[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT45 KNHC 120258
TCDAT5

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.

FORECAST POSITIONS AND MAX WINDS

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

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