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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
 
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