Eastern Atlantic tropical wave
A well-defined tropical wave located about 850 miles west-southwest of the Cape Verde Islands has become a little better organized this evening. Infrared satellite loops suggest that there is a rather concentrated area of deep convection along 8 to 9°N 32 to 33°W. There is also another area of active convection to the west of that, in the general vicinity of where ex-90L used to be.
A definite center is extremely difficult to locate tonight, even using shortwave infrared and RGB satellite animations. My best guess, which is highly uncertain, is 8N 32W. Although this system has become better organized this evening, the enormity of the cyclonic gyre with which it is embedded will likely keep any rapid consolidation to a minimum. All in all, based on CIMSS 850 mb vorticity data, this system is still poorly defined, and is embedded, as previously mentioned, in a large cyclonic gyre associated with an active ITCZ. Water vapor animations depict a large swath of very dry air just to the north of this complex weather disturbance, associated with the dry, dusty Saharan Air Layer that originates from the Sahara Desert of Africa, and gets periodically blown westward out into the Atlantic Ocean.
However, given the fact that the system will be moving generally W under the influence of a well-defined low- to mid-level tropospheric ridge, this dry air should not significantly impede development, since the steering flow will be primarily easterly, which will act to push the dry air away from the system. Latest CIMSS wind shear analysis indicates that the system is experiencing 10-20 kt of easterly to northeasterly shear, associated with a developing anticyclone just to the southwest of the Cape Verde Islands.
By about 36 hours, the system should start to gradually gain latitude and move WNW. Until then though, expect a continued W motion. By around 72 hours, a break in the Atlantic subtropical ridge is forecast to occur unanimously by all the models -- some keep it stronger, some weaker, but the general consensus is for at least a slight slackening of the south side of this ridge during this time, as a vigorous mid- to upper-level trough swings by across the mid-Atlantic states and New England.
There is still the possibility that the system could remain shallow and continue W, eventually entering the Caribbean. The NOGAPS keeps a stronger Bermuda-Azores high, which keeps the low-level flow generally easterly throughout the forecast period. This solution appears unlikely at this time, if not just for the fact that the system is likely to gradually deepen over the next couple of days.
The models are unanimously calling for tropical cyclogenesis from this feature, with the 12z CMC developing it east of the Leeward Islands in about four days. Interestingly, this particular model shows something of a Fujiwhara interaction between this system and another competing area of vorticity subsequent to this. This solution appears highly unlikely for now, but nevertheless the model has the right idea in developing this system. The 18z GFS indicates that cyclogenesis will occur right on the doorstep of the northern Leeward Islands in about four and a half days.
The 12z NOGAPS keeps the system weak, and as a consequence, it moves westward in tandem with the low-level easterly flow characteristic of the tropics. It develops it at 144 hours, while it is about 400 miles east of the central Leeward Islands. This model appears much too far south at this time, and hence I have discounted this track solution. But again, it does have the right idea with regards to eventual development from this system. Finally, the 12z ECMWF develops this system in about four days as it approaches the Leeward Islands.
Rather alarmingly, both the GFS and ECMWF foresee the synoptic scale pattern as being supportive of a stronger Atlantic subtropical ridge, strong enough to prevent the system from recurving harmlessly out to sea. While I do not like to speculate on long-range tracks, particularly with systems that do not even have a definite surface center, I am inclined to go with the solution provided by these models, based on the current large-scale pattern over North America.
Upper-level winds support continued slow development of this disturbance. I am not expecting any rapid development, however, and the NHC's current forecast of a 40% chance of tropical cyclogenesis occurring in 48 hours is a reasonable one. The earliest I would expect this system to become a tropical depression or tropical storm is Tuesday night, but more likely Wednesday afternoon.
Central Caribbean tropical wave
A tropical wave moving across the central Caribbean Sea continues to sporadically produce limited and disorganized shower activity. Upper-level winds are not conducive for development in this area, and are forecast to continue being largely zonal and strong, even as the system approaches Central America in about two days. I am not expecting any significant development of this wave, though it may bring heavy rains to Honduras, Nicaragua, El Salvador, and Costa Rica over the next couple of days as it continues moving generally W.
Saturday, July 31, 2010
Friday, July 30, 2010
Tropical weather analysis - July 30, 2010
Invest 90L
A strong tropical wave, dubbed "Invest 90L" by the National Hurricane Center, has become less organized throughout today. This isn't due to unfavorable atmospheric conditions. Rather, it is due to interaction with a larger tropical wave to the east, near the Cape Verde Islands. I suspect that this wave will continue to gradually ingest Invest 90L, and will likely be designated "Invest 91L" tomorrow morning. This system is very poorly organized, with shortwave infrared satellite animations indicating very little in the way of shower activity, much less any cyclonic vorticity. I can detect a faint, very weak and rapidly dissipating spin from 8 to 9°N and along 34 to 32°W. The rest of the convection in the area is associated primarily with the ITCZ, not the wave.
Latest CIMSS wind shear analysis from University of Wisconsin CIMSS depicts only 5-10 kt of northeasterly shear impinging upon the system. However, again, because of dangerously close proximity to the aforementioned tropical wave to the east, I am not expecting significant development of Invest 90L. Obviously though, it would be rather foolhardy for anyone to take their eyes off the system until the system completely loses its identity.
The Cape Verde wave should move W over the next couple of days, with a more WNW motion occurring in 60-72 hours as a shortwave trough weakens the ridge to the north. This system may pass north of the Leeward Islands and Puerto Rico in about five to seven days, but it is uncertain if this system will be able to recurve out to sea entirely. This system may be a long-range threat to the southeast or northeastern United States, but this is highly speculative, and residents living in those areas should not be overly concerned. Upper-level winds favor development over the next several days, but as the wave approaches the Lesser Antilles, strong westerly shear associated with the TUTT will return once again, which will make additional development or intensification highly improbable, and the system may even destroyed at that point.
Should it remain farther south, however, and enter the Caribbean, upper-level winds greatly favor intensification.
Eastern Caribbean tropical wave
A tropical wave located across the eastern Caribbean Sea continues to produce disorganized convection. Upper-level winds are currently unfavorable for development. This, along with the wave's fast forward speed, 20 to 25 mph (which is expected to continue for the next couple days, by the way), should prevent any appreciable organization from occurring until the system nears Central America in about two to three days. At that point, upper-level winds will also relax, and the system will have a small window of opportunity in which to become a tropical cyclone. I am not expecting this to happen, however.
Also, I still fully anticipate an active season. I cannot stress this enough. This season will be active -- in 1998, the most similar year to this one that I can find, the second named storm did not occur until August 20. That season ended up producing 14 named storms, 10 hurricanes, and 3 major hurricanes, and, most remarkably, four simultaneous hurricanes existed in the basin on September 25 -- Georges, Ivan, Jeanne, and Karl. The season also had three hurricanes impact the United States mainland: Bonnie, Earl, and Georges.
