Richard will move only slowly in the short-term. Some erratic motion cannot be ruled out, as well.

Models are split into two camps: the first set, comprised of the CMC, NOGAPS, and UKMET forecast a much weaker Richard. As a result, a shortwave trough and associated cold front that is forecast by the models to branch off from the base of a longwave trough over the high plains/upper midwest in about three days has little appreciable effect on the cyclone. Should this scenario verify, Richard would move into Central America in about two to three days, and eventually into northeast Mexico or south Texas. In fact, the CMC and NOGAPS even lose Richard's circulation over the rugged terrain of Central America.

The latter set of models, comprised of the GFS, HWRF, GFDL, and ECMWF, forecast a deeper, more vertically respectable Richard to respond more significantly to the trough. As a result, these models have Richard turning NW to NNW in about four days toward a developing weakness in the ridge due to a southward movement of said trough toward the Gulf Coast states. In this scenario, Richard would expected to be in the eastern Bay of Campeche in the Gulf of Mexico at that time.

Given current trends, which suggests slow organization (but Richard is still undoubtedly struggling), as well as the forecast of an improving upper wind and thermodynamic environment, which appears to be happening as we speak, I will side with the latter camp.

This track tends to agree pretty well with the NHC's current one, although I am a bit farther to the east of their forecast track at days four and five.

Richard does not pose a significant threat to the United States, as vertical shear profiles are forecast to be quite unfavorable across the Gulf of Mexico. In fact, dissipation of Richard well before "landfall" can be expected. However, Central America and the Yucatan Peninsula may not be so fortunate. The GFDL brings Richard up to Category 4 strength before it slams it into northeastern Belize in about 66-72 hours.

Interests in Honduras, Belize, the Yucatan Peninsula should closely follow the progress of this system over the next several days. Richard could also pose a long-range rainfall threat for from the central Texas to southwest Louisiana coasts, and interests in those areas should also follow the progress of Richard over the next few days.

Again, I want to reiterate that the US is unlikely to see anything more than a little bit of rain from Richard's remnants. Richard will NOT be an organized tropical system by the time it reaches any of the aforementioned areas. However, the rainfall could be locally heavy depending on how moist the environment gets with the approach of said cold front. Tropical systems are notorious for entraining moisture into approaching cold fronts, which can in turn create some serious rainfall and associated flood potential.

Since these areas are in a drought though, the rainfall is unlikely to cause significant flooding problems for Texas and Louisiana.

The most likely target for Richard's Central American landfall is Belize, and for its US landfall, western Louisiana, near Cameron.

Watches and warnings

A HURRICANE WATCH IS IN EFFECT FOR...
* HONDURAS FROM THE NICARAGUA/HONDURAS BORDER WESTWARD TO LIMON AND
THE BAY ISLANDS
* EAST COAST OF MEXICO FROM PUNTA GRUESA TO CHETUMAL

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* HONDURAS FROM THE NICARAGUA/HONDURAS BORDER WESTWARD TO LIMON AND
THE BAY ISLANDS

A TROPICAL STORM WATCH IS IN EFFECT FOR...
* HONDURAS FROM WEST OF LIMON TO THE GUATEMALA BORDER.
* BELIZE

Invest 90L

A strong tropical wave is located near the Cape Verde Islands. This wave has been dubbed "Invest 90L" by the National Hurricane Center. The associated shower activity has become a little better organized, but upper-level winds are only marginally conducive for development, and a less favorable upper wind environment is forecast over the next 12-24 hours. Indeed, southwesterly shear is rapidly approaching the system.

This system does not pose a threat to any land areas, and can be expected to move NW to NNW throughout the forecast period.

Elsewhere

Elsewhere in the tropics, the GFS and ECMWF are indicating that tropical cyclogenesis might occur in the eastern Caribbean in about 7-10 days. This could occur as the ITCZ lifts northward with the approach of the upward MJO.

Since the GFS has been particularly fluent in sniffing out long-range storms this year, I would not discount this possibility at all. However, upper-level winds will not be particularly favorable for development over that part of the Caribbean, so no significant intensification should be expected if this system does form.

