Invest 97 uncertainty

July 21, 2010

This morning, when the sun rose over the area of disturbed weather known as Invest 97, it was apparent to the duty National Hurricane Center forecaster that tropical cyclone formation was not imminent. The scheduled Hurricane Hunter mission was canceled and probability of 48 hour development was lowered to 60%, then 50% in the subsequent Tropical Weather Outlooks.  Overnight, thunderstorms would flare up in one location with a bit of rotation evident but die out a few hours later.  Similar to trying to track the movement of a person walking against the flow of a moving sidewalk, best guesses of a notional center inched forward rather than indicating steady movement. This has served to throw out the “straight-forward” scenario and made things complicated.

What little is clear: We have had a persistent area of thunderstorms north of the Dominican Republic that have moved away from the coast to the west-northwest and are now over and to the east of the Turks and Caicos islands.

As seen on the water vapor loop, a shear inducting upper-level low has moved steadily to the west at a pace slightly quicker than that of the thunderstorms’.  This, combined with the movement away from land creates conditions a bit more favorable for development.

What is not clear is whether we have a closed circulation at the surface.  Observations from the Turks & Caicos have not shown the ‘smoking gun’ of a westerly wind that a closed circulation would have. That would back up what’s suggested by the lower level clouds seen on visual satellite.  If there is a surface circulation, it is further to the east/north-east, obscured by the thunderstorm clouds.

For what is a relatively short term forecast, there is a large spread in track forecast guidance. As is clear from the satellite imagery of the region, low level flow is going around Florida before turning to the northwest to the upper Gulf coast.  Instead the 8 lane highway of southeast-northwest flow that spanned the Gulf into Florida, it’s more like four lanes with an on ramp that circles Florida.  Because of that, if the storm system does not develop quickly, it will get pushed to the west-southwest into Cuba or through the Florida Straits before turning towards the Louisiana.  That is the prognostication of some global forecast models. In sharp contrast though, the specialized dynamic hurricane models develop a storm fairly quickly, and move it fairly quickly to the northwest before landfall on the central Florida coast.  Cuba, Cape Canaveral, or somewhere in between are the possibilities offered a mere 48 hours out from this morning.

That of course, makes any sort of development/intensity forecast trickier than usual.  If we do have a tropical cyclone developing (i.e. a surface circulation is indeed present), then a weak-moderate tropical storm is the likely outcome prior to a south Florida landfall.  However, if one is not presently developing, then it’s an open question as to whether we’ll have one anytime soon.  Getting caught in rapid low-level flow would make it difficult for a circulation to close off. The result would be  rain and gusty winds in thunderstorms for the Keys and/or Cuba.  Assuming a run of the Florida straits (and not a Cuban death blow), some development would be possible further down the road.

As I mentioned, the Hurricane Hunter flight was canceled. The Gulfstream IV mission did launch, however. It took off from Tampa and is making a bit of a circle around the Bahamas and the southeast  edge of the storm before turning back home along the western Florida peninsula. Along the way, it is releasing dropsondes to collect upper-air data to be fed to computer model runs this evening.  If we had a developed storm, I would be more confident of this evening’s model runs reflecting greater certainty.  With the development/no-development game still on, I am not so sure. Fortunately, large-scale conditions are not currently favorable for any sort of rapid intensification regardless of where a potential storm would go, so the uncertainty is not dangerous.

As was the case last night, I anticipate my next post being early  tomorrow morning with any brief observations going to my Friendfeed


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