Lastly, it featured the mighty Hurricane Mitch, which killed anywhere from 11 to 20,000 people across portions of Central America due to days of prolonged torrential rainfall, which produced flash flooding and mudslides across mountainous areas.
While I'm not saying we're necessarily going to see an exact repeat of 1998, I do not want anyone to be foolish enough to drop their guard by thinking that the season is over. Most parameters still point to a sharp upswing in overall activity beginning the second week of August.
A strong tropical wave, dubbed "Invest 90L" by the National Hurricane Center, has become less organized throughout today. This isn't due to unfavorable atmospheric conditions. Rather, it is due to interaction with a larger tropical wave to the east, near the Cape Verde Islands. I suspect that this wave will continue to gradually ingest Invest 90L, and will likely be designated "Invest 91L" tomorrow morning. This system is very poorly organized, with shortwave infrared satellite animations indicating very little in the way of shower activity, much less any cyclonic vorticity. I can detect a faint, very weak and rapidly dissipating spin from 8 to 9°N and along 34 to 32°W. The rest of the convection in the area is associated primarily with the ITCZ, not the wave.
Latest CIMSS wind shear analysis from University of Wisconsin CIMSS depicts only 5-10 kt of northeasterly shear impinging upon the system. However, again, because of dangerously close proximity to the aforementioned tropical wave to the east, I am not expecting significant development of Invest 90L. Obviously though, it would be rather foolhardy for anyone to take their eyes off the system until the system completely loses its identity.
The Cape Verde wave should move W over the next couple of days, with a more WNW motion occurring in 60-72 hours as a shortwave trough weakens the ridge to the north. This system may pass north of the Leeward Islands and Puerto Rico in about five to seven days, but it is uncertain if this system will be able to recurve out to sea entirely. This system may be a long-range threat to the southeast or northeastern United States, but this is highly speculative, and residents living in those areas should not be overly concerned. Upper-level winds favor development over the next several days, but as the wave approaches the Lesser Antilles, strong westerly shear associated with the TUTT will return once again, which will make additional development or intensification highly improbable, and the system may even destroyed at that point.
Should it remain farther south, however, and enter the Caribbean, upper-level winds greatly favor intensification.
Eastern Caribbean tropical wave
A tropical wave located across the eastern Caribbean Sea continues to produce disorganized convection. Upper-level winds are currently unfavorable for development. This, along with the wave's fast forward speed, 20 to 25 mph (which is expected to continue for the next couple days, by the way), should prevent any appreciable organization from occurring until the system nears Central America in about two to three days. At that point, upper-level winds will also relax, and the system will have a small window of opportunity in which to become a tropical cyclone. I am not expecting this to happen, however.
Also, I still fully anticipate an active season. I cannot stress this enough. This season will be active -- in 1998, the most similar year to this one that I can find, the second named storm did not occur until August 20. That season ended up producing 14 named storms, 10 hurricanes, and 3 major hurricanes, and, most remarkably, four simultaneous hurricanes existed in the basin on September 25 -- Georges, Ivan, Jeanne, and Karl. The season also had three hurricanes impact the United States mainland: Bonnie, Earl, and Georges.
Lastly, it featured the mighty Hurricane Mitch, which killed anywhere from 11 to 20,000 people across portions of Central America due to days of prolonged torrential rainfall, which produced flash flooding and mudslides across mountainous areas.
While I'm not saying we're necessarily going to see an exact repeat of 1998, I do not want anyone to be foolish enough to drop their guard by thinking that the season is over. Most parameters still point to a sharp upswing in overall activity beginning the second week of August.
Thursday, July 29, 2010
Tropical weather analysis - July 29, 2010
Invest 90L
A large but well-defined perturbation in the Intertropical Convergence Zone (ITCZ) was designated Invest 90L this afternoon by the National Hurricane Center. Visible satellite animations suggest that this system is embedded within a large area of convection and cyclonic vorticity associated with the ITCZ. My best estimate for a "center" position, based on satellite data, is 7N 31W. Latest CIMSS 850 mb vorticity data also somewhat supports this.
Upper-level winds are favorable for additional development of this system, with CIMSS wind shear analysis indicating only 5-10 kt of east to northeasterly shear over the system. This favorable upper-level environment is forecast to persist throughout the next several days, with a fairly uniform easterly flow. The two largest impediments I can see for 90L is the large amount of dry air to the north, depicted rather nicely on water vapor loops, and the broad cyclonic gyre that the system is embedded in. However, given the very moist environment within the vicinity of the disturbance, along with the fact that the system should move generally W over the next couple of days, I am not expecting the dry air to completely halt any additional development of 90L.
Analysis of 12z steering from PSU's e-wall suggests that a break in the Atlantic subtropical ridge could occur in about 60-72 hours as a mid- to upper-level trough swings by to the north of the system. However, unless 90L develops significantly during the next three days, this weakness should not be strong enough to turn the system poleward so much so that it completely misses any and all land areas.
All of that having been said, I expect some slow additional development of the disturbance as it moves generally W over the next couple of days. As previously noted, a turn to the WNW could occur in 60-72 hours as the subtropical ridge weakens somewhat. The 18z GFS develops this system just north of Puerto Rico in 168 hours. The 12z CMC develops this in as little as two and a half to three days, and eventually makes it a hurricane. However, such rapid development does not appear likely at this time, and I am discounting both the track and intensity solution currently being provided by this model. Finally, the 12z ECMWF hints at tropical cyclogenesis occurring north of the Leeward Islands in about six days, but quickly loses the system as it moves it WNW toward the southeast United States coast.
I would not expect development to begin occurring in earnest until around 60 hours, as the system begins to gain some latitude. The National Hurricane Center is giving 90L a 20% chance of becoming a tropical cyclone within the next 48 hours, and this is a reasonable forecast.
Windward Islands tropical wave
A tropical wave is producing scattered showers and thunderstorms across portions of the Windward Islands and adjacent eastern Caribbean Sea.
However, the wave axis has not yet passed these islands. Upper-level winds are currently not favorable for development, but could become more favorable over the next several days as the wave moves generally W, well-embedded within the low-level easterly trade wind flow common of the tropics. Surface observations from the Windward Islands, along with buoy data, indicate that nothing of note is occurring at the surface in association with this wave.
I suspect that any development will be very slow to occur. And, though the upper wind flow may improve somewhat over the next several days, the system will run into Central America in about four to five days.
Regardless of development, locally heavy rainfall and strong gusty winds will accompany the wave as it passes through the Windward Islands tonight. None of the models, save the CMC, which is always too bullish with tropical cyclogenesis, significantly develop the wave.
A large but well-defined perturbation in the Intertropical Convergence Zone (ITCZ) was designated Invest 90L this afternoon by the National Hurricane Center. Visible satellite animations suggest that this system is embedded within a large area of convection and cyclonic vorticity associated with the ITCZ. My best estimate for a "center" position, based on satellite data, is 7N 31W. Latest CIMSS 850 mb vorticity data also somewhat supports this.
