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.
Subtropical Storm Theta Discussion Number 1
NWS National Hurricane Center Miami FL AL302020
300 AM GMT Tue Nov 10 2020
Over the past 12 h convection has slowly consolidated near the
center of a non-tropical area of low pressure over the far
northeast Atlantic. Earlier, there was a 2325 UTC ASCAT-B pass that
showed that the low-level circulation had become more detached
from the warm frontal boundary located to its northeast. In
addition, the ASCAT pass showed that winds have increased to near 45
kt on the southeast portion of the circulation near where coldest
convective cloud tops were located. Both a 2137 UTC SSMIS and 2324
UTC AMSU microwave pass showed convective banding under the cirrus
canopy near where the highest winds were observed. However, water
vapor satellite imagery still shows the presence of a broad
upper-level low tangled up with the low-level circulation embedded
in the convection. Therefore, Theta has been designated as a
subtropical storm with maximum wind of 45 kt.
Theta has been on a near due east heading over the last 12 h at
90/13 kt. Currently the cyclone is located south of a broad
deep-layer longwave trough. This trough is expected to bypass the
cyclone to the east and Theta will primarily be steered by
southwesterly flow on the northern periphery of a mid-level ridge
centered over the Cape Verde islands. This flow will steer Theta
slowly to the east-northeast over the next 3-5 days. The track
guidance is in good agreement with this solution, with only minor
differences in across-track spread. There are somewhat larger
differences in the along-track spread related to different forward
motion, but the latest NHC track forecast elected to stay close to
the track consensus at this time.
Thetas structure has evolved from a frontal cyclone to a
subtropical cyclone, though a frontal boundary remains nearby on the
northeast side of the circulation. This boundary is expected to
gradually dissipate and Theta is expected to fully transition to a
tropical storm in 24 h as convection erodes the upper-level cyclonic
flow overhead. The maximum sustained winds were increased slightly
in the first 12 h of the forecast but are kept at 50 kt through 120
h which is on the high end of the intensity guidance.
Theta is the 29th named storm in the record-breaking 2020 Atlantic
Hurricane Season, breaking the previous record of 28th named storms
in 2005.
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