Tropical Storm SAM Advisory do, 23-09

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Tropical Depression Eighteen Discussion Number   2
NWS National Hurricane Center Miami FL       AL182021
1100 PM AST Wed Sep 22 2021

The depression's organization has not improved since earlier this
afternoon.  An analysis of the ambiguities from 0000 UTC ASCAT-B
data suggests that the surface circulation is just barely closed,
with the center still attached to the northern end of a trough.
Peak wind retrievals from the scatterometer pass were 25-30 kt, so
the initial intensity remains 30 kt.

Based on the ASCAT data, the depression appears to be moving just
north of due west, or 275/13 kt.  Low- to mid-level ridging to the
north is expected to maintain the cyclone on a westward heading for
the next 36 hours, followed by a path toward the west-northwest
from day 2 through day 5.  The more notable part of the forecast is
the forward speed.  Global models are showing anomalously strong
500-mb ridging developing over the eastern and central Caribbean
Sea in 2 to 3 days, which is likely to block the depression's
forward progress toward the end of the forecast period.  In fact,
the current NHC forecast has the system moving west-northwestward
at only 7 or 8 kt well east of the Lesser Antilles on days 3 through
5.  All this means that it will probably be a few more days before
we have a better idea if and how the system might potentially
threaten areas farther west.  For the 5-day forecast period, there
is a normal amount of spread among the track guidance, and the
updated NHC forecast is very close to a blend of the TVCA and HCCA
consensus aids.  This solution is not too different from the
previous forecast.

Overall, an environment of low shear, warm sea surface
temperatures, and a moist, unstable atmosphere appears conducive
for strengthening.  However, there is some sort of signal being
conveyed by the global models (e.g., the GFS, ECMWF, and UKMET)
whereby the cyclone's surface circulation does not really tighten
up for another 24 to 48 hours.  Given the system's current
structure, this scenario seems plausible.  Therefore, the updated
NHC intensity forecast shows a slower rate of strengthening during
the first 48 hours, just a bit below the intensity consensus in
deference to the global model solutions.  After 48 hours, the
official forecast converges on top of the previous NHC forecast and
still brings the cyclone to hurricane, and then possibly major
hurricane, strength by the end of the forecast period.


FORECAST POSITIONS AND MAX WINDS

INIT  23/0300Z 10.2N  35.0W   30 KT  35 MPH
 12H  23/1200Z 10.4N  37.0W   30 KT  35 MPH
 24H  24/0000Z 10.9N  39.6W   35 KT  40 MPH
 36H  24/1200Z 11.4N  42.1W   45 KT  50 MPH
 48H  25/0000Z 12.0N  44.3W   55 KT  65 MPH
 60H  25/1200Z 12.6N  46.0W   70 KT  80 MPH
 72H  26/0000Z 13.2N  47.5W   80 KT  90 MPH
 96H  27/0000Z 14.5N  50.2W   90 KT 105 MPH
120H  28/0000Z 16.0N  53.3W  100 KT 115 MPH

$$
Forecaster Berg