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Tropical Storm Helene Forecast Discussion Number 7
2018-09-09 04:42:09| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Sat Sep 08 2018 252 WTNT43 KNHC 090242 TCDAT3 Tropical Storm Helene Discussion Number 7 NWS National Hurricane Center Miami FL AL082018 1100 PM AST Sat Sep 08 2018 Helene continues this evening with a large area of convective bands that wrap about three-quarters around the center. A blend of the subjective Dvorak classifications from TAFB and SAB with the CIMSS' SATCON gives an intensity of 50 kt, unchanged from the previous advisory. The tropical storm is situated just south of the upper-level subtropical ridge, which is inducing weak southeasterly vertical shear. That along with warm 28C waters and a moist, unstable atmosphere should support at least steady intensification for the next two days. By day three, however, the shear will increase out of the southwest as Helene moves close to an upper-level low. Simultaneously, the SSTs will drop to near 26C and the mid-troposphere should dry. The new intensity forecast shows a peak of 90 kt at 48 h, with gradual to steady weakening thereafter. This is based upon the IVCN multi-model consensus (minus the much weaker HMON solution) with a higher peak intensity than the previous advisory but a slightly faster decay. Helene's initial position was aided by the availability of a 2222Z ASCAT pass, which showed it to be somewhat farther south than earlier estimated. The initial motion is analyzed to be 260/11, as it is steered by a deep-layer ridge to its north. The tropical cyclone should gradually turn toward the west-northwest by Monday, then toward the north-northwest by Wednesday. The new track forecast - based on the TVCN multimodel consensus - is slightly south of the previous advisory throughout the forecast period, mainly because of the southward initial position. The aforementioned scatterometer pass allowed for a more accurate assessment of the initial size, which is somewhat larger than analyzed earlier. The new wind radii prediction indicates a larger Helene than the previous advisory, and is based upon the RVCN multi-model consensus. FORECAST POSITIONS AND MAX WINDS INIT 09/0300Z 13.2N 22.8W 50 KT 60 MPH 12H 09/1200Z 13.3N 24.7W 60 KT 70 MPH 24H 10/0000Z 13.8N 27.4W 70 KT 80 MPH 36H 10/1200Z 14.5N 30.2W 80 KT 90 MPH 48H 11/0000Z 15.3N 33.0W 90 KT 105 MPH 72H 12/0000Z 17.1N 37.4W 85 KT 100 MPH 96H 13/0000Z 19.5N 40.5W 65 KT 75 MPH 120H 14/0000Z 22.5N 42.5W 50 KT 60 MPH $$ Forecaster Landsea
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