[Tropical] Tropical Cyclone Discussion

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WTNT45 KNHC 110247
TCDAT5

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.

FORECAST POSITIONS AND MAX WINDS

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
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