Upper-level winds are favorable for additional development of this system, with CIMSS wind shear analysis indicating only 5-10 kt of east to northeasterly shear over the system. This favorable upper-level environment is forecast to persist throughout the next several days, with a fairly uniform easterly flow. The two largest impediments I can see for 90L is the large amount of dry air to the north, depicted rather nicely on water vapor loops, and the broad cyclonic gyre that the system is embedded in. However, given the very moist environment within the vicinity of the disturbance, along with the fact that the system should move generally W over the next couple of days, I am not expecting the dry air to completely halt any additional development of 90L.
Analysis of 12z steering from PSU's e-wall suggests that a break in the Atlantic subtropical ridge could occur in about 60-72 hours as a mid- to upper-level trough swings by to the north of the system. However, unless 90L develops significantly during the next three days, this weakness should not be strong enough to turn the system poleward so much so that it completely misses any and all land areas.
All of that having been said, I expect some slow additional development of the disturbance as it moves generally W over the next couple of days. As previously noted, a turn to the WNW could occur in 60-72 hours as the subtropical ridge weakens somewhat. The 18z GFS develops this system just north of Puerto Rico in 168 hours. The 12z CMC develops this in as little as two and a half to three days, and eventually makes it a hurricane. However, such rapid development does not appear likely at this time, and I am discounting both the track and intensity solution currently being provided by this model. Finally, the 12z ECMWF hints at tropical cyclogenesis occurring north of the Leeward Islands in about six days, but quickly loses the system as it moves it WNW toward the southeast United States coast.
I would not expect development to begin occurring in earnest until around 60 hours, as the system begins to gain some latitude. The National Hurricane Center is giving 90L a 20% chance of becoming a tropical cyclone within the next 48 hours, and this is a reasonable forecast.
Windward Islands tropical wave
A tropical wave is producing scattered showers and thunderstorms across portions of the Windward Islands and adjacent eastern Caribbean Sea.
However, the wave axis has not yet passed these islands. Upper-level winds are currently not favorable for development, but could become more favorable over the next several days as the wave moves generally W, well-embedded within the low-level easterly trade wind flow common of the tropics. Surface observations from the Windward Islands, along with buoy data, indicate that nothing of note is occurring at the surface in association with this wave.
I suspect that any development will be very slow to occur. And, though the upper wind flow may improve somewhat over the next several days, the system will run into Central America in about four to five days.
Regardless of development, locally heavy rainfall and strong gusty winds will accompany the wave as it passes through the Windward Islands tonight. None of the models, save the CMC, which is always too bullish with tropical cyclogenesis, significantly develop the wave.
Monday, July 26, 2010
Louisiana hurricane history Part 3b: Unnamed hurricane (Sep 11 - Sep 16)
This storm was the second in a series of three powerful hurricanes to make landfall in southeastern Louisiana during the 1860 Atlantic hurricane season. This system was first noted in the early morning hours of September 11 about 150 miles west-northwest of the Dry Tortugas. It made landfall on the morning of September 16 near the Mouth of the Mississippi River along the southwest end of Plaquemines Parish.
Storm history
As mentioned above, a Category 2 hurricane was first detected about 150 miles west-northwest of the Dry Tortugas near the Florida Keys, moving W. As with most systems in the 1800s, the complete history of this hurricane is not known. It is possible that the system moved in from the Bahamas and south Florida, but this presents a problem: why was such an impact not recorded, even during those technologically scarce times? The only possible explanation to this is that the system was only a tropical depression or tropical storm whilst traversing these regions, and it did not intensify significantly until it reached the eastern Gulf of Mexico. At that point though, rapid intensification would have had to have occurred in order for the storm to go from a tropical depression/storm to a Category 2 hurricane within the span of just a day or so. Another possibility is that the system was moving northward from the western Caribbean, across western Cuba, and then into the Gulf of Mexico before a ridge built back in to the north. However, this would be a rather atypical motion for the middle of September, and in fact, more typical of the middle of October.
Regardless of its origin, the hurricane continued generally westward as a Category 2 until the morning of September 12, at which point it turned to the west-northwest as the subtropical ridge to the north weakened somewhat, likely due to the approach of a mid-latitude trough from the west. The hurricane turned to the northwest late on the afternoon of September 13 as it encountered a further weakness in the aforementioned ridge. Around 24 hours later, the hurricane turned northward, now well-embedded within the southerly flow along the western periphery of the subtropical ridge, and the aforementioned trough and its associated cold front to the west. The hurricane made landfall across lower Plaquemines Parish at 0600 GMT September 15, then turned further poleward, to the north-northeast, at around 1200 GMT that same day. At this time, the hurricane is still estimated to have been a Category 2, located just offshore the southwestern coast of Mississippi. The hurricane quickly made a second landfall near Bay St. Louis, still a Category 2.
The hurricane continued north-northeastward under the influence from the trough, and ultimately dissipated somewhere over the eastern United States. It was last identified as a 45 kt (50 mph) tropical storm at 0600 GMT near 33.0°N 88.0°W, still moving quickly north-northeastward under the influence of the climatological mid-latitude westerlies.
Figure 1. Track of the hurricane.
Louisiana
Gale force winds were reported for around 20 hours across extreme southeast Louisiana as the hurricane ravaged the area. Additionally, and quite rare in a tropical cyclone, hail was reported. The only other tropical cyclone I can think of that has produced hail (though there were undoubtedly others as well) is 1995's Hurricane Marilyn.
Every building in Balize, an old settlement located about five miles from Pillottown, (the latter of which is located along extreme lower Plaquemines Parish, on the southeast side facing the Gulf), was destroyed by the hurricane, and the third Bayou St. John lighthouse was damaged beyond repair.
A storm surge in excess of several feet was reported across lower Plaquemines Parish, which inundated the area and killed several people. The tied was reported as being six feet above the high tide. All wharves along the southern shore of Lake Pontchartrain were destroyed.
Mississippi
Across Mississippi, where the hurricane's second landfall occurred, a lighthouse in the area, along with a hotel, were swept away by the storm surge.
Total damages from the hurricane exceeded $1 million.
Storm history
As mentioned above, a Category 2 hurricane was first detected about 150 miles west-northwest of the Dry Tortugas near the Florida Keys, moving W. As with most systems in the 1800s, the complete history of this hurricane is not known. It is possible that the system moved in from the Bahamas and south Florida, but this presents a problem: why was such an impact not recorded, even during those technologically scarce times? The only possible explanation to this is that the system was only a tropical depression or tropical storm whilst traversing these regions, and it did not intensify significantly until it reached the eastern Gulf of Mexico. At that point though, rapid intensification would have had to have occurred in order for the storm to go from a tropical depression/storm to a Category 2 hurricane within the span of just a day or so. Another possibility is that the system was moving northward from the western Caribbean, across western Cuba, and then into the Gulf of Mexico before a ridge built back in to the north. However, this would be a rather atypical motion for the middle of September, and in fact, more typical of the middle of October.
