21 August 2014

Tropical disturbance slowly organizing, model tracks shift

Since yesterday's post, the area of interest east of the Lesser Antilles has gotten slightly better organized, and more interestingly, the models have all shifted their forecasts north.  Keep in mind that this is not yet a tropical depression, so there is tremendous uncertainty in any forecast you see (hard to forecast "it", when "it" isn't even a tropical cyclone).  And even with a bona fide tropical cyclone, model forecasts beyond five days contain a lot of uncertainty.

Visible satellite image from 8:15am EDT.  I added a red L to help locate the center of the disorganized disturbance.  (NRLMRY)
The northwestward motion that we've been seeing is due to a weakness in the subtropical ridge... or in other words, rather than being pushed west or west-northwest like storms typically would in this location, it's moving north much quicker than usual.  It is forecast to pass over the northern Leeward Islands early Friday morning, but shouldn't be more than a gusty and wet day there.

Though too complicated to explain in a caption, the steering feature I'm describing can be found in the area north of Puerto Rico and Hispaniola.  The line contours are the current 500mb heights, and the shaded contours are the height anomalies. (tropicaltidbits.com)
As far as the latest model forecasts go, a storm entering the Gulf of Mexico now looks like a long-shot, whereas yesterday it looked relatively likely.  Models  seem to have a better grip on the structure of that subtropical ridge and show a northward turn sometime around Sunday when the "storm" is near Hispaniola.  BUT, any change in the strength of that ridge will make a huge difference in track... if it builds, the storm would start moving more west, and if it weakens further, the storm will nudge further north.

Tracks from the 06Z model runs... the first four are global models and the last four are regional hurricane models. (U.Albany)

Of historical note, today is the 7-year anniversary of Hurricane Dean's landfall on Mexico's Yucatan peninsula.  Dean hit near Chetumal with sustained winds estimated at 170+ mph.  It remains the strongest landfalling hurricane in the Atlantic since Andrew hit south Florida in 1992.

Enhanced infrared satellite image of Hurricane Dean making landfall during the early morning hours of August 21, 2007. (NRLMRY)

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