Ernesto weakens slightly

August 27, 2006

The strengthening trend of Ernesto has reversed. The 5 AM forecast discussion from the National Hurricane Center, stated that “Ernesto was likely undergoing rapid intensification”. However, the reconnaisance plane, after collecting observations that showed the storm at minimal hurricane strength, did not stronger winds. As such, the 11 AM discussion, relying on the final observations from the plane three hours earlier assessed the hurricane to be at steady state. Two hours later the next mission entered the storm and found that the central pressure had risen ten millibars, a sign of weakening. It did not however, have time to fully sample the storm, so the 2 PM advisory, kept Ernesto as a hurricane with a note that Ernesto may have weakened. At around 3:00 EDT the plane found flight level winds that suggest winds of 65 mph at the surface. Becuase of that the 5 PM advisory may feature Ernesto as a tropical storm (edited as I realized I was looking at a wrong column in the observations when I was looking for wind speeds). The plane is not flying overland, so it probably isn’t going to be able to sample the northeast quadrant winds. Nonetheless, the center fix at 3 PM EDT shows that the pressure remains 10 millibars higher than earlier, so it is likely that Ernesto will be rated as a tropical storm in the next advisory

There are two possible reasons for the weakening. One is the storms proximity to the mountainous terrain of Haiti; nearly the whole northeast quadrant is over that country and it appears that the center is going to clip the western peninsula. Another is shear that went undiagnosed in the analyses (the mid-level shear that crippled Chris, for example, was not apparent until it took its toll).

If the weakening is solely due to the terrain of Haiti, Ernesto should get some relief later this evening as it continues northwest and gets most of its circulation over water. However, that relief is likely to be shortlived as it will then start to run into the mountains on the eastern side of Cuba. As I mentioned in my previous post, the mountains there have historically taken lowered the strength of hurricanes down to tropical storms, (or to a minimum hurricane if the storm was a major hurricane to begin with). Owing to the events of the past six hours and the increased likelihood of full interaction with the Cuban mountains, the 11 AM forecast, which brought Ernesto out of Cuba may be quite overdone and could be reduced significantly in the 5 PM forecast. However, an alternative scenario would be for another northward reformation of the center to occur, that adjusts Ernesto’s path such that it it only clips the northeastern corner of the country.

The future after passing through or near Cuba remains hazy. The spread of the most respected models goes from a landfall at the southernmost part of peninsular Florida (offered by NOGAPS and the GFS) to a continued northwest movement across the Gulf of Mexico to Panama City (GFDL’s solution). The somewhat in-between forecast is offered by the UKMET, which continues to forecast Ernesto to get some time over open water before curving to the northeast towards Tampa. The track forecast will likely be unchanged, or perhaps slightly adjusted to the current motion of the storm.

As I noted in my last post, there is a possibility that the models on the eastern side of the envelope are under-estimating the strength and extent of the ridging that may protect Peninsular Florida (therebt creating a threat to the panhandle). The flight of the Gulfstream IV this afternoon should put more certainty into the situation and some models may shift this evening depending on what upper air observations show.

The best case scenario would be for Ernesto to have an uninterrupted trip northwest into the mountains of Cuba, with significant time overland before turning north into peninsular Florida. That would make Ernesto only a rain event for Florida. However, if a northward reformation of the center were to disturb a straight-line northwest movement, there is a disconcerting scenario where Ernesto gets turned to the west by the ridging north of Cuba, and enjoys increased values of heat potential in the Florida Straits (see the TCHP chart for the Gulf of Mexico), which would give Ernesto ample time to strengthen before heading to the Panhandle or curving back into Pensinsular Florida (and of course affecting the Florida Keys along the way).

Due to the Florida Straits scenario, residents in the Keys need to keep an extremely close eye on the progress of Ernesto and be ready to evacuate on short notice. As before, residents of the entire gulf coast of Florida need to maintain watch as well. The next set of Ernesto advisories comes out at 5 PM, with an intermediate update at 8 PM, followed by another full package at 11 PM EDT. I’ll have a very late night update thereafter, featuring what the upper air observation enriched 00Z models are forecasting.


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