Tropical weather analysis - October 13, 2010

Paula

Extremely tiny Hurricane Paula has weakened steadily throughout the day today, and is now only a Category 1 hurricane. Here is the latest information on this system as of the 11:00 PM CDT advisory from the National Hurricane Center:

Wind: 80 mph, with higher gusts
Movement: NE at 3 mph
Pressure: 993 mb
Location: 21.9°N 85.4°W
Category: 1 (Saffir-Simpson Hurricane Wind Scale)

Analysis of infrared satellite animations suggest that Paula's circulation might be becoming somewhat exposed along the southwestern edge of the CDO. Indeed, high cloud motions observed on said imagery indicate that the hurricane is experiencing southwesterly vertical shear associated with a mid- to upper-level trough and associated cold front currently draped across the central Gulf of Mexico. CIMSS analysis indicates that this shear is prohibitively strong, at 30 kt.

Upper-level outflow is nonexistent to the southwest and quickly going in that direction on the western side as a consequence of the aforementioned shear.

Paula should continue to gradually spin down due to continued southwesterly then westerly vertical shear, and it would not surprise me at all to see Paula dissipate in as little as 36 hours if it manages to track directly over the mountainous terrain of Cuba. Since I suspect Paula will track just north of the Cuban coast after passing the hurricane warning area, I will not forecast such rapid dissipation at this time, but I do expect that Paula will have dissipated by 60 hours.

Analysis of water vapor imagery implies that Paula is already embedded within the mid-latitude southwesterly flow associated with the aforementioned trough. As such, the hurricane is now recurving, albeit at a painfully slow forward motion for the time being, as the approach of said trough has briefly weakened the steering currents surrounding the storm, as is usually the case just prior to and just after recurvature.

Based on various steering data I've analyzed, both real time and forecast, it appears that the trough responsible for Paula's current recurvature will not be strong enough to fully recurve Paula out to sea. All of the models, with the exception of the GFDL, which loses Paula north of Hispaniola in five days, suggest that that a severely weakened or dissipated Paula will eventually move southward back into the western Caribbean Sea underneath the northerly flow associated with the backside of the trough, which is forecast to eventually become a powerful Nor'easter.

Some, such as the NOGAPS, go even further and forecast Paula, or its remnants, to move NW into the southern Gulf of Mexico along the southwestern periphery of a building ridge across the Gulf of Mexico in about five days or so.

Needless to say, confidence in the forecast track beyond two days is not high.

Right now, I my forecast track is just a bit to the south (about 75-100 miles) of the 18z GFDL for the first 48 hours, and just north of NHC's current forecast track during that time.

After 48 hours, my forecast track agrees pretty strongly with that of the NHC, albeit a tad farther westward at the end of the forecast period.

Watches and warnings

A HURRICANE WARNING IS IN EFFECT FOR...
* THE CUBAN PROVINCE OF PINAR DEL RIO

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* THE CUBAN PROVINCES OF LA HABANA AND CUIDAD DE LA HABANA

A TROPICAL STORM WATCH IS IN EFFECT FOR...
* THE FLORIDA KEYS FROM CRAIG KEY WESTWARD...INCLUDING THE DRY
TORTUGAS

Heavy rainfall in excess of 4 to 8 inches, with locally higher amounts, are possible across portions of western and central Cuba over the next couple of days, along with sustained hurricane force winds along the western and extreme west-central sections of the country, and residents in those areas should closely monitor the progress of Paula over the next couple days.

A storm surge in excess of 3 to 6 feet is possible across extreme western Cuba, which will be accompanied by large, destructive waves.

Elsewhere

Elsewhere in the tropics, most of the global models are indicating another tropical cyclone will develop in the Caribbean, this one north of Panama. The time frame is about a week, which, though still quite far out, is one where we need to start paying attention, especially given the model consensus with this.

The GFS suggests that an anticyclonic environment will develop across the southwest Caribbean at about that time, so this will need to be watched closely.