Regardless of its origin, the hurricane continued generally westward as a Category 2 until the morning of September 12, at which point it turned to the west-northwest as the subtropical ridge to the north weakened somewhat, likely due to the approach of a mid-latitude trough from the west. The hurricane turned to the northwest late on the afternoon of September 13 as it encountered a further weakness in the aforementioned ridge. Around 24 hours later, the hurricane turned northward, now well-embedded within the southerly flow along the western periphery of the subtropical ridge, and the aforementioned trough and its associated cold front to the west. The hurricane made landfall across lower Plaquemines Parish at 0600 GMT September 15, then turned further poleward, to the north-northeast, at around 1200 GMT that same day. At this time, the hurricane is still estimated to have been a Category 2, located just offshore the southwestern coast of Mississippi. The hurricane quickly made a second landfall near Bay St. Louis, still a Category 2.
The hurricane continued north-northeastward under the influence from the trough, and ultimately dissipated somewhere over the eastern United States. It was last identified as a 45 kt (50 mph) tropical storm at 0600 GMT near 33.0°N 88.0°W, still moving quickly north-northeastward under the influence of the climatological mid-latitude westerlies.
Figure 1. Track of the hurricane.
Louisiana
Gale force winds were reported for around 20 hours across extreme southeast Louisiana as the hurricane ravaged the area. Additionally, and quite rare in a tropical cyclone, hail was reported. The only other tropical cyclone I can think of that has produced hail (though there were undoubtedly others as well) is 1995's Hurricane Marilyn.
Every building in Balize, an old settlement located about five miles from Pillottown, (the latter of which is located along extreme lower Plaquemines Parish, on the southeast side facing the Gulf), was destroyed by the hurricane, and the third Bayou St. John lighthouse was damaged beyond repair.
A storm surge in excess of several feet was reported across lower Plaquemines Parish, which inundated the area and killed several people. The tied was reported as being six feet above the high tide. All wharves along the southern shore of Lake Pontchartrain were destroyed.
Mississippi
Across Mississippi, where the hurricane's second landfall occurred, a lighthouse in the area, along with a hotel, were swept away by the storm surge.
Total damages from the hurricane exceeded $1 million.
Friday, July 23, 2010
Tropical weather analysis - July 23, 2010
Bonnie
Bonnie weakened to a tropical depression while traversing southern Florida this afternoon. Thelast of the visible satellite images place the center at 26N 83W, over the eastern Gulf of Mexico. The National Hurricane Center had it at 26.4N 82.5W just a little over an hour ago, so this motion merely serves to reflect an extrapolation of that position. In any case, the aforementioned imagery also reveals a highly disorganized tropical cyclone, with the small area of deep convection displaced to the north and northwest of the poorly defined surface center, which appears to be located about 40-50 miles southeast of the aforementioned, shapeless convective activity.
This extreme disorganization is due to strong southeasterly vertical wind shear in excess of 30 kt. This shear is associated with a large, persistent, and powerful upper-level low centered near 27N 93W, and a large and powerful upper-level ridge centered across the mid-Atlantic states. Bottom line? Bonnie is embedded within a very hostile environment for intensification, with water vapor imagery depicting a large swath of dry air to the west. This dry air is gradually being entrained into the poorly defined surface center due to clockwise flow around the aforementioned ridge. Given the noticeable lack of upper-level divergence and low-level convergence associated with the cyclone, along with the aforementioned dry air and a continued hostile upper wind environment, I am not expecting Bonnie to be able to intensify. In fact, the most likely scenario is for the system to gradually weaken, ultimately opening up into a disorganized trough of low pressure prior to moving ashore along the northern Gulf Coast in the next 24-36 hours.
The track forecast is rather straightforward. Water vapor imagery depicts a strong and solid southeasterly flow impinging upon the cyclone. Embedded within general southeasterly flow between the upper low and the aforementioned anticyclone, Bonnie should continue moving WNW for the next 12 hours or so, after which point a turn to the NW should occur. By this time, a weakness should become apparent within the western Atlantic subtropical ridge as a mid- to upper-level trough and its associated cold front will be nudged a bit to the southeast by a well-defined upper-level low currently moving across northwestern North Dakota. This should effectively erode the ridge and allow the cyclone to turn more poleward during this time. That motion should continue until well after landfall occurs along the northern Gulf Coast in about 24-30 hours, and the cyclone could still be moving NW as it enters eastern or central Arkansas in about 60 hours.
The intensity forecast is also relatively simple. The 18z GFS indicates that the current southeasterly flow will gradually veer to the northeast, associated with the aforementioned ridge. There could be a brief relaxation of the shear over the next six hours, as indicated by the GFS, and then again in 30 hours as the system approaches the northern Gulf Coast. I'm in general agreement with the NHC's 5:00 PM EDT forecast track, albeit slightly farther to the right when landfall occurs. I expect that landfall will occur along the Louisiana/Mississippi border in just around 30 hours.
Watches and warnings
A tropical storm warning is in effect from:
- Destin, Florida to Morgan City, Louisiana, including New Orleans and Lake Pontchartrain
However, I imagine that these warnings were given as a course of least regret, just in case the cyclone unexpectedly strengthens due to an unforeseen relaxation of the vertical shear. I am not actually expecting tropical storm force winds anywhere within the warning area. I still expect Bonnie to open into an elongated trough within the next 12 hours, given the hostile environment.
Bonnie's main legacy will be 4 to 6 foot waves atop the oil spill region. A persistent fetch of SE to E winds will begin to blow across that region beginning in about 12 hours. This could potentially bring oil into the already devastated Louisiana marshlands, along with other areas of coast that have already experienced oil on their shores.
Elsewhere
There are no other threat areas to discuss in the tropics at the moment. Former Invest 98L moved inland near Tampico early this morning. A weak area of low pressure could still form near the Cape Verde Islands on Sunday, as indicated by both the GFS and the ECMWF, but both, particularly the latter, have become far less aggressive in this forecast than they were previously. The ECMWF brings this system off Africa at a higher latitude than the GFS does. I am inclined to agree with the solution of the GFS, given current convective trends over western Africa. Though upper-level winds favor development in this area, a dry environment awaits the system in the eastern Atlantic, which will act to inhibit any significant development. In fact, this is probably why the aforementioned models do not strengthen this feature, and quickly lose it within 12 hours of exiting the coast.
It should be noted that we are forecast, as per the EWP model, to enter a rather dry period across the Atlantic basin as the downward motion pulse of the MJO -- a 30 to 60 day eastward propagation of moisture originating over the Indian Ocean. The upward MJO pulse tends to favor greater atmospheric instability, and hence, enhances convection. The downward motion tends to do the exact opposite. However, tropical cyclogenesis can still occur during downward MJO pulses -- for instance, 2005's Harvey and Irene all formed during the downward MJO, the latter of which ultimately became a Category 2 hurricane that threatened the Carolinas.
