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Tropical Storm Helene Forecast Discussion Number 25
2018-09-13 16:39:21| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Thu Sep 13 2018 266 WTNT43 KNHC 131439 TCDAT3 Tropical Storm Helene Discussion Number 25 NWS National Hurricane Center Miami FL AL082018 1100 AM AST Thu Sep 13 2018 Helene continues with very limited deep convection, only observed this morning in the northwestern quadrant. Dvorak estimates from TAFB and SAB are dropping, and a blend of these with the CIMSS SATCON indicates an intensity of 60 kt. Thus Helene has weakened to a tropical storm. Strong tropospheric vertical shear along with only lukewarm waters are likely contributing toward Helene's weakening. The shear should further increase during the next two days, though the sea surface temperatures will rebound up to 27C along with an increase in low-level moisture. A slow weakening is thus expected. Beginning in about three days, Helene will commence baroclinic transition, and it is expected to become an extratropical cyclone by 96 hours. This forcing should preclude any additional weakening through the remainder of the forecast period. The official intensity forecast is similar to the previous advisory. The prediction is based on a blend of the stronger HWRF dynamical model and the weaker SHIPS/LGEM statistical schemes for the next couple of days, and is based on a blend of the GFS/ECMWF/UKMET global models at the extended lead times. Helene is moving northward at about 12 kt between the subtropical ridge to its east and a deep-layer trough to its west. The system should accelerate and turn toward the northeast over the next few days as it rounds the subtropical ridge and starts getting picked up by the mid-latitude westerlies. The official track forecast is based upon the variable consensus technique (TVCN) and is just north of the previous forecast through three days. The guidance suite remains tightly clustered and indicates that Helene will pass near the Azores in 2 or 3 days. Interests in those islands should closely monitor the progress of Helene. FORECAST POSITIONS AND MAX WINDS INIT 13/1500Z 24.8N 37.3W 60 KT 70 MPH 12H 14/0000Z 27.2N 37.1W 55 KT 65 MPH 24H 14/1200Z 30.8N 36.6W 50 KT 60 MPH 36H 15/0000Z 34.2N 35.5W 50 KT 60 MPH 48H 15/1200Z 37.2N 33.3W 50 KT 60 MPH 72H 16/1200Z 40.5N 26.0W 50 KT 60 MPH 96H 17/1200Z 44.0N 18.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 120H 18/1200Z 51.5N 9.0W 50 KT 60 MPH...POST-TROP/EXTRATROP $$ Forecaster Landsea
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