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Tropical Depression Eighteen Forecast Discussion

WTNT43 KNHC 230238

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.  


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

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