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Tropical Storm Helene Forecast Discussion Number 6
2018-09-08 23:02:38| National Hurricane Center (Atlantic)
Issued at 500 PM AST Sat Sep 08 2018 912 WTNT43 KNHC 082102 TCDAT3 Tropical Storm Helene Discussion Number 6 NWS National Hurricane Center Miami FL AL082018 500 PM AST Sat Sep 08 2018 During the past few hours, Helene has developed a convective band that wraps about three-quarters of the way around the storm, and the intensity of the inner core convection has increased. In addition, microwave imagery shows an elliptical inner ring feature. Satellite intensity estimates at 18Z were in the 35-50 kt range, and given the recent increase in organization the initial intensity is increased to 50 kt. Helene should continue to strengthen through at least 72 h in a light vertical shear environment over sea surface temperatures near 27C. This part of the intensity forecast has been increased over the previous forecast and lies near the intensity consensus. However, there remain a couple of alternate forecast scenarios. The first is that Helene could rapidly intensify and become stronger than currently forecast, and the rapid intensification index of the SHIPS model has about a 35 percent chance of 25 kt of strengthening in the next 24 h. The second is that a large plume of African dust and associated dry air that has spread over the Cabo Verde Islands starts entraining into the cyclone and inhibits intensification. The former alternative is more likely than the latter at this time. After 72 h, Helene is expected to encounter increasing southwesterly shear associated with a mid- to upper-level trough over the central Atlantic, which should cause the system to weaken significantly. The initial motion is 275/11. There again is little change in the forecast guidance or the forecast track, with Helene expected to move westward to west-northwestward for the next 3 days or so on the south side of the subtropical ridge, then turn northwestward in response to the aforementioned trough. FORECAST POSITIONS AND MAX WINDS INIT 08/2100Z 13.6N 21.7W 50 KT 60 MPH 12H 09/0600Z 13.7N 23.5W 55 KT 65 MPH 24H 09/1800Z 14.1N 26.2W 65 KT 75 MPH 36H 10/0600Z 14.7N 29.1W 75 KT 85 MPH 48H 10/1800Z 15.5N 32.0W 85 KT 100 MPH 72H 11/1800Z 17.5N 37.0W 85 KT 100 MPH 96H 12/1800Z 19.5N 40.5W 65 KT 75 MPH 120H 13/1800Z 23.0N 42.5W 55 KT 65 MPH $$ Forecaster Beven
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