However, given that the upward MJO has been rather hesitant to leave our area throughout most of the season, I rather am skeptical of the EWP's forecast. We will see, though.
On the other hand, the CFS model foresees a far less robust and long-lasting downward pulse.
Finally, the GFS is forecasting a rather significant downward MJO pulse, but an extrapolation of this forecast points to a general lessening of this motion by the second week of August. I rather like the forecast from the CFS, given current convective trends across the basin. However, given that both the EWP and the GFS are forecasting a significant downward pulse, I cannot simply ignore them. Hence, I will split the difference and forecast a downward MJO pulse for the next two weeks, after which point we should return to a neutral MJO pulse, followed by an upward motion pulse sometime during the second week of August.
Remember, just because we have only seen one significant tropical cyclone this year: Alex, doesn't mean that the season is a bust. Please do not be lulled into a false sense of security. In 1998, the most similar analog I can think of, the second storm didn't develop until August 19, and we ended up having 14 named storms, 10 hurricanes, and 3 major hurricanes that season, including notable ones like Bonnie, Georges, and Mitch.
The season isn't over by a longshot, and I still fully anticipate an above average season, even if we don't necessarily end up getting the 18 storms that CSU predicts.
Bonnie weakened to a tropical depression while traversing southern Florida this afternoon. Thelast of the visible satellite images place the center at 26N 83W, over the eastern Gulf of Mexico. The National Hurricane Center had it at 26.4N 82.5W just a little over an hour ago, so this motion merely serves to reflect an extrapolation of that position. In any case, the aforementioned imagery also reveals a highly disorganized tropical cyclone, with the small area of deep convection displaced to the north and northwest of the poorly defined surface center, which appears to be located about 40-50 miles southeast of the aforementioned, shapeless convective activity.
This extreme disorganization is due to strong southeasterly vertical wind shear in excess of 30 kt. This shear is associated with a large, persistent, and powerful upper-level low centered near 27N 93W, and a large and powerful upper-level ridge centered across the mid-Atlantic states. Bottom line? Bonnie is embedded within a very hostile environment for intensification, with water vapor imagery depicting a large swath of dry air to the west. This dry air is gradually being entrained into the poorly defined surface center due to clockwise flow around the aforementioned ridge. Given the noticeable lack of upper-level divergence and low-level convergence associated with the cyclone, along with the aforementioned dry air and a continued hostile upper wind environment, I am not expecting Bonnie to be able to intensify. In fact, the most likely scenario is for the system to gradually weaken, ultimately opening up into a disorganized trough of low pressure prior to moving ashore along the northern Gulf Coast in the next 24-36 hours.
The track forecast is rather straightforward. Water vapor imagery depicts a strong and solid southeasterly flow impinging upon the cyclone. Embedded within general southeasterly flow between the upper low and the aforementioned anticyclone, Bonnie should continue moving WNW for the next 12 hours or so, after which point a turn to the NW should occur. By this time, a weakness should become apparent within the western Atlantic subtropical ridge as a mid- to upper-level trough and its associated cold front will be nudged a bit to the southeast by a well-defined upper-level low currently moving across northwestern North Dakota. This should effectively erode the ridge and allow the cyclone to turn more poleward during this time. That motion should continue until well after landfall occurs along the northern Gulf Coast in about 24-30 hours, and the cyclone could still be moving NW as it enters eastern or central Arkansas in about 60 hours.
The intensity forecast is also relatively simple. The 18z GFS indicates that the current southeasterly flow will gradually veer to the northeast, associated with the aforementioned ridge. There could be a brief relaxation of the shear over the next six hours, as indicated by the GFS, and then again in 30 hours as the system approaches the northern Gulf Coast. I'm in general agreement with the NHC's 5:00 PM EDT forecast track, albeit slightly farther to the right when landfall occurs. I expect that landfall will occur along the Louisiana/Mississippi border in just around 30 hours.
Watches and warnings
A tropical storm warning is in effect from:
- Destin, Florida to Morgan City, Louisiana, including New Orleans and Lake Pontchartrain
However, I imagine that these warnings were given as a course of least regret, just in case the cyclone unexpectedly strengthens due to an unforeseen relaxation of the vertical shear. I am not actually expecting tropical storm force winds anywhere within the warning area. I still expect Bonnie to open into an elongated trough within the next 12 hours, given the hostile environment.
Bonnie's main legacy will be 4 to 6 foot waves atop the oil spill region. A persistent fetch of SE to E winds will begin to blow across that region beginning in about 12 hours. This could potentially bring oil into the already devastated Louisiana marshlands, along with other areas of coast that have already experienced oil on their shores.
Elsewhere
There are no other threat areas to discuss in the tropics at the moment. Former Invest 98L moved inland near Tampico early this morning. A weak area of low pressure could still form near the Cape Verde Islands on Sunday, as indicated by both the GFS and the ECMWF, but both, particularly the latter, have become far less aggressive in this forecast than they were previously. The ECMWF brings this system off Africa at a higher latitude than the GFS does. I am inclined to agree with the solution of the GFS, given current convective trends over western Africa. Though upper-level winds favor development in this area, a dry environment awaits the system in the eastern Atlantic, which will act to inhibit any significant development. In fact, this is probably why the aforementioned models do not strengthen this feature, and quickly lose it within 12 hours of exiting the coast.
It should be noted that we are forecast, as per the EWP model, to enter a rather dry period across the Atlantic basin as the downward motion pulse of the MJO -- a 30 to 60 day eastward propagation of moisture originating over the Indian Ocean. The upward MJO pulse tends to favor greater atmospheric instability, and hence, enhances convection. The downward motion tends to do the exact opposite. However, tropical cyclogenesis can still occur during downward MJO pulses -- for instance, 2005's Harvey and Irene all formed during the downward MJO, the latter of which ultimately became a Category 2 hurricane that threatened the Carolinas.
However, given that the upward MJO has been rather hesitant to leave our area throughout most of the season, I rather am skeptical of the EWP's forecast. We will see, though.
On the other hand, the CFS model foresees a far less robust and long-lasting downward pulse.
Finally, the GFS is forecasting a rather significant downward MJO pulse, but an extrapolation of this forecast points to a general lessening of this motion by the second week of August. I rather like the forecast from the CFS, given current convective trends across the basin. However, given that both the EWP and the GFS are forecasting a significant downward pulse, I cannot simply ignore them. Hence, I will split the difference and forecast a downward MJO pulse for the next two weeks, after which point we should return to a neutral MJO pulse, followed by an upward motion pulse sometime during the second week of August.
