Tropical weather analysis - August 24, 2010

Danielle

Danielle, after a period of rapid intensification yesterday afternoon and evening, has weakened just as quickly today, dropping below hurricane status as of the 5:00 PM EDT advisory from the National Hurricane Center. Speaking of the 5:00 PM advisory, here is the information they put out during that time:

Wind: 70 mph, with higher gusts
Pressure: 993 mb
Movement: WNW at 18 mph
Location: 17.5°N 48.2°W
Category: Tropical storm

The last of the visible satellite imagery along with animation of shortwave infrared imagery, suggest that the cyclone is once again becoming better organized, with the center now pretty much fully underneath the steadily increasing deep convection.

Additionally, a recent SSMI/S microwave satellite overpass, along with CIMSS vorticity analysis indicates that the low- and mid-level centers are fairly well aligned in the vertical, with only perhaps a very slight eastward decoupling of the centers. More importantly, the aforementioned SSMI/S overpass suggests that Danielle is in the process of reforming an eye and eyewall, though the former is not currently visible on satellite imagery.

Danielle is currently experiencing about 10 knots of westerly vertical wind shear. It appears that over the last 18 hours, the well-defined 200 mb anticyclone, observed on the link above, has detached from the circulation of Danielle, and, rather than aiding it, it has been shearing it. Additionally, wind shear has decreased over the system the last six hours.

Based on all of this, Danielle's weakening throughout the day should be relatively short-lived, and I expect she will reattain hurricane status by the time of the next advisory. And if not at that time, definitely at the 5:00 AM one. Based on the 18z GFS, the anticyclone should become reestablished over the storm in about 18-24 hours, so I expect some more steady intensification to begin tomorrow evening, and into Thursday morning. Shear could increase again during the 72-96 hour time frame, as the cyclone comes underneath the influences of upper southwesterlies associated with a longwave trough currently moving across the central and southern United States. However, this shear may not be quite as detrimental as one would expect if Danielle moves more NE with acceleration, as it will be moving in the same direction as the shear vector.

As far as track is concerned, I expect that, based on water vapor animations, CIMSS steering data, and 0z steering data from PSU's e-wall, Danielle will continue to track more westerly (specifically, WNW, though a slight W motion cannot be ruled out at this time, either) for the next 12 hours or so, as the weakness in the ridge evident on water vapor imagery and steering data from CIMSS to the north of her, along 25N from 50 to 45W, doesn't appear to be appreciably affecting the cyclone just yet.

My forecast track is a blend of the 18z GFS and 12z CMC, which calls for a continued WNW motion for the next 12 hours or so, followed by a NW turn and an associated decrease in forward speed as the system approaches the aforementioned weakness. This general motion, albeit with some possible oscillations to the left or right, should continue through at least day three. Thereafter, models diverge, with some turning Danielle N quicker, and some slower. The GFS, in a sharp contrast from what it has been doing the past several days, foresees a sharp westward turn near the end of the forecast period, which could potentially mean a landfall in the mid-Atlantic or New England in about seven days. However, I will not go that far west at this time, since the models have the luxury of flipflopping at random, while I am forced to remain consistent.

All in all, I expect a NW to begin in about 12 hours, and this should persist until around 108 hours, at which point a turn to the N is likely. However, given that the usual uncertainty in long-range forecasts, as well as the fact that the GFS trended westward at 18z, and the 12z NOGAPS tracks the center directly over Bermuda at day five, interests in Bermuda should closely follow the progress of Danielle over the next several days.

The tropical storm force wind probability map from the National Hurricane Center indicates that there is a 30% chance of Bermuda receiving tropical storm force winds from Danielle over the next five days, and that island is not out of the woods by any means. I still suspect Danielle will pass east of Bermuda, but this is not certain, and it could come closer to the island than I was thinking last night.

Interests along the mid-Atlantic and New England coasts should also be monitoring Danielle, as should Atlantic Canada.

Invest 96L

A strong tropical wave, "Invest 96L", is located near 13N (nearing 14N) 26W. This system continues to display signs of organization on satellite imagery, though deep convection is currently minimal, and a bit to the west of the 850 mb vorticity maximum.

CIMSS shear data indicates that this system is experiencing 20 knots of southwesterly shear. However, there are no apparent signs of this shear on satellite imagery. The likely catalyst for the recent wane in convection is some slight dry entrainment noted on water vapor imagery. This dry air is associated with a pocket of Saharan Air Layer (SAL) located to the west of 96L.

The dry air notwithstanding, conditions appear favorable for continued slow development over the next couple of days, and this system could become a tropical depression during the next couple of days. The GFS suggests that this will develop in just about 12 hours. I suspect it will take a little more time than that, based on current trends and forecast shear from the GFS. Instead, I expect this will undergo tropical cyclogenesis in about 18 to 24 hours. If dry air continues to hinder this system, however, it could take even longer. It should be noted that the models have shifted considerably southward today, and now foresee 96L being a long-range threat to the southeast United States.

The weaker it stays, the more southerly it should go. Given that the system will be passing over the cold water wake generated by Danielle, it should remain below hurricane status until around day four. None of the models, save the GFS, show this system becoming a hurricane during the next five days. I tend to agree with the models on the track, and the United States may not be so lucky with this one.

Elsewhere

Elsewhere, showers and thunderstorms across the central Gulf of Mexico are associated with a surface trough. This system is currently very poorly organized, and any development should be slow to occur as the system moves W at 5 to 10 mph over the next several days. A turn to the NW could occur on day three as the ridge weakens somewhat. Residents from northeast Mexico to the upper Texas coast should closely monitor the progress of this system over the next several days. I am not really expecting development here, and none of the models show it either.

The 18z GFS is forecasting that a third tropical cyclone will develop behind Danielle and future Earl late on day five. This is a sign that the tropics are heating up, and residents in hurricane prone areas should review their preparedness plans. We're still likely to churn out at least 15 storms this season, and it's not over by a longshot.