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Tropical Storm THETA

Tropical Storm Theta Discussion Number   5
NWS National Hurricane Center Miami FL       AL302020
300 AM GMT Wed Nov 11 2020

Convection with Theta has waned some since the previous advisory, 
with the coldest cloud tops now focused in a band on the 
northeastern side of the partially exposed circulation. Despite this 
decrease in convective coverage, a 2304 UTC ASCAT-B pass showed a 
large region of 50 kt winds with at least one 55 kt barb. 
Accounting for a bit of undersampling of this instrument and 
maintaining continuity with the previous advisory, the maximum 
sustained winds have been kept at 60 kt for this advisory.

Theta has slowed a bit tonight on the same east-northeast heading at 
075/10 kt. The steering philosophy in the first part of the forecast 
has remained the same, with Theta moving off to the east-northeast 
while it continues to round the northern edge of a mid-level ridge 
providing the steering flow.  The latest track guidance has slowed 
down the forward motion a bit more this cycle, and the new track 
forecast splits the difference between the previous forecast and 
some of the more reliable global model guidance that is slower and 
south of the NHC track. At the end of the forecast period both the 
ECMWF and GFS are forecasting Theta to become a shallow vortex as 
the remaining convection is stripped away, and this could cause 
Theta to slow down in the weaker low-level flow and move more 
slowly to the northeast at the end of the forecast period.

Despite moving over gradually decreasing sea surface temperatures 
over the next 48-72 h, upper-level temperatures are also forecast to 
cool per GFS-SHIPS, which should provide enough instability for 
moderate to deep convection. In addition, vertical wind shear may 
decrease some in the 48-72 h period as Theta remains in light flow 
along an upper-level trough axis. Therefore, the latest NHC  
intensity forecast remains on the high end of the guidance for the 
first three days, but now shows a faster weakening trend thereafter 
as vertical wind shear increases out of the north and strips the 
remaining convection away. At 5 days the forecast now makes Theta a 
remnant low since it appears the circulation will be too shallow to 
take advantage of more favorable baroclinic conditions that would 
ordinarily allow for extratropical transition.

INIT  11/0300Z 29.4N  35.5W   60 KT  70 MPH
 12H  11/1200Z 29.7N  34.2W   60 KT  70 MPH
 24H  12/0000Z 30.1N  32.5W   55 KT  65 MPH
 36H  12/1200Z 30.5N  30.4W   55 KT  65 MPH
 48H  13/0000Z 31.0N  27.7W   55 KT  65 MPH
 60H  13/1200Z 31.7N  25.2W   55 KT  65 MPH
 72H  14/0000Z 32.2N  23.2W   55 KT  65 MPH
 96H  15/0000Z 32.8N  21.4W   50 KT  60 MPH
120H  16/0000Z 34.5N  20.1W   40 KT  45 MPH...POST-TROP/REMNT LOW
Forecaster Papin/Brown