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Tropical Storm Helene Forecast Discussion Number 5
2018-09-08 16:44:34| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Sat Sep 08 2018 532 WTNT43 KNHC 081444 TCDAT3 Tropical Storm Helene Discussion Number 5 NWS National Hurricane Center Miami FL AL082018 1100 AM AST Sat Sep 08 2018 The structure of Helene continues to improve, with convective banding wrapping around almost completely around the center of the cyclone in visible satellite imagery. Infrared imagery suggests this convection is not very strong at the moment, though, and this may be why recent scatterometer data indicates that the storm has not strengthened since the last advisory. The initial intensity remains 40 kt based on the scatterometer winds. All indications are that Helene should strengthen through at least 72 h in a light vertical shear environment over sea surface temperatures near 27C. This part of the intensity forecast is similar to the previous forecast and lies near the intensity consensus. However, there are a couple of alternate forecast scenarios. The first is that Helene could rapidly intensify and become stronger than currently forecast. The second is that a large plume of African dust and associated dry air that has spread over the northern Cabo Verde Islands starts entraining into the cyclone and inhibits intensification. The former alternative seems more likely than the latter at this time, and if RI begins later intensity forecasts will need to be increased. 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. Helene's center appears to have again re-formed, this time a little to the south. The somewhat uncertain initial motion is 265/11. Other than a slight nudge to the south due to the initial position, there is little change in the forecast track for Helene. The cyclone should moved westward to west-northwestward for the next 3-4 days on the south side of the subtropical ridge, then turn northwestward in response to the aforementioned trough. FORECAST POSITIONS AND MAX WINDS INIT 08/1500Z 13.4N 20.6W 40 KT 45 MPH 12H 09/0000Z 13.6N 22.4W 50 KT 60 MPH 24H 09/1200Z 13.9N 24.9W 60 KT 70 MPH 36H 10/0000Z 14.6N 27.8W 70 KT 80 MPH 48H 10/1200Z 15.3N 30.8W 80 KT 90 MPH 72H 11/1200Z 17.0N 36.0W 80 KT 90 MPH 96H 12/1200Z 19.0N 40.0W 70 KT 80 MPH 120H 13/1200Z 22.5N 42.5W 60 KT 70 MPH $$ Forecaster Beven
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