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Subtropical Storm Theta Forecast Discussion Number 1
2020-11-10 03:59:41| National Hurricane Center (Atlantic)
Issued at 300 AM GMT Tue Nov 10 2020 000 WTNT45 KNHC 100259 TCDAT5 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. 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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