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Tropical Storm Fernanda Forecast Discussion Number 34
2017-07-20 10:46:30| National Hurricane Center (East Pacific)
Issued at 200 AM PDT Thu Jul 20 2017 000 WTPZ41 KNHC 200846 TCDEP1 Tropical Storm Fernanda Discussion Number 34 NWS National Hurricane Center Miami FL EP062017 200 AM PDT Thu Jul 20 2017 Fernanda is beginning to experience moderate southwesterly vertical wind shear, which has resulted in a pronounced degradation of the convective cloud pattern since the previous advisory. Recent AMSU and SSMI/S microwave satellite data indicate that the shear has tilted the vortex toward the north, resulting in the initial position being located a little south of the satellite fix positions and closer to the southwestern edge of the coldest cloud tops. The initial intensity has been lowered to 60 kt based on a blend of consensus T-numbers of T3.5/55 kt and CI-numbers of T4.0/65 kt from both TAFB and SAB. Fernanda's motion is now 290/08 kt. The weakening cyclone is expected to continue moving generally west-northwestward along the southern periphery of a broad deep-layer subtropical ridge for about the next 72 hours. By days 4 and 5, a faster westward motion is forecast as the shallow vortex comes under the influence of the brisk low-level easterly trade wind flow. The latest GFS model run appears to keep Fernanda too strong and vertically deep after 48 h, which has resulted in a track well north of any of the other model guidance. As a result, the new NHC forecast has been shifted a little south of the previous advisory track through 72 hours, and is then close to the middle of the guidance envelope after that. Fernanda should steadily succumb to the effects of cooler water, drier air, and shear increasing to 30 kt throughout the forecast period, resulting in weakening and degeneration into a remnant low by around 48 hours. It is possible that the system could even open up into a tropical wave by 96-120 hours when it is passing just north of the Hawaiian Islands. The official intensity forecast is similar to the previous advisory and closely follows the IVCN consensus model. FORECAST POSITIONS AND MAX WINDS INIT 20/0900Z 18.2N 137.7W 60 KT 70 MPH 12H 20/1800Z 18.5N 138.9W 50 KT 60 MPH 24H 21/0600Z 19.0N 140.7W 40 KT 45 MPH 36H 21/1800Z 19.5N 142.5W 35 KT 40 MPH 48H 22/0600Z 20.1N 144.2W 30 KT 35 MPH...POST-TROP/REMNT LOW 72H 23/0600Z 21.6N 147.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 24/0600Z 22.7N 152.2W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 25/0600Z 23.8N 158.0W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Stewart
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