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Tropical Storm Theta Discussion Number  18
NWS National Hurricane Center Miami FL       AL302020
900 AM GMT Sat Nov 14 2020
 
Nearly all of the deep convection associated with Theta has 
dissipated since the previous advisory, with only cold, high cirrus 
clouds remaining in the northeastern quadrant. A late-arriving 
13/2255Z partial ASCAT-C pass indicated a few 35-kt wind vectors 
were present in the southeastern quadrant, so the initial intensity 
has been lowered to 35 kt for this advisory, which is consistent 
the latest TAFB current intensity estimate of T2.5/35 kt.

The initial motion estimate is due east, or 090/08 kt. Theta is 
expected to move eastward along the northern periphery of a 
deep-layer ridge for the next 24 hours or so, a motion that will 
also be accompanied by a gradual decrease in forward speed. By 36 
hours, a much weaker and more vertically shallow Theta is forecast 
to turn sharply northward ahead of a frontal system. In the 48-72 
hour period, the weakening cyclone expected to accelerate 
northeastward ahead of the front, passing to the  north of the 
Canary Islands this weekend. Theta is forecast to dissipate by 96 
hours near the Madeira Island. The new NHC track forecast is similar 
to the previous advisory, and lies close to the consensus model 
TVCN.
 
Strong shear in excess of 35 kt currently affecting Theta is 
forecast to steadily increase to more than 40 kt by 12 h and beyond. 
In addition, entrainment of very dry mid-level air should combine 
with the increasing shear and prevent the reformation of persistent 
organized deep convection near the center. Although the global 
models show that some marginal instability is expected to linger 
through the forecast period, only intermittent small bursts of 
modest convection away from the center could develop. However, the 
general trend in the model guidance calls for Theta to slowly spin 
down, and the official intensity forecast follows that scenario, 
calling for the cyclone to weaken to a remnant low later today, then 
becoming extratropical before dissipating in 3-4 days.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  14/0900Z 32.0N  20.8W   35 KT  40 MPH
 12H  14/1800Z 31.8N  19.5W   30 KT  35 MPH...POST-TROP/REMNT LOW
 24H  15/0600Z 31.8N  18.3W   30 KT  35 MPH...POST-TROP/EXTRATROP
 36H  15/1800Z 32.2N  17.7W   25 KT  30 MPH...POST-TROP/EXTRATROP
 48H  16/0600Z 33.2N  17.4W   20 KT  25 MPH...POST-TROP/EXTRATROP
 60H  16/1800Z 34.9N  16.6W   20 KT  25 MPH...POST-TROP/EXTRATROP
 72H  17/0600Z 37.2N  15.6W   20 KT  25 MPH...POST-TROP/EXTRATROP
 96H  18/0600Z...DISSIPATED
 
$$
Forecaster Stewart
 
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