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.