Tropical weather analysis - October 12, 2010

Paula

Hurricane Paula became a Category 2 hurricane early this afternoon. As of the latest NHC advisory (7:00 PM CDT), here is the latest information on Paula:

Wind: 100 mph, with higher gusts
Movement: N at 9 mph
Pressure: 981 mb
Location: 19.6°N 86.0°W
Category: 2 (Saffir-Simpson Hurricane Wind Scale)

Infrared satellite animations show that Paula is a very small but well-organized hurricane, with a well-defined poleward (north) and equatorial (west) outflow channels due to an amplifying upper trough currently moving across the deep south. However, outflow remains quite restricted to the east, and particularly to the south, due to 20 kt of southerly vertical wind shear analyzed by University of Wisconsin CIMSS. This shear is being induced by a well-defined mid- to upper-level ridge to the east.

An eye is not currently present using conventional satellite imagery, but it was earlier, and all indications are that Paula has maintained her strength over the last couple of hours.

Doppler radar out of Cancun, Mexico shows that Paula has a very small eye, and earlier reconnaissance reports indicates that the eye is no more than, say, 10-15 miles wide. Indeed, if I hadn't been forecasting these systems for three years, I would not, as a layman, be able to readily tell just where Paula's eye is using the aforementioned radar imagery.

I've never seen a Caribbean hurricane as small as Paula, except for perhaps Hurricane Iris in 2001, and even that's iffy since the latter had a larger radius of tropical storm force winds.

Steering analysis from CIMSS indicate that Paula is moving northward under the influence of the aforementioned anticyclone to the east.

Once this shear gets into the core, then rapid weakening is possible, given the hurricane's small size. Shear is forecast to gradually increase over the next 24 hours, and beyond that time, rapidly increase. However, shear should be light enough over the next 12 hours or so to allow a little more intensification of Paula. I will not go as high as the current NHC intensity forecast, however. Instead, I'll go up to 90 kt/105 mph, though it would not be surprising given current trends if Paula has already peaked.

After 24 hours, the shear will increase dramatically, as previously noted, and will also veer from southerly to southwesterly by 36-48 hours, and remain that way throughout the forecast period. This, combined with the likelihood of dry air entrainment from the Gulf of Mexico, along with interaction with the Cuban landmass, suggests that Paula may not even last five days as a tropical cyclone.

In fact, some of the models, including the reliable ECMWF, insist on losing Paula's circulation over the next 1-3 days or have a severely weakened tropical cyclone meandering in the northwest Caribbean Sea for several days, and this is a possible scenario. However, I feel that Paula will maintain enough vertical depth to feel the effects of the trough currently moving across the deep south.

One important development in the synoptic scale steering pattern that has transpired over the last 12 hours or so is that the aforementioned deep-layer trough and accompanying cold front has transitioned from a meridional flow to a more zonal one.

This is evident by analysis of water vapor imagery, mid-level steering from PSU's e-wall, and finally, CIMSS real-time steering data.

This suggests that Paula may move north for a little bit longer than was earlier anticipated, and as a result, I would expect a slight northward shift in the NHC forecast track at 11:00 PM. Nothing drastic, mind you, just a slight northward shift. South Florida is still very likely to avoid a landfall from this storm, and the Florida Keys probably will too. Nevertheless, interests in those areas should still monitor the progress of Paula closely over the next several days.

The current transition to a more zonal flow notwithstanding, Paula should still ultimately miss the US and generally follow the NHC's forecast track, since all of the global models foresee the trough reamplifying over the next day or two as it moves across the southeast United States.

Right now, I agree pretty strongly with the NHC's current forecast track, though I am a little bit further north of it after 24 hours and beyond.

Interests in the Yucatan Peninsula, the Cayman Islands, western and central Cuba, the Florida Keys, south Florida, and the Bahamas should closely follow the progress of Paula over the next several days.

In the meantime, heavy, flooding rains are possible across the eastern portion of the Yucatan Peninsula and western Cuba over the next day or two, along with the likelihood of sustained tropical storm force winds for the former and sustained hurricane force winds for the latter.

The eastern Yucatan Peninsula can expect sustained tropical storm force winds throughout the morning tomorrow, with western Cuba experiencing those winds Wednesday afternoon. Hurricane force sustained winds are likely across western Cuba Wednesday night and into Thursday morning.

Additionally, storm surge flooding will be a serious concern for those areas, as well.

