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Tropical Storm Theta Forecast Discussion Number 18
2020-11-14 09:36:56| National Hurricane Center (Atlantic)
Issued at 900 AM GMT Sat Nov 14 2020 806 WTNT45 KNHC 140836 TCDAT5 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. FORECAST POSITIONS AND MAX WINDS INIT 14/0900Z 32.0N 20.8W 35 KT 40 MPH 12H 14/1800Z 31.8N 19.5W 30 KT 35 MPH...POST-TROP/REMNT LOW 24H 15/0600Z 31.8N 18.3W 30 KT 35 MPH...POST-TROP/EXTRATROP 36H 15/1800Z 32.2N 17.7W 25 KT 30 MPH...POST-TROP/EXTRATROP 48H 16/0600Z 33.2N 17.4W 20 KT 25 MPH...POST-TROP/EXTRATROP 60H 16/1800Z 34.9N 16.6W 20 KT 25 MPH...POST-TROP/EXTRATROP 72H 17/0600Z 37.2N 15.6W 20 KT 25 MPH...POST-TROP/EXTRATROP 96H 18/0600Z...DISSIPATED $$ Forecaster Stewart
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