Tropical weather analysis - June 25, 2010

Began writing at approximately 3:19 PM eastern

Invest 93L

Invest 93L continues to display signs of organization, with visible satellite animations depicting a well-organized system. A recent AMSUB pass indicates a well-defined surface circulation, with some convective banding noted, particularly along the eastern flank of the circulation. This circulation might not be closed, however, as surface observations across northern Honduras indicates a lack of prolonged westerly winds (there was a report of some westerly winds across La Mesa and Puerto Cortez at around 8 AM central, but that was it -- again, no prolonged reports of westerly winds). An earlier ship report from the vessel Celebrity Solstice indicated a west wind just offshore the northern coast of Honduras, but has since veered around to the southeast. If there exists a closed surface circulation with this system, it's not a particularly well-defined one yet. An air force reserve hurricane hunter aircraft is currently investigating this system. In any case, I have the center, based on visible imagery and the aforementioned AMSUB pass, at 16N 83W.

Heavy rainfall is likely across portions of Honduras, Nicaragua, Belize, the Yucatan Peninsula, and western Cuba over the next couple of days. I would imagine that these rains could bring some flash flooding and mudslides to Honduras and Nicaragua, given their notorious vulnerability to heavy rains. CIMSS analysis depicts moderate northeasterly shear impinging upon the system.

The culprit of this shear is a well-defined upper-level anticyclone currently centered about 100 miles offshore the northern coast of Honduras. This shear has caused a noticeable elongation of the cloud pattern along the northeastern portion of the circulation.

93L appears, based on satellite animations, to be moving NW at about 5 kt. Recent steering analysis from CIMSS also supports this. This slow movement will allow for 93L to continue gradually deepening, though do place emphasis on the word "gradually" in this case, as there isn't a whole lot of low-level convergence associated with this system, and the system isn't vertically stacked, and the associated vorticity is still elongated. It is best noted at 850 mb, however, and I think this is the most relevant part for now, as the vorticity has improved markedly in this area over the last six hours, a sign of a deepening system. I expect, based on 12z steering currents from PSU, that the current NW motion will continue, eventually increasing to around 10 kt.

This should bring the system into the eastern Yucatan Peninsula by around 36 to 42 hours. Most of the model guidance also reflects this likelihood. As the system enters the western Gulf of Mexico in about 72 hours, there are some indications within the models that the steering flow will collapse for around 24-30 hours, likely due in part to the potent upper-level trough that should ultimately recurve this system toward the northern Gulf Coast (the possibility exists that the trough bypasses the system and 93L instead moves more westward, making landfall somewhere in Mexico or southern and central Texas, but I do not forecast this for now), and a binary interaction between 93L and Eastern Pacific Hurricane Darby. The deeper 93L becomes, the further east Darby will move near the end of the forecast period, and thus, the greater the interaction between the two systems.

By sometime on day four, the models restrengthen the steering currents, though I would imagine that this would occur late on day four, given the possibility of the aforementioned interaction with Darby. At this point, the models do not have a good handle on this system, with some of the models even initializing this system properly. The only models that appear to have properly initialized the system are the GFDL, HWRF, UKMET, and ECMWF. Right now, the majority of the guidance aims the system toward the Mexico or Texas coast in about five days. Based on the fact that I still see this system becoming a relatively deep one, I feel that these models are too far west.

Nevertheless, the current consensus, particularly since the reliable ECMWF is a part of it, cannot be completely ignored (though this consensus is probably due in part to lack of reconnaissance data, which will be fed into these models later, providing us with better and more accurate data). Since I do not want to lean too heavily against the model consensus, I will average the consensus and the outliers (GFDL and HWRF) and forecast a landfall along the Texas/Louisiana border, though this could occur a bit further east, to the west of Vermilion Bay.

However, there is greater than normal uncertainty in this forecast the present time, and residents from Mexico to Florida, particularly from Corpus Christi to Vermilion Bay, should carefully monitor the progress of this system over the next several days.

Upper-level winds appear conducive for at least some slow additional development of this system for the next 18 to 24 hours. Thereafter, 93L could catch up to the anticyclone, which may effectively stall offshore the Yucatan Peninsula during this time. This could provide the system with an environment conducive enough to attain winds of 45 kt before moving inland along the eastern Yucatan Peninsula in 36 hours. There is a possibility, assuming both how strong the system is at landfall here, and how slow it moves after going inland, that the system could decouple across the Yucatan Peninsula, with the low-level center continuing westward, and the mid-level center advecting northeastward in advance of the aforementioned upper-level trough. I do not expect this to happen at the moment, as the forward speed should increase to around 10 kt before and after landfall, but this possibility will be very carefully monitored nonetheless.

After the system enters the western Gulf of Mexico in about 72 hours, upper-level winds should be at least marginally conducive for some additional development, though this is uncertain, given that two very reliable models, the GFS and the ECMWF, foresee two exact opposite scenarios with regards to the vertical shear profiles across the Gulf, with the former showing an anticyclone environment and associated meridional flow, while the latter indicates a largely zonal flow across the Gulf.

It is difficult to tell which is right at this point, so I will average the forecast out. However, it will take 93L at least 24 hours to reinvigorate its circulation after passage over the Yucatan Peninsula, so one should not expect too much intensification of the system as it enters the Gulf. However, conditions appear favorable enough to see a minimal hurricane. There are some indications, even within the GFS, that westerly to southwesterly shear could increase near the coast before landfall, which could act to weaken the system a bit before landfall.

Invest 94L

A well-defined tropical wave was designated Invest 94L by the National Hurricane Center today. Visible satellite animations suggest a strongly sheared system, with most of the associated convection streaming northeastward, away from the surface center. This system is currently undergoing around 30 kt of southwesterly shear, as per CIMSS analysis. This shear is associated with the subtropical jet. The system, expectantly, is not at all aligned in the vertical, with the low- and mid-level centers displaced at least 50 miles to the northeast, due to the aforementioned vertical shear.

Based on 850 mb vorticity data from University of Wisconsin CIMSS, along with visible satellite animations and a recent AMSUB pass, I place the broad surface center at 20N 59W. We have had no recent ASCAT passes over the broad center, but recent ship reports indicate that the center is certainly not closed. Animation of water vapor imagery indicates that a weak upper-level low is centered near 24N 63W. This upper low has broken off from the subtropical jet, and is responsible at least in part for the shear that 94L is currently experiencing. This is a very similar situation, minus the subtropical jet, that occurred with 2008's Hurricane Hanna. Hanna also formed in a similar location to 94L.

Based on visible imagery and CIMSS steering maps, 94L is currently moving NW at about 15 to 20 kt. This fast motion, along with the shear, will make development impossible in the near-term. The models suggest that this swift forward speed should generally continue throughout the forecast period. Indeed, the system might be well-embedded within the westerlies by 108 to 120 hours. However, the upper flow pattern is forecast to improve within about 24 hours as the aforementioned upper low retrogrades westward.

This more favorable pattern aloft should persist until at least 96 hours, when some southwesterly shear could again impinge upon the system. All in all, I expect that this system could eventually develop into a tropical depression, and I give this a 40% chance of happening within the next few days. The NHC foresees a 20% chance of development within the next 48 hours, which is reasonable.

Anything that forms here will only pose a threat to Bermuda, as steering forecasts suggest that the subtropical ridge will weaken further, and the system could turn N by 72 hours. None of the computer models, save the CMC, develops this system, though the GFS is hinting at development.