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Tropical Depression Sixteen Discussion Number   1
NWS National Hurricane Center Miami FL       AL162021
1100 PM AST Sat Sep 18 2021
 
Satellite images and surface observations indicate that the area of 
low pressure that NHC has been monitoring to the east of the 
northern Leeward Islands now has a well-defined center and 
sufficiently organized deep convection to be classified a tropical 
depression.  The depression has a small area of deep convection near 
its center and curved bands on its north and east sides.  The 
initial intensity is estimated to be 30 kt, but this is a little 
below the latest Dvorak classifications and could be conservative.

The depression is moving west-northwestward at 12 kt.  The cyclone 
should continue west-northwestward at about the same pace during the 
next few days as it moves in the flow on the southwest side of a 
mid-level ridge.  After that time, the ridge is expected to break 
down and the steering currents are forecast to become weak.  Not 
surprisingly, the spread in the models increases around that time, 
but most of the solutions show a slow turn toward the north toward 
broad troughing associated with Post-Tropical Cyclone Odette.  The 
GFS is on the southern side of the guidance envelope and the ECMWF 
on the northern side. The NHC track forecast lies between those 
models near the consensus aids.

Some strengthening seems possible during the next 12 to 24 hours and 
the system is expected to become a tropical storm during that time 
frame.  However, an increase in southerly and then southwesterly 
vertical wind shear should cause the system to level off in strength 
from 24 hours through the end of the forecast period.  The models 
are in fairly good agreement on this overall scenario, and the NHC 
intensity forecast is in line with the majority of the guidance. 

Based on the track, intensity, and wind radii forecast, no tropical 
storm watches or warnings are required for the northern Leeward 
Islands, Virgin Islands or Puerto Rico.  However, interests there 
should monitor the progress of the system as locally heavy rain is 
possible on Monday and Tuesday when it is expected to pass to the 
north of the area.


FORECAST POSITIONS AND MAX WINDS
 
INIT  19/0300Z 16.4N  53.1W   30 KT  35 MPH
 12H  19/1200Z 17.3N  54.8W   35 KT  40 MPH
 24H  20/0000Z 18.5N  57.3W   40 KT  45 MPH
 36H  20/1200Z 19.4N  59.8W   40 KT  45 MPH
 48H  21/0000Z 20.3N  61.9W   35 KT  40 MPH
 60H  21/1200Z 21.3N  64.1W   35 KT  40 MPH
 72H  22/0000Z 22.3N  65.5W   35 KT  40 MPH
 96H  23/0000Z 24.4N  66.5W   35 KT  40 MPH
120H  24/0000Z 26.3N  66.2W   35 KT  40 MPH
 
$$
Forecaster Cangialosi
 
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