23 August 2014

Tropical Depression 4 forms north of Haiti

The disturbance we've been watching struggle across the Atlantic for the past 13 days is now north of Haiti... and has just been upgraded to Tropical Depression 4.  Aircraft reconnaissance missions have been carried out frequently, but up until now, it has either been lacking enough centralized thunderstorm activity or a closed circulation that extends all the way to the surface.  Both are requirements for a tropical cyclone (that generic term includes tropical depressions, tropical storms, and hurricanes). 

Because it has remained so poorly organized, models have had a hard time with it.  Stronger cyclones are steered by different layers of the atmosphere than weaker cyclones, so if a model is too bullish on the intensity in the analysis or forecast, it will likely get the track wrong.  For the past several days, this track forecast has revolved around whether or not a weakness in the subtropical ridge is strong enough to allow the disturbance to slide northward into it, or if it will not "feel" that weakness and continue a west-northwest heading.

The difference between those scenarios is rather big: it either hits south Florida or recurves over the Bahamas and stays well away from the US... with some 'wiggle room' in between.  I would still estimate a 10-15% chance of the south Florida option happening, which is noteworthy because it would only be 3 days away and the Bahamas are infamous for producing rapid intensifiers.  The wind shear is low, the SST is very warm, the only obvious obstacle now is its proximity to Hispaniola.

The 12Z suite of "late" models, including the 20 GFS ensemble members.

The eastern Bahamas are now under a tropical storm warning, and the official NHC forecast (with cone of uncertainty) is shown below.  Their forecast brings it up to tropical storm intensity tonight, and then hurricane intensity on Tuesday... right in the middle of the pack in the figure above.  If you're in south Florida, this should not be ignored.  There is a slight possibility that soon-to-be Cristobal could make landfall there as a minimal hurricane in just three days.  If you're in coastal SC or NC, you should also be paying very close attention to this in the coming days.

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