Atlantic Basin Tropical Cyclones

Tropical Depression Eighteen Forecast Discussion

WTNT43 KNHC 230836

Tropical Depression Eighteen Discussion Number   3
NWS National Hurricane Center Miami FL       AL182021
500 AM AST Thu Sep 23 2021

Satellite images indicate that the tropical depression has been
more or less steady in strength overnight.  Curved bands remain
most organized to the north and west of the center with some dry
air wrapping into the eastern half of the circulation.  The initial
intensity is held at 30 kt as a compromise between the earlier
ASCAT data that showed peak winds around 25 kt and 2.5/35 kt Dvorak
classifications from TAFB and SAB.

The initial motion is estimated to be 280/13 kt.  The cyclone is
located on the south side of a mid-level ridge, and that should
continue to steer the system westward but at a slightly slower pace
during the next couple of days.  After that time, the ridge is
expected to weaken and shift eastward. In response, the depression
is forecast to turn west-northwestward to northwestward and slow
down some more.  The new NHC track forecast is largely an update of
the previous one and lies near the middle of the guidance envelope,
near the consensus aids.  This forecast is also between the GFS
model on the north side of the guidance and the ECMWF on the south

Environmental conditions all seem to be favorable for the storm to
gain strength during the next several days.  The cyclone is expected
to traverse warm 28 deg C waters and move in an environment of low
wind shear (less than 10 kt) and fairly high moisture. Nearly all
of the models respond by showing steady strengthening during the
next several days, and so does the official forecast.  This
prediction lies near the IVCN and HCCA aids, which are usually the
most skillful.  The official forecast shows the system becoming a
tropical storm later today, a hurricane in a couple of days, and a
major hurricane by the end of the forecast period.  In fact, some of
the models suggest that the storm could get even stronger than
currently forecast.


INIT  23/0900Z 10.5N  36.4W   30 KT  35 MPH
 12H  23/1800Z 10.8N  38.4W   35 KT  40 MPH
 24H  24/0600Z 11.3N  40.9W   40 KT  45 MPH
 36H  24/1800Z 11.9N  43.3W   50 KT  60 MPH
 48H  25/0600Z 12.5N  45.2W   60 KT  70 MPH
 60H  25/1800Z 13.0N  46.9W   70 KT  80 MPH
 72H  26/0600Z 13.7N  48.3W   80 KT  90 MPH
 96H  27/0600Z 15.0N  50.8W   90 KT 105 MPH
120H  28/0600Z 16.9N  53.9W  100 KT 115 MPH

Forecaster Cangialosi

Source link