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Tropical Storm Gert Forecast Discussion Number 7
2017-08-14 16:38:32| National Hurricane Center (Atlantic)
Issued at 1100 AM EDT Mon Aug 14 2017 000 WTNT43 KNHC 141438 TCDAT3 Tropical Storm Gert Discussion Number 7 NWS National Hurricane Center Miami FL AL082017 1100 AM EDT Mon Aug 14 2017 Gert has become better organized this morning. A recent 1336 UTC GMI overpass shows a large band of convection wrapping around the circulation from the southwest to the northwest quadrant. However, imagery from the GMI overpass, as well as an earlier SSMIS overpass, suggest that the mid-level center is displaced somewhat south or southeast of the low-level center, due to continued northerly shear of about 15 kt. Based on the improved structure and increased subjective Dvorak classifications from TAFB and SAB, the initial intensity has been raised to 50 kt. Gert is now moving toward the north, and the initial motion estimate is 360/07 kt. Very little change has been made to the track forecast. Gert should continue to move around the western periphery of the subtropical ridge for the next day or so, before rapidly accelerating eastward ahead of a trough. Since Gert has already turned northward, the NHC forecast is on the eastern side of the guidance envelope for the first 48 h, but very near the multi-model consensus after that time. Despite the presence of moderate northerly vertical wind shear, there is good agreement among the intensity guidance that Gert will continue to strengthen for the next 48 h. The HWRF is an outlier in showing Gert becoming a major hurricane around that time, while DSHP, LGEM, and CTCI only forecast modest intensification. Since SHIPS diagnostics from the GFS and ECMWF suggest that the shear will never decrease below 10 kt, the NHC forecast gives more weight to the modest intensification scenario. All of the global models show extratropical transition (ET) beginning around 48 h, and completing by about 72 h. After ET completes, Gert is still expected to steadily weaken before being absorbed by a larger extratropical low in about 4 or 5 days. The 72 h and 96 h track, intensity, and wind radii forecasts incorporated guidance from the Ocean Prediction Center. FORECAST POSITIONS AND MAX WINDS INIT 14/1500Z 30.3N 72.2W 50 KT 60 MPH 12H 15/0000Z 31.5N 72.2W 55 KT 65 MPH 24H 15/1200Z 33.4N 71.3W 60 KT 70 MPH 36H 16/0000Z 35.8N 68.8W 65 KT 75 MPH 48H 16/1200Z 38.4N 64.0W 70 KT 80 MPH 72H 17/1200Z 43.0N 50.0W 65 KT 75 MPH...POST-TROP/EXTRATROP 96H 18/1200Z 50.0N 36.0W 45 KT 50 MPH...POST-TROP/EXTRATROP 120H 19/1200Z...DISSIPATED $$ Forecaster Zelinsky
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