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Tropical Storm Theta Forecast Discussion Number 5
2020-11-11 03:47:52| National Hurricane Center (Atlantic)
Issued at 300 AM GMT Wed Nov 11 2020 000 WTNT45 KNHC 110247 TCDAT5 Tropical Storm Theta Discussion Number 5 NWS National Hurricane Center Miami FL AL302020 300 AM GMT Wed Nov 11 2020 Convection with Theta has waned some since the previous advisory, with the coldest cloud tops now focused in a band on the northeastern side of the partially exposed circulation. Despite this decrease in convective coverage, a 2304 UTC ASCAT-B pass showed a large region of 50 kt winds with at least one 55 kt barb. Accounting for a bit of undersampling of this instrument and maintaining continuity with the previous advisory, the maximum sustained winds have been kept at 60 kt for this advisory. Theta has slowed a bit tonight on the same east-northeast heading at 075/10 kt. The steering philosophy in the first part of the forecast has remained the same, with Theta moving off to the east-northeast while it continues to round the northern edge of a mid-level ridge providing the steering flow. The latest track guidance has slowed down the forward motion a bit more this cycle, and the new track forecast splits the difference between the previous forecast and some of the more reliable global model guidance that is slower and south of the NHC track. At the end of the forecast period both the ECMWF and GFS are forecasting Theta to become a shallow vortex as the remaining convection is stripped away, and this could cause Theta to slow down in the weaker low-level flow and move more slowly to the northeast at the end of the forecast period. Despite moving over gradually decreasing sea surface temperatures over the next 48-72 h, upper-level temperatures are also forecast to cool per GFS-SHIPS, which should provide enough instability for moderate to deep convection. In addition, vertical wind shear may decrease some in the 48-72 h period as Theta remains in light flow along an upper-level trough axis. Therefore, the latest NHC intensity forecast remains on the high end of the guidance for the first three days, but now shows a faster weakening trend thereafter as vertical wind shear increases out of the north and strips the remaining convection away. At 5 days the forecast now makes Theta a remnant low since it appears the circulation will be too shallow to take advantage of more favorable baroclinic conditions that would ordinarily allow for extratropical transition. FORECAST POSITIONS AND MAX WINDS INIT 11/0300Z 29.4N 35.5W 60 KT 70 MPH 12H 11/1200Z 29.7N 34.2W 60 KT 70 MPH 24H 12/0000Z 30.1N 32.5W 55 KT 65 MPH 36H 12/1200Z 30.5N 30.4W 55 KT 65 MPH 48H 13/0000Z 31.0N 27.7W 55 KT 65 MPH 60H 13/1200Z 31.7N 25.2W 55 KT 65 MPH 72H 14/0000Z 32.2N 23.2W 55 KT 65 MPH 96H 15/0000Z 32.8N 21.4W 50 KT 60 MPH 120H 16/0000Z 34.5N 20.1W 40 KT 45 MPH...POST-TROP/REMNT LOW $$ Forecaster Papin/Brown
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