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Tropical Depression Eleven Discussion Number   1
NWS National Hurricane Center Miami FL       AL112020
500 PM AST Tue Aug 11 2020
 
Visible satellite imagery and ASCAT-C data from earlier in the day
have shown that the area of low pressure NHC has been monitoring
over the tropical Atlantic has developed a less-elongated
circulation with a well-defined center.  For the most part, deep
convection has persisted with the system since about this time
yesterday, save a brief period of warming cloud tops this morning.
The low now meets the criteria of a tropical cyclone, and
advisories have been initiated on Tropical Depression Eleven with
30-kt winds, in line with the latest Dvorak Current Intensity
numbers from TAFB and SAB.
 
The depression is moving westward, or 280/14 kt, to the south of a
large mid-tropospheric high centered over the central Atlantic.
This pattern is expected to evolve rather quickly, with a break
developing in the ridge over the central Atlantic by 48 hours.
This change should allow the depression to begin making more
poleward progress, moving west-northwestward from 36 hours until
the end of the forecast period.  The track models are in good
agreement on this scenario, as well as the system's forward speed,
and bring the center of the cyclone near or just to the north of
the northern Leeward Islands in 4-5 days.  This first NHC forecast
lies just to the north of the multi-model consensus cluster
through day 3, out of respect for the northern-lying ECMWF model,
and then is close to HCCA on days 4 and 5.
 
Conventional satellite imagery and Saharan Air Layer analyses
suggest that the center of the depression is being shielded from
much drier air to its north and west.  However, as has been the
case for a few days, at least 15 kt of easterly shear has been
pushing deep convection to the western side of the circulation.
This shear is expected to decrease over the next day or two, which
should allow for gradual strengthening to begin by 36 hours, and a
peak in the cyclone's intensity should occur in about 3 days.  For
this period, the NHC intensity forecast is a little above HCCA and
the IVCN intensity consensus.  After that time, westerly or
southwesterly shear is forecast to develop and increase to 20-30 kt
by days 4 and 5, which is likely to induce significant weakening.
In fact, it's notable that the conditions become hostile enough that
the global models are showing the system opening up into a trough
near the northern Leeward Islands by day 5, which is a plausible
alternate scenario.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  11/2100Z 11.7N  40.0W   30 KT  35 MPH
 12H  12/0600Z 12.0N  42.1W   30 KT  35 MPH
 24H  12/1800Z 12.4N  44.5W   30 KT  35 MPH
 36H  13/0600Z 13.0N  46.9W   35 KT  40 MPH
 48H  13/1800Z 13.9N  49.4W   40 KT  45 MPH
 60H  14/0600Z 15.1N  51.8W   45 KT  50 MPH
 72H  14/1800Z 16.4N  54.3W   45 KT  50 MPH
 96H  15/1800Z 18.5N  59.5W   40 KT  45 MPH
120H  16/1800Z 21.0N  64.0W   35 KT  40 MPH
 
$$
Forecaster Berg
 
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