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Ernesto: What's Going On?
      #71405 - Sun Aug 27 2006 12:41 PM

Early this afternoon, Hurricane Ernesto sits just southwest of the southwestern Haitian coastline, slowly moving off to the west-northwest. It continues to gradually become better organized as the shear being imparted upon it by the upper low to its west has greatly diminished over the past 24 hours, but interactions with land are serving to keep intensity in check for the time being. The big question with Ernesto isn't the current track, however, but the future track -- both short and long term.

Overnight, much of the model guidance shifted significantly to the right, highlighting the Florida Peninsula as being under the gun for an eventual impact from Ernesto. Breaking with standard operating practices, the NHC jumped on this change and wholeheartedly shifted their forecast track as of the 5a ET advisory this morning. Then, the GFS and GFDL shifted back to the west, highlighting the Florida panhandle, but now the GFS at 12Z today has shifted even further to the east, suggesting a track over Miami and up the length of the peninsula. Other models continue to jump back and forth as well. Needless to say, without some sort of continuity, it's hard to pinpoint any one area as under the gun from Ernesto into next week, but current indications suggest it will be somewhere in Florida.

What is causing this uncertainty and all of these jumps? It's not from a better representation of the storm; all of the models except the GFDL maintain a very weak storm as they have throughout Ernesto's lifecycle. It stems from uncertainty in what the upper level feature currently over the intermountain west is going to do over the next few days. It also stems from uncertainty in the short-term track of Ernesto. Let's start with this latter point.

Initial projections for Ernesto had the subtropical ridge over Florida and the western Atlantic building in to the north of storm, allowing it to move on a more westerly path across the Caribbean. Problem is, Ernesto isn't south of it yet. An upper low located to its north, a feature that has been there through its history, has not weakened or moved out of the region to the east as had been forecast. This is keeping a bit of a weakness in the ridge and allowing Ernesto to move more to the northwest now, coupled perhaps with the continual reorganization of the storm that we have seen over the past few days. How this feature evolves today and tomorrow will determine whether or not Ernesto will ever fully get underneath that subtropical ridge and start to turn back more toward the west or west-northwest. Most of the model guidance does not believe that this will be the case and, given the shear zone setting up between the ridge and Ernesto, I don't either. Trough fracture has not occurred and does not appear that it will occur in a region that allows Ernesto to significantly bypass this upper low. To compound matters, this upper low is starting to serve as an evacuation mechanism for the outflow from the storm, aiding in its intensification.

What this does is places southern Florida and most of the eastern 2/3rds of Cuba at significant risk for an impact from Ernesto. While shocking at first, with more data to evaluate now as we head into the early afternoon hours, the model guidance trends and NHC track shift are starting to make a little more sense. The upper low over the north-central US is slowly progressing eastward, though is having a hard time breaking down the western extent of the subtropical ridge right now. This is projected to change, though there's not a whole lot to the west of this upper low to drive it into the ridge. Despite this, model guidance responds by creating enough of a weakness to gradually turn Ernesto to the northeast around 29-30 degrees N. This weakness isn't going to be a big one, but if Ernesto is far enough east or strong enough in the Gulf, it should feel this weakness. Question is, where is it going to be in 4 days?

A couple of scenarios can play out from here.

1: First, model guidance is accurate, with the storm continuing northwest all the way from its current location to south Florida, turning north and northeast somewhere between the eastern coast and 100 miles west of the western coast of the state. This suggests that the upper low to the north of Ernesto will keep enough of a weakness in the steering flow for the storm to track northwest now, while the upper low over the north-central US will be strong enough to force the subtropical ridge to move and/or breakdown.

2: A different scenario takes the storm further west, with Ernesto turning more toward the west-northwest over Cuba, potentially from a combination of impacts from the upper ridge over the western Atlantic as well as not as vertically deep of a storm due to continued interaction with Cuba. Also, outflow from Ernesto starts to diabatically modify its own environment, as it somewhat has already, and helps amplify that ridge despite interaction with Cuba. Historically, this scenario is not well-captured by any of the global models. The trough still comes to get Ernesto, but with only a gradual turn and acceleration and a more likely impact from Pensacola to the Big Bend.

So, which one is more likely? Only time will tell. Despite this, everyone in Florida -- and potentially further north along the southeast coastline for a rain threat -- needs to watch Ernesto closely today and tomorrow, as the situation is currently very fluid and subject to rapid change based upon the evolution of the upper pattern and the intensity of the storm. While Texas and likely Louisiana are safe from an impact as of now, I don't feel comfortable excluding Mississippi and Alabama from a potential threat as of yet given the nature of the situation. It is a situation with less confidence than normal, thus I feel those regions outside the NHC cone still need to pay attention to Ernesto at least for another day.

Residents of the Florida Keys should begin to make evacuation plans for the early morning hours tomorrow in advance of the storm. Residents of the southern half of the Peninsula should begin reviewing their hurricane preparedness plans, just in case. Residents further north along the west coast of Florida north of Tampa need to begin thinking about making preparations for a potential direct imapct from Ernesto sometime late in the workweek. veryone else just needs to stay abreast of the situation for the time being.

More as events warrant.

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