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Tropical Storm THETA

Tropical Storm Theta Discussion Number   9
NWS National Hurricane Center Miami FL       AL302020
300 AM GMT Thu Nov 12 2020

The cloud appearance of Theta briefly became a bit more symmetric 
earlier tonight, though the colder cloud tops that attempted to wrap 
around the low-level center have recently decayed, possibly due to 
entrainment of dry mid-latitude air. A 2243 UTC ASCAT-B pass 
and a subsequent ASCAT-C pass both found the max winds with Theta 
a little stronger than previously estimated with maximum values 
exceeding 50 kt. These data support an initial intensity of 55 kt, 
and this could even be a little conservative.

Over the last 6 h Theta appears to have made a slight jog left of 
the previous forecast track, perhaps related to the convection 
wrapping around the center, but the longer term motion is still 
east-northeast at about 10 kt. Over the next day, Theta is expected 
to continue to move east-northeastward as it rounds the north side 
of a mid-level ridge centered over the Cape Verde islands. Between 
48 h to 72 h Theta's track is forecast to bend east and then 
east-southeast as the mid-level ridge to the south weakens a bit, 
and mid to upper-level northerly flow from a weak mid-level ridge 
attempts to build in to the west. The latest track guidance shows a 
fair amount of spread in how quickly Theta makes this bend to the 
east-southeast, and this is a large source of uncertainty in the 
forecast. The NHC track forecast splits the difference between the 
faster ECMWF and slower HMON/HWRF guidance, and is not far from the 
TVCN consensus.

The intensity forecast of Theta has been adjusted upward for the 
first 48 h due to the higher initial intensity. GFS-SHIPS 
diagnostics indicate the vertical wind shear will gradually 
decrease while Theta moves through an unstable temperature profile 
conducive for moderate to deep convection. Thereafter, the 
aforementioned upper-level northerly flow will lead to an increase 
in the vertical wind shear and import very stable mid-latitude air 
over Theta’s circulation. Most of the reliable global model 
guidance shows Theta succumbing to these negative effects and 
becoming a remnant low by 96 h. The official intensity forecast 
shows the same, and is on the high side of the guidance but not far 
from any of the models since the spread is quite low.
INIT  12/0300Z 31.1N  31.4W   55 KT  65 MPH
 12H  12/1200Z 31.5N  29.8W   55 KT  65 MPH
 24H  13/0000Z 32.0N  27.8W   55 KT  65 MPH
 36H  13/1200Z 32.1N  25.4W   55 KT  65 MPH
 48H  14/0000Z 31.8N  23.4W   50 KT  60 MPH
 60H  14/1200Z 31.4N  21.9W   45 KT  50 MPH
 72H  15/0000Z 31.2N  20.5W   40 KT  45 MPH
 96H  16/0000Z 31.2N  19.8W   30 KT  35 MPH...POST-TROP/REMNT LOW
120H  17/0000Z 35.6N  18.9W   30 KT  35 MPH...POST-TROP/REMNT LOW
Forecaster Papin/Zelinsky