Watches and warnings

A HURRICANE WARNING IS IN EFFECT FOR...
* THE COAST OF MEXICO FROM PUNTA GRUESA NORTHWARD TO CABO CATOCHE...
INCLUDING COZUMEL
* THE PROVINCE OF PINAR DEL RIO CUBA

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* THE COAST OF MEXICO FROM CABO CATOCHE TO SAN FELIPE

Tropical weather analysis - October 11, 2010

Paula

Tropical Storm Paula formed this afternoon in the western Caribbean, east of Honduras. As of the latest NHC advisory (11:00 PM EDT), here is the latest information on Paula:

Wind: 70 mph, with higher gusts
Movement: NW at 10 mph
Pressure: 994 mb
Location: 16.8N 84.6W
Category: Tropical storm

Infrared satellite imagery indicates that Paula is a healthy and intensifying tropical cyclone, with very cold convection over the center and what appears to be the early stages of an eyewall wrapping cyclonically around the center. However, an eye or warm spot is not yet evident using conventional satellite imagery. Unfortunately, there have been no recent microwave satellite passes over the center of the cyclone to confirm this suspicion.

Regardless, it is quite obvious that Paula is an intensifying tropical cyclone. The storm has managed to develop a well-defined poleward and equatorial outflow channel, namely along the northern and western semicircles. Indeed, outflow is also becoming apparent to the south of the center. It remains restricted to the east due to weak southeasterly shear induced by an upper-level high pressure system centered near northwestern Colombia.

Upper-level winds appear favorable for Paula to continue intensifying over at least the next 48 hours. Thereafter, increasing southwesterly to southerly vertical shear will begin to impinge upon the system. This, along with a very dry airmass across the Gulf of Mexico, should begin to weaken Paula after 48 hours. The GFDL brings the system to 75 kt near the northeastern Yucatan Peninsula in 36 hours. While this track seems a tad too fast, the model certainly has the right idea in forecasting such a degree of intensification. In fact, for once, I think the GFDL is being too conservative. I'll go a bit higher, and forecast Paula to become an 85 kt Category 2 hurricane in the western Caribbean in about 48 hours.

It is certainly not out of the question that Paula could become a major hurricane over the next two days or so, especially if current trends continue.

Paula is being steered generally NW by a low- mid-level ridge to the east. It is possible, based on analysis of said imagery along with water vapor imagery, that Paula could turn a little to the right over the next couple hours before bending back to a straight NW course.

In the short-term, Paula should continue moving in this general direction, bringing the center in the general direction of the Yucatan Peninsula in the next 36-48 hours. Thereafter, the track forecast becomes quite complicated, and is dependent entirely upon the strength and amplification of a deep-layer trough forecast by the models to move across the southeast United States.

Depending on the model of your choice, the effects of this trough on Paula's motion varies. Some move the cyclone quickly poleward as Paula comes underneath the influence of the deep-layer westerly flow associated with the aforementioned trough, quickly ejecting it toward western Cuba by about 48 hours.

Others foresee Paula slowing to a crawl and either meandering in the western Caribbean Sea for several days, or moving very slowly eastward, never to leave the Caribbean throughout the forecast period (five days).

Right now, my current thinking is that, based upon current intensification trends and water vapor analysis, Paula will move a bit more poleward than some of the models are thinking, as a well-defined weakness is evident at 400 mb due to a shortwave trough currently moving across the Texas/Louisiana area.

This trough should drop a bit more southward over the next 12-18 hours as a well-defined upper-level low located across the central plains has moved quite far to the southeast over the last several hours. Since Paula is intensifying, it will be strong enough to feel this deep-layer trough. And, as a consequence, move more poleward.

For my forecast track, out of respect for the usual reliability of these two global models, as well as given current trends, and of course, the ever-present uncertainty in long-range weather forecasting, particularly with systems such as Paula, I will compromise between the faster GFS and the slower ECMWF.

This comes out to a forecast of Paula moving NW over the next 36-48 hours, toward the eastern Yucatan Peninsula, and then slowing down a bit and moving very slowly eastward, possibly just north of due E during the remainder of the forecast period.

This track is highly uncertain until the synoptic scale situation becomes clearer. Given the uncertainty, interests in Honduras, Belize, the Yucatan Peninsula, Jamaica, the Cayman Islands, western Cuba, south Florida, and the western Bahamas should closely follow the progress of Paula over the next several days.