Remember, just because we have only seen one significant tropical cyclone this year: Alex, doesn't mean that the season is a bust. Please do not be lulled into a false sense of security. In 1998, the most similar analog I can think of, the second storm didn't develop until August 19, and we ended up having 14 named storms, 10 hurricanes, and 3 major hurricanes that season, including notable ones like Bonnie, Georges, and Mitch.
The season isn't over by a longshot, and I still fully anticipate an above average season, even if we don't necessarily end up getting the 18 storms that CSU predicts.
Thursday, July 22, 2010
Tropical weather analysis - July 22, 2010
Tropical Depression Three
A vigorous tropical wave located across the Turks and Caicos Islands, previously labeled Invest 97L by the National Hurricane Center, has organized into Tropical Depression Three.
Visible satellite animations confirm what the National Hurricane Center has said, a closed surface circulation is clearly evident. The partially exposed center of the newly developed depression isn't particularly difficult to locate, and the NHC puts it at 21.9N 75.0W. In addition, the maximum sustained winds are estimated to be near 35 mph. A reconnaissance aircraft will investigate the cyclone later today and give us a better assessment of the intensity of the system. The NHC has the motion estimated at WNW/15 mph, and satellite loops agree quite well with this.
The cyclone is still in its formative stages, and as a result, is not well organized. However, it is steadily improving as the upper flow slowly abates with the westward retreat of the persistent upper-level low that has been vehemently shearing the system (and still is, to a degree) over the last several days. Indeed, analysis of vorticity data from University of Wisconsin CIMSS suggests that the low- and mid-level centers have become more symmetrically aligned in the vertical, further confirming a gradual reduction in the vertical wind shear. Visible satellite imagery indicates that the center is located near the western edge of the deep convective mass, and the imagery also reveals less of a shearing pattern than it did just two hours ago.
Additionally, high arc clouds are streaming toward the center, rather than away from it as was previously the case. This indicates that the overall circulation is slowly becoming better defined.
Water vapor animations, used in conjunction with visible satellite loops, strongly suggest that the aforementioned upper low is moving faster than TD3. In fact, it may soon move far enough away to ventilate it, thereby generating a poleward upper-level outflow channel. Should this occur, it will assist in quicker intensification prior to the system reaching south Florida or the Florida Keys in about 18 hours or so. The depression is currently traversing very warm Sea Surface Temperatures (SSTs) of 29-30°C, well above the minimum threshold of 26C typically needed for tropical cyclogenesis.
Additionally, the cyclone is poised to traverse an area of increased oceanic heat content across the central Bahamas within the next six hours or so, which will assist in the intensification process.
Rather interestingly, latest analysis from CIMSS indicates the beginnings of an upper-level anticyclone, which is currently centered about 50 miles to the southeast of the depression. No doubt, this development has occurred because of the rapidly lessening influence of the persistent upper low. The 6z GFS suggests that this anticyclone will remain collocated with TD3 for at least the next 18 hours, after which point it is forecast to detach from the cyclone and retreat northeastward and gradually open, ultimately becoming absorbed into the western Atlantic subtropical ridge. My current thinking is that the anticyclone will benefit the depression enough to bring it to 45 mph before it approaches southern Florida or the keys. Obviously, the intensity could certainly be higher than this, depending on how well-established, persistent, and strong the anticyclone becomes.
The system, should it go through the Florida Straits, rather than through south Florida, should emerge into the eastern Gulf of Mexico in around 24 hours. Upper-level winds are not forecast to be ideal for intensification, and may only be marginally favorable. However, the shear will primarily be southerly, which would tend to have less of an impact on the cyclone than it ordinarily would. Overall, atmospheric and oceanic conditions conditions appear favorable enough for a 65 mph tropical storm, but this will depend upon the evolution of the upper wind environment across the Gulf.
A large weakness is evident in the subtropical ridge to the north of the cyclone, which is responsible for the current WNW motion. I expect this general motion to continue throughout most of the forecast period, though a more poleward turn could occur by around 60 hours as the system nears the coast of central Louisiana. Just how much of a poleward turn this system takes will depend largely upon the evolution of the aforementioned upper low, which is forecast to continue retreating westward into the Gulf, and eventually move inland by around 54 hours. This is impossible to tell at this point, so residents from Galveston eastward to Biloxi should carefully monitor the progress of this system over the next several days. Another factor regarding the eventual Gulf Coast landfall location of the depression will depend upon a cold front currently moving across the central United States.
My current thinking is that this trough and associated front will at least dent the western extent of the Atlantic subtropical ridge during the next two days. All in all, based upon model analysis as well as other data, I expect the cyclone to ultimately make landfall along the west-central Louisiana coast, just west of Vermilion Bay, though this is obviously not certain. Again, residents across the aforementioned areas should closely monitor the progress of this system over the coming days.
My forecast track agrees well with the NHC's current one, except it's just a little bit to the east.
Invest 98L
Invest 98L continues churning across the Bay of Campeche. Visible satellite animations imply the continued development of deep convection near the center, with some bands of heavy thunderstorms to the north of the circulation. These rains are impacting portions of central Mexico to the south of Tampico, an area just recently battered by Hurricane Alex and Tropical Depression Two. Based on animation of visible satellite imagery and 850 mb vorticity data from CIMSS, I assume that the surface center is located near 20N 95W. An upper-level anticyclone continues to be centered aloft, which favors continued development until the system moves inland. Land interaction is the only inhibiting factor I can see to tropical cyclogenesis, and the system should move inland to the south of Tampico by tonight or early tomorrow morning. The farther south this system goes, the less time it will have over water.
I give this system a 50% chance of becoming a tropical depression prior to moving inland. Were it not for land interaction, I'd go higher. Residents from Veracruz to Tampico should closely monitor the progress of this system. Regardless of development, locally heavy rainfall will overspread the aforementioned areas, and possibly even extreme southern Texas over the next day or two.
Elsewhere
The 12z ECMWF isn't out yet, but the 0z run was still indicating the possibility of our first Cape Verde storm on July 25. The 6z GFS also continues to reflect this possibility. Upper-level winds still appear rather conducive for anything that might happen to form in this area.
A vigorous tropical wave located across the Turks and Caicos Islands, previously labeled Invest 97L by the National Hurricane Center, has organized into Tropical Depression Three.
Visible satellite animations confirm what the National Hurricane Center has said, a closed surface circulation is clearly evident. The partially exposed center of the newly developed depression isn't particularly difficult to locate, and the NHC puts it at 21.9N 75.0W. In addition, the maximum sustained winds are estimated to be near 35 mph. A reconnaissance aircraft will investigate the cyclone later today and give us a better assessment of the intensity of the system. The NHC has the motion estimated at WNW/15 mph, and satellite loops agree quite well with this.
