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Tropical Depression Thirteen Discussion Number   4
NWS National Hurricane Center Miami FL       AL132020
500 PM AST Thu Aug 20 2020
 
Recent late afternoon visible satellite imagery has shown evidence 
of a low-level swirl that is racing away from the persistent area 
of deep convection.  It is unclear if that is the only center of 
circulation or a swirl rotating around the broader circulation.  
The initial position for this advisory is a compromise between the 
earlier estimated center location and the swirl seen in satellite 
imagery.  The initial intensity has been held at 30 kt, based on 
the earlier ASCAT data.
 
The initial motion estimate is a somewhat uncertain 290/18 kt. The 
cyclone is located south of a subtropical ridge that is forecast to 
build westward over the western Atlantic through early next week.  
This ridge should steer the system west-northwestward over the next 
several days, and the suite of dynamical track models continue to be 
agreement on this overall scenario.  The lastest NHC track foreast 
is similar to the previous advisory and again lies between the HFIP 
corrected consensus and the TVCA multi-model consensus.  It should 
be noted that since the system is still lacking in organization, 
there could be some center reformations that result in some shifts 
in the track forecast.
 
Although the depression has changed little in strength since it
formed yesterday, the overall environment ahead of the system
favors gradual strengthening.  The cyclone is forecast to remain
over warm water and in an area of light to moderate vertical wind
shear.  Most of the guidance suggests a little more favorable
upper-level wind pattern once the system is north of the Greater
Antilles, but a track farther south would result in more land
interaction, which increases the intensity forecast uncertainty.
The updated NHC intensity forecast shows a slightly slower rate of 
strengthening over the next couple of days, but is unchanged at the 
latter periods.  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.

FORECAST POSITIONS AND MAX WINDS
 
INIT  20/2100Z 16.7N  53.9W   30 KT  35 MPH
 12H  21/0600Z 17.5N  57.2W   30 KT  35 MPH
 24H  21/1800Z 18.3N  60.5W   35 KT  40 MPH
 36H  22/0600Z 19.1N  63.5W   40 KT  45 MPH
 48H  22/1800Z 20.0N  66.7W   45 KT  50 MPH
 60H  23/0600Z 20.9N  70.4W   50 KT  60 MPH
 72H  23/1800Z 22.2N  74.1W   60 KT  70 MPH
 96H  24/1800Z 25.1N  80.4W   65 KT  75 MPH
120H  25/1800Z 28.3N  84.2W   65 KT  75 MPH
 
$$
Forecaster Brown
 
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