In the meantime, heavy rainfall is possible across portions of Honduras, Nicaragua, Jamaica, Belize, the Cayman Islands, the Yucatan Peninsula, and western Cuba over the next several days. These rains have the potential to produce life-threatening flash floods and mudslides across these areas.

Watches and warnings

A HURRICANE WARNING IS IN EFFECT FOR...
* THE COAST OF MEXICO FROM PUNTA GRUESA NORTHWARD TO CABO CATOCHE...
INCLUDING COZUMEL

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* THE COAST OF HONDURAS FROM LIMON EASTWARD TO THE
HONDURAS/NICARAGUA BORDER...INCLUDING THE BAY ISLANDS
* THE COAST OF MEXICO FROM CHETUMAL NORTHWARD TO SOUTH OF PUNTA
GRUESA
* THE COAST OF MEXICO FROM CABO CATOCHE TO SAN FELIPE

A TROPICAL STORM WATCH IS IN EFFECT FOR...
* THE COAST OF BELIZE

Tropical weather analysis - October 9, 2010

Otto Tropical Storm Otto continues to move rapidly northeastward over the central Atlantic Ocean. As of the latest (11:00 PM AST) advisory from the National Hurricane Center, here is the latest information on Otto: Wind: 70 mph, with higher gusts Movement: ENE at 38 mph Pressure: 988 mb Location: 33.9N 48.7W Category: Tropical storm Infrared satellite loops indicate that Otto continues to look less and less like a tropical cyclone, as it begins to undergo extratropical transition. Indeed, deep convection is elongated about a sharp SW to NE axis due to strong southwesterly shear associated with a mid- to upper-level trough located to the west. In addition to the shear, conventional satellite imagery implies that there is a substantial area of cool, stable air just to the west of the cyclone. The aforementioned vertical shear is acting to pump said dry air into Otto's circulation, hampering thunderstorm activity near the center and promoting extratropical transition. The official NHC forecast calls for Otto to complete extratropical transition within 24 hours, and I tend to concur with this forecast. Otto is currently being steered by an upper trough to the west and a ridge to the east. By 18-24 hours, the global models indicate that Otto will begin moving more to the ENE as it feels less influence from the trough and more influence from the ridge. By mid day two or early day three, most of the models, with the exception of the GFDL and NOGAPS, slow the system down and swing it south to southeastward. I will compromise between the two extremes and forecast a slow but steady south-southeasterly motion during that time. This track also happens to agree well with the current NHC one. Otto could stick around in the vicinity of the eastern Azores for some time, and interests in and around those islands should closely monitor the progress of this system over the next several days. Interests in the Madeira Islands should follow Otto closely as well. Invest 98L A small but steadily organizing area of low pressure located about 160 miles southeast of the Honduras/Nicaragua border, labeled "Invest 98L" by the National Hurricane Center, is a threat to develop into a tropical depression. Analysis of shortwave infrared satellite imagery indicates that the center is located along 13N 81W. In addition, infrared satellite loops show that deep convection is firing near the estimated center position, and this convection shows some organization. An upper-level outflow channel has become established to the west and north of the center, with some evidence along the eastern semicircle as well. Outflow remains restricted in the southern and eastern quadrants, however, especially the former. Surface observations and satellite data indicate that the low-level circulation associated with this system is becoming better-defined. Upper-level winds are gradually easing over Invest 98L, and the GFS suggests that the upper wind environment will only further improve, particularly by 24 hours. The environment might not be anticyclonic, but it will at least be of light shear, enough to allow steady intensification of the system. The shear is forecast to remain generally light throughout forecast period. At later periods, the primary inhibiting factor arguing against significant strengthening of 98L is the presence of very dry, stable air across the western Caribbean and Gulf of Mexico, noted on water vapor imagery. Regardless, dry air will not be an issue for at least the next few days, and I concur with the NHC's assessment of a 60% chance of tropical cyclogenesis in the next 48 hours. Regardless of development, 98L will spread heavy rainfall across portions of Central America, Jamaica, and the Cayman Islands over the next couple of days. These rains could cause life-threatening flash floods and mudslides across these areas, and interests in the aforementioned areas should closely follow the progress of Invest 98L over the next couple of days. 98L is currently moving very slowly to the NW, underneath the influence of a weak low- to mid-level ridge. I expect this motion to more or less continue for at least the next two to three days. Thereafter, the models diverge, with some taking 98L more equatorial toward the Belize/Yucatan Peninsula area, with others taking it more northward, closer to western Cuba. It is impossible at this point to tell which of these solutions is correct, especially given the location of the system and the time of year. Caribbean systems are notorious for meandering during October as steering currents remain generally weak. The GFDL brings 98L up to a Category 2 hurricane in four days as it nears western Cuba, but this solution is hard to accept at this time. However, if 98L remains in this portion of the Caribbean long enough due to weak steering, it certainly has the potential to become a hurricane.

