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Tropical Depression Thirteen Discussion Number   1
NWS National Hurricane Center Miami FL       AL132020
1100 PM AST Wed Aug 19 2020
 
The low pressure system that NHC has been monitoring over the 
central tropical Atlantic has now developed a well-defined center of 
circulation and maintained enough organized deep convection to be 
classified as a tropical depression, the thirteenth cyclone of the 
2020 Atlantic hurricane season.  The depression already has some 
banding features on its north and west sides as evident in 
geostationary satellite and microwave images. The initial intensity 
is set at 30 kt based on ASCAT data from around 0100 UTC and a 
T2.0/30 kt Dvorak classifications from TAFB.
 
The depression has been moving fairly quickly to the west-northwest,
with the latest initial motion estimated to be 295/17 kt.  It should
be noted that the initial motion is somewhat uncertain given that
the system has only recently formed.  A subtropical ridge currently
over the central Atlantic is expected to build westward during the
next several days and should be the primary steering feature for
the depression through the forecast period.  This pattern should
keep the depression on a fairly quick west-northwest track during
the next several days, taking the cyclone near the northern 
Leeward Islands by Friday night and near the Greater Antilles 
and southeastern Bahamas this weekend.  The models are in fairly 
good agreement, but there is some north-south spread with the GFS 
being on the southern side of the guidance envelope and the ECMWF 
on the northern end.  The NHC track forecast lies down the middle 
of the guidance suite.
 
The environmental conditions appear generally favorable for the
depression to strengthen, with the wind shear expected to remain
relatively low while the system moves over warm SSTs and remains in
a moist airmass.  These conditions should promote gradual
strengthening and it seems quite likely that the cyclone will be a
tropical storm when it moves near or north of the northern Leeward 
Islands in a couple of days.  The bigger question is how 
much interaction will there be with the Greater Antilles.  If the 
depression moves on the south side of the guidance envelope, 
further strengthening would be limited due to land interaction.  
Conversely, if the system gains more latitude and moves north of 
these highly topographic islands, it could have the opportunity for 
more significant intensification. The NHC intensity forecast, which 
is of low confidence, is roughly near the middle of the guidance 
envelope.
 
Key Messages:
 
1. Tropical storm conditions are possible across portions of the
northern Leeward Islands by Friday night, and Tropical Storm Watches
have been issued for some of these islands.  Heavy rainfall is 
likely across this area beginning late Friday.
 
2.  There is a risk of tropical storm conditions in the Virgin
Islands and Puerto Rico Friday night and Saturday and Tropical 
Storm Watches could be required for these islands tomorrow.  
Interests there should closely monitor the progress of this system.
 
3. 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 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  20/0300Z 14.6N  47.9W   30 KT  35 MPH
 12H  20/1200Z 16.1N  50.8W   30 KT  35 MPH
 24H  21/0000Z 17.4N  54.6W   35 KT  40 MPH
 36H  21/1200Z 18.4N  58.4W   40 KT  45 MPH
 48H  22/0000Z 19.1N  62.1W   45 KT  50 MPH
 60H  22/1200Z 19.9N  65.9W   50 KT  60 MPH
 72H  23/0000Z 20.8N  69.7W   55 KT  65 MPH
 96H  24/0000Z 22.9N  76.7W   60 KT  70 MPH
120H  25/0000Z 26.2N  82.2W   60 KT  70 MPH
 
$$
Forecaster Cangialosi
 
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