Tropical Depression THIRTEEN (Text)

Tropical Depression Thirteen Discussion Number   5
NWS National Hurricane Center Miami FL       AL132020
1100 PM AST Thu Aug 20 2020

A NOAA Hurricane Hunter aircraft has been flying all around the 
circulation of the depression this evening, providing valuable 
information on the low-level structure.  The strongest winds are 
primarily northeast of the center, with an elongated surface 
circulation and a mid-level swirl near the southern end. While the 
plane couldn't locate a definite center, there's enough uncertainty 
and curvature in the plane's wind field data to hold onto the 
system as 30-kt tropical depression for now.  Another mission should 
be in the area around 1200 UTC.  A late-arriving scatterometer pass 
confirms both the disorganization of the cyclone and the maximum 

The initial motion continues about the same as before, 290/19.  A 
strong subtropical ridge over the western Atlantic should remain 
north of the depression during the next few days, steering the 
cyclone at a fast pace to the west-northwest.  After the weekend, 
the ridge weakens some over the eastern Gulf of Mexico, leading to
depression probably turning more to the northwest.   The guidance 
has trended to the south and west after 24 hours tonight, led by 
the GFS and HWRF models.  I'm not inclined to make significant 
changes since the models are about to ingest the information from 
the reconnaissance mission, plus the initial disorganization of the 
center, but the new track is still adjusted slightly to the south 
and west, north of much of the guidance.

The depression should be moving through mixed conditions over the 
next few days. While the water is very warm, GFS forecasts show that 
there is significant mid-level shear that could continue to mix in 
nearby dry air toward the center.  This shear will probably relax 
late this weekend or early next week, but there is very poor 
agreement on how much shear remains and the timing of this event.  
In addition, a track any farther south would result in potentially 
mountainous land interaction, which also increases the intensity 
forecast uncertainty.  No significant changes were made to the 
previous wind speed forecast, and the overall confidence in both 
the track and intensity forecasts remain lower than normal.
Key Messages:
1. Tropical storm conditions are possible across portions of the
northern Leeward Islands, the Virgin Islands, and Puerto Rico
Friday night and Saturday, and Tropical Storm Watches have been
issued for some of these islands.  Heavy rainfall is likely across
this area beginning late Friday and could cause mudslides and flash
and urban flooding through Sunday.
2.  The details of the long-range track and intensity forecasts are
more uncertain than usual since the system could move over portions
of the Greater Antilles this weekend.  However, this system could
bring some storm surge, rainfall and wind impacts to portions of
Hispaniola, Cuba, the Bahamas, and Florida this weekend and early
next week. Interests there should monitor this system's progress
and updates to the forecast over the next few days.
INIT  21/0300Z 17.3N  56.4W   30 KT  35 MPH
 12H  21/1200Z 17.9N  59.1W   30 KT  35 MPH
 24H  22/0000Z 18.6N  62.4W   35 KT  40 MPH
 36H  22/1200Z 19.4N  65.7W   40 KT  45 MPH
 48H  23/0000Z 20.3N  69.2W   45 KT  50 MPH
 60H  23/1200Z 21.3N  72.8W   50 KT  60 MPH
 72H  24/0000Z 22.6N  76.4W   55 KT  65 MPH
 96H  25/0000Z 26.0N  82.5W   65 KT  75 MPH
120H  26/0000Z 29.0N  85.5W   65 KT  75 MPH
Forecaster Blake

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Page last modified: Thursday, 31-Dec-2020 12:09:37 UTC