Tropical weather analysis - October 6, 2010

Otto

Subtropical Storm Otto developed today from what was previously Invest 97L. As of the latest NHC advisory (5:00 PM AST) from the National Hurricane Center, here is the latest information on Otto:

Wind: 60 mph, with higher gusts
Pressure: 990 mb
Movement: N at 5 mph
Location: 23.2°N 68.3°W
Category: Subtropical storm

Infrared satellite loops indicate that Otto may be beginning to acquire tropical characteristics, as an equatorial outflow channel has recently appeared and expanded on the southern side of the storm. This indicates that the system is spreading out warm air in the upper levels, signature of a warm-cored system. Deep convection is also firing closer to the center, but is still located relatively distant from it.

In addition, AMSU temperature profiles from CIMSS and cyclone phase diagrams from FSU suggest that the cyclone is gradually improving its mid- upper-level warm core.

Water vapor imagery indicates that the upper low responsible for Otto's subtropical characteristics is gradually weakening and deamplifying, and is quite elongated at present. Consequently, the surrounding airmass has moistened, indicating less dry mid- to upper-level cold air and more warm moist air. As a result, Otto should gradually acquire tropical characteristics over the next 6-12 hours.

Conditions appear favorable for continued intensification, especially now that the upper low is moving out of the picture. I expect that by morning, Otto will have fully acquired tropical characteristics. There is now a distinct possibility that Otto becomes a hurricane, as most of the models are reflecting this. I agree rather well with the NHC's current intensity forecast:

FORECAST POSITIONS AND MAX WINDS

INITIAL 06/2100Z 23.2N 68.3W 50 KT
12HR VT 07/0600Z 23.8N 68.5W 55 KT
24HR VT 07/1800Z 24.5N 67.9W 60 KT
36HR VT 08/0600Z 25.5N 66.4W 65 KT
48HR VT 08/1800Z 26.8N 63.7W 65 KT
72HR VT 09/1800Z 30.3N 55.8W 55 KT
96HR VT 10/1800Z 34.5N 44.5W 40 KT...POST-TROP/EXTRATROP
120HR VT 11/1800Z 39.0N 31.0W 35 KT...POST-TROP/EXTRATROP

$$
FORECASTER STEWART

Otto is forecast to be extratropical in about four days as it interacts with a large and powerful baroclinic system to the west.

Otto is located in a weak steering regime between a well-defined and slow-moving longwave trough to the west and the subtropical ridge to the east. Such a steering pattern favors a continued slow generally N motion over the next 18-24 hours, though there could be some wobbles to the NW during this time as well. Thereafter, the global models unanimously predict a gradual acceleration to the NE as Otto becomes well-embedded within the southwesterly steering regime associated with the aforementioned trough, which is forecast to move closer to Otto over the next day or so.

In about two and a half to three days, the models diverge, with some accelerating Otto more NNE, with others accelerating him ENE. I still feel the trough is amplified enough to bring Otto along the eastern edge of the guidance envelope. The HWRF, UKMET, and ECMWF models are on the western edge of the guidance, forecasting Otto to move well west of the Azores. I tend to favor the more eastern camp, composed of the NGFDL, GFDL, and GFS models.

I am unsure what is causing the GFS to move Otto sharply east near the end of the forecast period, and so I will discount that solution for now. My forecast track takes Otto within 100 miles west of the western end of the Azores in about five days, and interests there should closely follow the progress of this system.