The cyclone is still in its formative stages, and as a result, is not well organized. However, it is steadily improving as the upper flow slowly abates with the westward retreat of the persistent upper-level low that has been vehemently shearing the system (and still is, to a degree) over the last several days. Indeed, analysis of vorticity data from University of Wisconsin CIMSS suggests that the low- and mid-level centers have become more symmetrically aligned in the vertical, further confirming a gradual reduction in the vertical wind shear. Visible satellite imagery indicates that the center is located near the western edge of the deep convective mass, and the imagery also reveals less of a shearing pattern than it did just two hours ago.
Additionally, high arc clouds are streaming toward the center, rather than away from it as was previously the case. This indicates that the overall circulation is slowly becoming better defined.
Water vapor animations, used in conjunction with visible satellite loops, strongly suggest that the aforementioned upper low is moving faster than TD3. In fact, it may soon move far enough away to ventilate it, thereby generating a poleward upper-level outflow channel. Should this occur, it will assist in quicker intensification prior to the system reaching south Florida or the Florida Keys in about 18 hours or so. The depression is currently traversing very warm Sea Surface Temperatures (SSTs) of 29-30°C, well above the minimum threshold of 26C typically needed for tropical cyclogenesis.
Additionally, the cyclone is poised to traverse an area of increased oceanic heat content across the central Bahamas within the next six hours or so, which will assist in the intensification process.
Rather interestingly, latest analysis from CIMSS indicates the beginnings of an upper-level anticyclone, which is currently centered about 50 miles to the southeast of the depression. No doubt, this development has occurred because of the rapidly lessening influence of the persistent upper low. The 6z GFS suggests that this anticyclone will remain collocated with TD3 for at least the next 18 hours, after which point it is forecast to detach from the cyclone and retreat northeastward and gradually open, ultimately becoming absorbed into the western Atlantic subtropical ridge. My current thinking is that the anticyclone will benefit the depression enough to bring it to 45 mph before it approaches southern Florida or the keys. Obviously, the intensity could certainly be higher than this, depending on how well-established, persistent, and strong the anticyclone becomes.
The system, should it go through the Florida Straits, rather than through south Florida, should emerge into the eastern Gulf of Mexico in around 24 hours. Upper-level winds are not forecast to be ideal for intensification, and may only be marginally favorable. However, the shear will primarily be southerly, which would tend to have less of an impact on the cyclone than it ordinarily would. Overall, atmospheric and oceanic conditions conditions appear favorable enough for a 65 mph tropical storm, but this will depend upon the evolution of the upper wind environment across the Gulf.
A large weakness is evident in the subtropical ridge to the north of the cyclone, which is responsible for the current WNW motion. I expect this general motion to continue throughout most of the forecast period, though a more poleward turn could occur by around 60 hours as the system nears the coast of central Louisiana. Just how much of a poleward turn this system takes will depend largely upon the evolution of the aforementioned upper low, which is forecast to continue retreating westward into the Gulf, and eventually move inland by around 54 hours. This is impossible to tell at this point, so residents from Galveston eastward to Biloxi should carefully monitor the progress of this system over the next several days. Another factor regarding the eventual Gulf Coast landfall location of the depression will depend upon a cold front currently moving across the central United States.
My current thinking is that this trough and associated front will at least dent the western extent of the Atlantic subtropical ridge during the next two days. All in all, based upon model analysis as well as other data, I expect the cyclone to ultimately make landfall along the west-central Louisiana coast, just west of Vermilion Bay, though this is obviously not certain. Again, residents across the aforementioned areas should closely monitor the progress of this system over the coming days.
My forecast track agrees well with the NHC's current one, except it's just a little bit to the east.
Invest 98L
Invest 98L continues churning across the Bay of Campeche. Visible satellite animations imply the continued development of deep convection near the center, with some bands of heavy thunderstorms to the north of the circulation. These rains are impacting portions of central Mexico to the south of Tampico, an area just recently battered by Hurricane Alex and Tropical Depression Two. Based on animation of visible satellite imagery and 850 mb vorticity data from CIMSS, I assume that the surface center is located near 20N 95W. An upper-level anticyclone continues to be centered aloft, which favors continued development until the system moves inland. Land interaction is the only inhibiting factor I can see to tropical cyclogenesis, and the system should move inland to the south of Tampico by tonight or early tomorrow morning. The farther south this system goes, the less time it will have over water.
I give this system a 50% chance of becoming a tropical depression prior to moving inland. Were it not for land interaction, I'd go higher. Residents from Veracruz to Tampico should closely monitor the progress of this system. Regardless of development, locally heavy rainfall will overspread the aforementioned areas, and possibly even extreme southern Texas over the next day or two.
Elsewhere
The 12z ECMWF isn't out yet, but the 0z run was still indicating the possibility of our first Cape Verde storm on July 25. The 6z GFS also continues to reflect this possibility. Upper-level winds still appear rather conducive for anything that might happen to form in this area.
Wednesday, July 21, 2010
Tropical weather analysis - July 21, 2010
Invest 97L
Invest 97L continues to struggle with strong vertical wind shear associated with a persistent upper-level low that has branched off from the Tropical Upper Tropospheric Trough (TUTT). The last of the visible satellite images suggest that the exposed low-level center is located at 21N 72W. 97L continues to battle 20 kt of southwesterly shear, associated with the aforementioned upper low. Animation of water vapor imagery and various CIMSS data depicts this upper low rather well from 350 mb upward to around 150 mb. This upper low is keeping all of the convection well to the east of the wave axis, resulting in a highly disorganized system.
However, this upper low has retreated westward over the last six hours, and as a result, shear has decreased over Invest 97L. The GFS continues to persistently indicate that the upper low will continue moving westward away from 97L, being replaced by an upper high, which would reduce the vertical shear over the system and allow for some upper-level ventilation to occur. Water vapor imagery supports the forecast of the GFS, indicating that the upper low has continued to gradually hook westward.
Theoretically, this low should ultimately outrun 97L, leaving a more favorable upper wind environment in place. The 18z GFS indicates that upper-level conditions should begin to significantly improve within about 12 hours. This agrees well with the evolution of the low on water vapor imagery and analysis of upper-air data. This, combined with a large area of dry air to the west, being channeled into the poorly defined low-level center by the upper low, makes it highly unlikely that 97L will significantly intensify tonight, and probably even into the early morning hours on Thursday.
By late morning and into the afternoon, however, the upper flow should vastly improve, as previously noted. Thereafter, upper-level winds appear favorable for some steady development until the system reaches south Florida in around 42-48 hours. Thereafter, the system will emerge into the eastern Gulf of Mexico, where upper-level winds appear to be more conducive than they were this time yesterday, though the aforementioned upper low, as it continues retreating westward, then northwestward and inland, may impart some southerly shear over the system.
I do not expect this shear to be strong enough to be a significant deterrent to development in the Gulf, however.
Given the system's current state of disorganization, it is quite improbable that the system will be able to be any stronger than a minimal tropical storm when it impacts southern Florida. The most likely scenario, however, is a strengthening low pressure system impacting south Florida with gusty winds, rough surf, and heavy rainfall.
