Tropical Storm THETA Advisory mar, 10-11

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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.

Theta’s 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.


FORECAST POSITIONS AND MAX WINDS

INIT  10/0300Z 28.8N  40.3W   45 KT  50 MPH...SUBTROPICAL STORM
 12H  10/1200Z 29.0N  38.6W   50 KT  60 MPH...SUBTROPICAL STORM
 24H  11/0000Z 29.1N  36.3W   50 KT  60 MPH...TROPICAL STORM
 36H  11/1200Z 29.5N  34.3W   50 KT  60 MPH
 48H  12/0000Z 30.0N  32.6W   50 KT  60 MPH
 60H  12/1200Z 30.5N  30.7W   50 KT  60 MPH
 72H  13/0000Z 31.0N  28.7W   50 KT  60 MPH
 96H  14/0000Z 32.2N  24.3W   50 KT  60 MPH
120H  15/0000Z 33.3N  20.6W   50 KT  60 MPH

$$
Forecaster Papin/Brown