Last night I had mentioned the possibility that Otto could become absorbed into a large extratropical low pressure system over the next few days, but this solution now appears unlikely. While interaction with this low will ultimately cause Otto to transition into an extratropical storm, global models suggest that Otto will remain a separate entity, distinct from the aforementioned baroclinic low throughout the forecast period.

Because Otto will be extratropical as it approaches the Azores, impacts will be felt well away from the center, particularly to the northeast. Hence, the center of the cyclone should not be focused on in this instance. Otto could also ultimately impact western Europe sometime next week.

Elsewhere

Elsewhere in the tropics, several of the global models continue to indicate the possibility of tropical cyclogenesis in the southwestern Caribbean in the next 5-7 days. However, they have become less aggressive today. If this becomes an established trend, it may mean that this system will not become as strong as previously thought. This possibility will have to be carefully assessed over the next few days.

The exact details of the track are uncertain, but right now I'd expect a track similar to Nicole.

Tropical weather analysis - October 5,, 2010

Invest 97L

An area of low pressure, located just north of Puerto Rico, dubbed "Invest 97L" by the National Hurricane Center, is producing a large and elongated area of showers and thunderstorms to the north and east of the estimated center. Analysis of shortwave infrared satellite imagery, buoy observations, and various other data indicate that the system does not yet have a well-defined surface circulation. However, the low-level center appears to be gradually becoming better-defined, as noted by low-cloud motions observed on satellite imagery.

The center appears to be located near 22N 66W according to the various data I've been able to analyze.

Infrared satellite imagery shows that 97L is still disorganized, but the convective bands to the north and east of the center appear to be becoming a bit more collocated with the center, a sign of increasing organization.


CIMSS analysis depicts 20 kt of westerly vertical shear blowing just to the north of the circulation center. This shear is responsible for the current subtropical appearance of the storm on satellite.

In addition, water vapor imagery shows that a large area of dry air surrounds 97L, and its counter-clockwise circulation is acting to pull in some of this dry air, which is also inhibiting any appreciable convective activity from developing and persisting near the center.

AMSU data from CIMSS along with cyclone phase diagrams from FSU indicate that the system currently lacks a well-defined warm core, with only a shallow warm core at best noted in the imagery.

Nevertheless, 97L appears to be gradually acquiring subtropical characteristics, and this system could become a subtropical cyclone at any time over the next 12-18 hours.

Based on the various steering data I've analyzed, I expect 97L to move NNW to NW over the next 36 hours or so with a gradual decrease in forward speed. Thereafter, the models unanimously forecast a stall as a well-defined longwave trough currently over the western Atlantic moves eastward and weakens the steering currents. After 48 hours, steering currents will once again become well-established as 97L/Otto becomes well-embedded within the southwesterly steering regime of said trough.

As a consequence, 97L can be expected to move NNE to ENE with a gradual acceleration after 48 hours. The models are split in their respective tracks. While it's impossible to tell at this juncture which camp of models is correct, I'll go with the easternmost camp, comprised of the NOGAPS and GFDL models, which would eventually bring the system near the western end of the Azores. This is based on the amplification of the aforementioned trough, which I feel will be quite significant.

By day four, the models forecast 97L/Otto to lose tropical characteristics (or subtropical characteristics) and become extratropical as it interacts with a large baroclinic system associated with the western Atlantic trough. 97L should ultimately become absorbed into this low, potentially impacting western Europe in about 7 days.

Vertical shear is gradually easing, and is forecast to lessen further over the next couple of days. In fact, the 18z GFS suggests that a weakly anticyclonic environment will develop over the system during the next day or so, and persist until 48 hours. Any potential transition to a tropical cyclone will occur then, when the trough will be relatively distant from the center and the anticyclone (high pressure ridge) established aloft, pumping warm air into the system at the upper-levels.

I do expect 97L to make the transition to a fully tropical cyclone sometime over the next 36 hours.

By 48 hours, vertical shear is forecast to increase sharply as the trough and associated cold front moves closer to the system. It is at this point that the system will lose its anticyclone.

It is possible that 97L becomes a hurricane, but this looks highly unlikely.

Elsewhere

Elsewhere in the tropics, the GFS, ECMWF, and NOGAPS forecast the development of a tropical depression in the western Caribbean Sea in about 5-7 days. The eventual path of such a system is uncertain.