Invest 96 may become second tropical depression of Atlantic season

July 7, 2010

Observers of  weather in the Tropical Atlantic may have spent the past few days feel like they were in a real-life ensemble forecast model.  In ensemble models, a given set of initial conditions in slight variations, to produce a multitude of forecasts. In our real life scenario, the initial conditions were Invest 93/Alex with Invest 96 being a variation thereof.

This area of disturbed weather, then in the Caribbean, was first mentioned in the early Sunday morning Tropical Weather Outlook and given a 10% chance of developing into a tropical cyclone within 48 hours. From there, the probability went up to 50% on Monday morning, then down to 30% before rebounding back to 50% in the current TWO. Looking from the bottom frame of this image and working up and to the right, one can see the progression of this system in the Gulf of Mexico and western Caribbean.

The Navy recently issued a Tropical Cyclone Formation Alert (TCFA) for the area. TCFA’s are issued using objective criteria that suggest the probability of tropical cyclone formation in the next 24 hours.  While TCFAs are by no means infallible, they usually end up verifying.

A scatterometer overpass this morning indicated a closed circulation at the surface, which was also evident in visual satellite imagery before thunderstorm activity began to cover it. Looking at the visual satellite loop we can see that the thunderstorm activity isn’t aligned with the surface circulation while  the infrared loop shows convective activity is not particularly deep.  That would be an argument against classifying the system as a tropical depression.  However, it is one of those things, that while in theory is black and white/straightforward, that is really subjective and dependent on the judgment of the forecaster on duty.

Hurricane Hunter missions were penciled in for yesterday and today, but both were canceled in the respective mornings as prospects of development did not appear particularly high.  A NOAA P-3, however, examined the system on a research mission yesterday and is doing the same today.

The most recent SHIPS guidance, assuming that a tropical depression was present, brought this up to tropical storm strength before landfall (which would be in the vicinity of Brownsville Texas).  Lack of pre-existing organization would seem to be the only key impediment for this system.

Regardless of further development, this will bring unwanted rain to Texas where it is not particularly needed.   The Alex report from the National Weather Service Office in Brownsville shows that 5-10 inches of rain fell in the area during the passage of the hurricane and most of Texas was similarly affected; many areas are already under flood watches or warnings.   As residents of North Carolina and Virginia affected by the double shot of Dennis and Floyd back in 1998 can attest, the passage of a tropical system over land already saturated by one is a recipe for serious flooding.

Updates will be posted to my Friendfeed as warranted.


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