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Hurricane Helene Forecast Discussion Number 11
2018-09-10 04:37:50| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Sun Sep 09 2018 951 WTNT43 KNHC 100237 TCDAT3 Hurricane Helene Discussion Number 11 NWS National Hurricane Center Miami FL AL082018 1100 PM AST Sun Sep 09 2018 Satellite images indicate that Helene continues to strengthen. During the past several hours, a large band of deep convection has completely wrapped around the center resulting in the formation of a large ragged eye. Although there are well-defined outer bands in the southern portion of the circulation, there are a few dry slots just beyond the inner core. The latest Dvorak classifications have increased to 4.5/77 kt from both TAFB and SAB, and therefore, the initial intensity is adjusted upward to 75 kt. Helene is currently in a low wind shear environment and over fairly warm 27-28 deg C SSTs. These conditions are expected to prevail for another 24-36 hours, so additional strengthening seems likely during that period. Beyond that time, however, the environment is expected to gradually become less favorable for Helene with south-southwesterly shear increasing, mid-level humidities falling, and SSTs lowering along the future path. All of these conditions suggest a steady weakening trend beginning in a couple of days, and the NHC intensity forecast follows the trend in the latest model guidance. The hurricane continues to pull away from the Cabo Verde Islands, and the initial motion estimate is 285/11 kt. A continued west-northwest motion at about the same forward speed is expected for about two days while a mid-level ridge remains the primary steering feature. After that time, a significant weakness in the ridge, caused by a cut off deep-layer low pressure system, should promote a northward turn with a gradual increase in forward speed. The models are in relatively good agreement on this scenario, but they differ on when and where Helene makes the turn. The NHC track forecast is adjusted a little to the east at the latter forecast times, but it still favors the left side of the guidance envelope to be near the HCCA model. FORECAST POSITIONS AND MAX WINDS INIT 10/0300Z 13.9N 27.2W 75 KT 85 MPH 12H 10/1200Z 14.4N 29.2W 85 KT 100 MPH 24H 11/0000Z 15.1N 31.8W 90 KT 105 MPH 36H 11/1200Z 15.7N 34.2W 90 KT 105 MPH 48H 12/0000Z 16.4N 36.1W 85 KT 100 MPH 72H 13/0000Z 19.0N 38.7W 70 KT 80 MPH 96H 14/0000Z 23.5N 40.5W 60 KT 70 MPH 120H 15/0000Z 29.6N 41.2W 55 KT 65 MPH $$ Forecaster Cangialosi
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