Track wise, I expect the system to move generally WNW until around 72 hours, at which point a large weakness in the western Atlantic subtropical ridge is forecast to develop unanimously by nearly all of the computer models. This should bring the system inland somewhere from southeast Louisiana to Destin in about four days.
The CMC continues to suggest that 97L will make landfall along the upper Texas coast, but there is currently no valid reason to assume that this scenario is a reasonable one, since even the CMC keeps a remarkably weak ridge to the east of the system, which wouldn't allow it to move that far west.
None of the models significantly develop 97L, and most actually lose it while it is traversing Florida. The reason for this is unclear, but it could be that the upper low, which will be in the Gulf of Mexico by this time, will pump dry continental air into the circulation, along with natural disruption from passage overland, and finally, southerly shear. This possibility will have to be carefully assessed over the next couple of days, as the models may well be onto something here.
For now, the NHC is giving this system a 40% chance of developing into a tropical cyclone in the next 48 hours, and this is a reasonable forecast.
Invest 98L
The tropical disturbance previously located across the western Caribbean Sea has recently been designated Invest 98L by the National Hurricane Center.
Infrared satellite loops indicate the development of deep convection near the center, which I estimate is at 20N 93W based on animation of infrared RGB imagery.
There is also some weak vorticity noted at 850 mb, though the strongest signature is located inland over the western Yucatan Peninsula, to the east of the deepest convection. Additionally, the overall vorticity is broad and not well organized at the moment. However, upper-level winds are very favorable for development, and an anticyclone aloft is noted based on CIMSS wind shear data. The GFS keeps this anticyclone intact until 54 hours, at which point the system should be inland. Ergo, I believe it is quite possible that we will see a tropical cyclone out of this area prior to it moving inland near Tampico, Mexico. The farther south it goes, however, the less time it will have over water. This situation looks similar to 2005's Tropical Storm Bret, and also to Hurricane Alex earlier this year.
Indeed, this area might have a greater chance to develop than 97L does. The primary inhibiting factor will be land interaction.
Elsewhere
Elsewhere in the tropics, the reliable ECMWF continues to indicate the development of our first Cape Verde system, or at least a strong low pressure area, near the northern Cape Verde Islands on July 25. This model, along with the GFS, has been hinting at the possibility of tropical cyclogenesis in this area for awhile.
This appears to be a reasonable forecast, and upper-level winds favor development of anything that does happen to form here.
Invest 97L continues to struggle with strong vertical wind shear associated with a persistent upper-level low that has branched off from the Tropical Upper Tropospheric Trough (TUTT). The last of the visible satellite images suggest that the exposed low-level center is located at 21N 72W. 97L continues to battle 20 kt of southwesterly shear, associated with the aforementioned upper low. Animation of water vapor imagery and various CIMSS data depicts this upper low rather well from 350 mb upward to around 150 mb. This upper low is keeping all of the convection well to the east of the wave axis, resulting in a highly disorganized system.
However, this upper low has retreated westward over the last six hours, and as a result, shear has decreased over Invest 97L. The GFS continues to persistently indicate that the upper low will continue moving westward away from 97L, being replaced by an upper high, which would reduce the vertical shear over the system and allow for some upper-level ventilation to occur. Water vapor imagery supports the forecast of the GFS, indicating that the upper low has continued to gradually hook westward.
Theoretically, this low should ultimately outrun 97L, leaving a more favorable upper wind environment in place. The 18z GFS indicates that upper-level conditions should begin to significantly improve within about 12 hours. This agrees well with the evolution of the low on water vapor imagery and analysis of upper-air data. This, combined with a large area of dry air to the west, being channeled into the poorly defined low-level center by the upper low, makes it highly unlikely that 97L will significantly intensify tonight, and probably even into the early morning hours on Thursday.
By late morning and into the afternoon, however, the upper flow should vastly improve, as previously noted. Thereafter, upper-level winds appear favorable for some steady development until the system reaches south Florida in around 42-48 hours. Thereafter, the system will emerge into the eastern Gulf of Mexico, where upper-level winds appear to be more conducive than they were this time yesterday, though the aforementioned upper low, as it continues retreating westward, then northwestward and inland, may impart some southerly shear over the system.
I do not expect this shear to be strong enough to be a significant deterrent to development in the Gulf, however.
Given the system's current state of disorganization, it is quite improbable that the system will be able to be any stronger than a minimal tropical storm when it impacts southern Florida. The most likely scenario, however, is a strengthening low pressure system impacting south Florida with gusty winds, rough surf, and heavy rainfall.
Track wise, I expect the system to move generally WNW until around 72 hours, at which point a large weakness in the western Atlantic subtropical ridge is forecast to develop unanimously by nearly all of the computer models. This should bring the system inland somewhere from southeast Louisiana to Destin in about four days.
The CMC continues to suggest that 97L will make landfall along the upper Texas coast, but there is currently no valid reason to assume that this scenario is a reasonable one, since even the CMC keeps a remarkably weak ridge to the east of the system, which wouldn't allow it to move that far west.
None of the models significantly develop 97L, and most actually lose it while it is traversing Florida. The reason for this is unclear, but it could be that the upper low, which will be in the Gulf of Mexico by this time, will pump dry continental air into the circulation, along with natural disruption from passage overland, and finally, southerly shear. This possibility will have to be carefully assessed over the next couple of days, as the models may well be onto something here.
For now, the NHC is giving this system a 40% chance of developing into a tropical cyclone in the next 48 hours, and this is a reasonable forecast.
Invest 98L
The tropical disturbance previously located across the western Caribbean Sea has recently been designated Invest 98L by the National Hurricane Center.
Infrared satellite loops indicate the development of deep convection near the center, which I estimate is at 20N 93W based on animation of infrared RGB imagery.
There is also some weak vorticity noted at 850 mb, though the strongest signature is located inland over the western Yucatan Peninsula, to the east of the deepest convection. Additionally, the overall vorticity is broad and not well organized at the moment. However, upper-level winds are very favorable for development, and an anticyclone aloft is noted based on CIMSS wind shear data. The GFS keeps this anticyclone intact until 54 hours, at which point the system should be inland. Ergo, I believe it is quite possible that we will see a tropical cyclone out of this area prior to it moving inland near Tampico, Mexico. The farther south it goes, however, the less time it will have over water. This situation looks similar to 2005's Tropical Storm Bret, and also to Hurricane Alex earlier this year.
Indeed, this area might have a greater chance to develop than 97L does. The primary inhibiting factor will be land interaction.
Elsewhere
Elsewhere in the tropics, the reliable ECMWF continues to indicate the development of our first Cape Verde system, or at least a strong low pressure area, near the northern Cape Verde Islands on July 25. This model, along with the GFS, has been hinting at the possibility of tropical cyclogenesis in this area for awhile.
This appears to be a reasonable forecast, and upper-level winds favor development of anything that does happen to form here.
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