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Hurricane Helene Forecast Discussion Number 13
2018-09-10 16:36:25| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Mon Sep 10 2018 274 WTNT43 KNHC 101436 TCDAT3 Hurricane Helene Discussion Number 13 NWS National Hurricane Center Miami FL AL082018 1100 AM AST Mon Sep 10 2018 Helene's cloud pattern has become much better organized during the past few hours, with a clear 20 n mi wide eye seen in Meteosat infrared imagery, and a clear eye noted in a 1135 UTC AMSU overpass. A Dvorak classifications from TAFB and SAB were both T5.0/90 knots at 1200 UTC, and that is the initial intensity for this advisory. Helene will remain in an environment that should support some additional strengthening for the next 12 hours or so, with very low shear and SSTs of 27C or above. After that time, the SSTs cool quickly and the shear increases dramatically by 36 hours, which should induce steady weakening. Late in the period, the shear continues, and while SSTs increase the atmosphere dries out, and the intensity is held steady at day 5. The new NHC intensity forecast is above most of the guidance through 24 hours to account for the recent intensification of the storm, but then is close to or slightly above HCCA and IVCN. The initial motion estimate is 285/14. The subtropical ridge north of Helene will gradually weaken as an upper-level trough digs southward around 40W. This pattern will cause Helene to slow down and gradually recurve during the forecast period, with a faster northward motion expected at days 4 and 5 as Helene interacts with the aforementioned upper-level trough. While there is significant variability between the various global models in the exact evolution of the upper-level pattern, the track model guidance is in generally good agreement on this evolution, although the UKMET, its ensemble mean and the GEFS mean are notable outliers to the right. The new NHC track was not changed much from the previous one and lies near the middle of the guidance envelope and is close to HCCA. FORECAST POSITIONS AND MAX WINDS INIT 10/1500Z 14.6N 30.0W 90 KT 105 MPH 12H 11/0000Z 15.2N 32.0W 100 KT 115 MPH 24H 11/1200Z 15.9N 34.3W 100 KT 115 MPH 36H 12/0000Z 16.8N 36.1W 90 KT 105 MPH 48H 12/1200Z 18.0N 37.7W 75 KT 85 MPH 72H 13/1200Z 21.5N 39.5W 60 KT 70 MPH 96H 14/1200Z 27.0N 40.5W 50 KT 60 MPH 120H 15/1200Z 32.5N 40.5W 50 KT 60 MPH $$ Forecaster Brennan
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