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Tropical Depression Thirteen Discussion Number   2
NWS National Hurricane Center Miami FL       AL132020
500 AM AST Thu Aug 20 2020

There has been little change in the organization of Tropical 
Depression Thirteen during the past several hours.  An area of 
ragged but persistent convection continues near the estimated 
center position, and there is some outer banding in the southern 
semicircle.  Dvorak-based satellite intensity estimates have 
changed little since the last advisory, so the initial intensity 
remains 30 kt.

The initial motion is a somewhat uncertain 295/18.  The subtropical 
ridge to the north of the cyclone is expected to build westward to 
the north of the Greater Antilles during the next several days, and 
this should steer the cyclone generally west-northwestward through 
the forecast period.  The track guidance remains in fairly good 
agreement with this scenario, but there remains a spread between 
the GFS/UKMET on the south side of the guidance and the 
Canadian/HMON on the north side.  Overall, the envelope has shifted 
a little to the south since the previous advisory and the new 
forecast track, which lies just to the north of the consensus 
models, is also nudged southward.  The new track calls for the 
cyclone to pass near the Leeward Islands in 36-48 h, near or north 
of Puerto Rico and the Virgin Islands in 48-60 h, and then near or 
over Hispaniola and the southeastern Bahamas by 72 h.

Overall, the environment looks generally favorable for 
strengthening, with the cyclone expected to encounter light- to 
moderate shear during forecast period.  However, the guidance 
responds to this environment with a wide range of solutions.  The 
HWRF/HMON forecast the cyclone to intensify into a major hurricane 
by 120 h.  On the other hand, the GFS and ECMWF show the system 
degenerating into an open wave by 120 h.  The UKMET and Canadian 
models are between these extremes.  The weak GFS solution appears 
to be due to forecast dry air entrainment, which is a possibility 
as satellite imagery suggests Saharan dust not far from the 
cyclone.  Between these factors and the possibility of land 
interaction, the low-confidence intensity forecast is changed 
little from the previous advisory.
 
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 later today.
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  20/0900Z 15.2N  49.8W   30 KT  35 MPH
 12H  20/1800Z 16.4N  52.7W   30 KT  35 MPH
 24H  21/0600Z 17.5N  56.4W   35 KT  40 MPH
 36H  21/1800Z 18.4N  60.2W   40 KT  45 MPH
 48H  22/0600Z 19.1N  63.9W   45 KT  50 MPH
 60H  22/1800Z 19.8N  67.1W   50 KT  60 MPH
 72H  23/0600Z 20.8N  71.0W   55 KT  65 MPH
 96H  24/0600Z 23.0N  78.0W   60 KT  70 MPH
120H  25/0600Z 26.0N  83.0W   60 KT  70 MPH
 
$$
Forecaster Beven
 
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