Atlantic Basin Tropical Cyclones

Tropical Storm Peter Forecast Discussion


588 
WTNT41 KNHC 210835
TCDAT1

Tropical Storm Peter Discussion Number  11
NWS National Hurricane Center Miami FL       AL162021
500 AM AST Tue Sep 21 2021

Peter is a ragged and strongly sheared tropical storm.  Satellite 
images and surface observations indicate that the low-level center 
is fully exposed and located about 100 miles west-southwest of the 
edge of the main area of deep convection.  This very asymmetric 
structure is due to strong west-southwesterly shear associated with 
an upper-level low to the northwest of Peter.  The initial intensity 
is held at 45 kt based on the Dvorak CI-numbers from TAFB and SAB, 
but this could be a little generous.  An Air Force Hurricane Hunter 
aircraft is scheduled to investigate the storm later this morning.

Since the storm has decoupled, the low-level center has been moving 
just south of due west at 265/10 kt.  This motion has brought the 
storm to the southwest of the previous track.  The models insist 
that Peter will turn back to the west-northwest soon and continue 
moving in that direction during the next couple of days as the storm 
moves in the flow on the southwest side of a mid-level ridge.  After 
that time, a turn to the north and then northeast is expected as 
Peter moves toward broad troughing over the north Atlantic.  The 
latest run of the ECMWF has come into line with the remainder of the 
guidance, and the new forecast is to the west of, and slower than, 
the previous one.  This prediction lies on the south side of the 
guidance envelope during the first 24 hours of the forecast.

Peter is expected to remain in fairly hostile conditions during the 
next few days with strong westerly shear continuing and dry air 
likely entraining into the circulation.  These negative factors for 
the storm should cause a slow decay, and that is reflected in the 
NHC forecast.  In fact, some of the guidance suggests that Peter 
could succumb to the hostile conditions and open into a trough later 
this week.  The NHC intensity forecast is the same as the previous 
one and in line with the majority of the guidance.

Key Messages:

1. Rainfall around the southern periphery of Tropical Storm Peter 
could lead to areas of urban and small stream flooding through 
Tuesday across northern Hispaniola, Puerto Rico, portions of the 
Virgin Islands, and the Northern Leeward Islands.


FORECAST POSITIONS AND MAX WINDS

INIT  21/0900Z 19.6N  63.8W   45 KT  50 MPH
 12H  21/1800Z 20.3N  65.3W   40 KT  45 MPH
 24H  22/0600Z 21.2N  66.9W   40 KT  45 MPH
 36H  22/1800Z 22.2N  67.9W   35 KT  40 MPH
 48H  23/0600Z 23.2N  68.2W   35 KT  40 MPH
 60H  23/1800Z 24.0N  68.0W   30 KT  35 MPH
 72H  24/0600Z 25.2N  67.2W   30 KT  35 MPH
 96H  25/0600Z 27.2N  65.8W   30 KT  35 MPH
120H  26/0600Z 29.2N  64.2W   30 KT  35 MPH

$$
Forecaster Cangialosi




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