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Subtropical Storm Ernesto Forecast Discussion Number 4
2018-08-16 04:34:08| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Wed Aug 15 2018 844 WTNT45 KNHC 160234 TCDAT5 Subtropical Storm Ernesto Discussion Number 4 NWS National Hurricane Center Miami FL AL052018 1100 PM AST Wed Aug 15 2018 Although inner-core convection has increased since the previous advisory, outer banding features have become more fragmented and the overall cloud pattern has become elongated north-to-south. There are also no signs of any upper-level anticyclonic outflow, an indication that Ernesto is still a subtropical cyclone. The initial intensity is being maintained at 35 kt based on a 2328Z ASCAT pass that showed peak winds of 32 kt in the southeastern quadrant, along with a radius of maximum winds of 70-80 nmi. This intensity is consistent with a ST2.5/35 kt classification from TAFB. The initial motion is now north-northeastward or 025/09 kt. Ernesto has rounded the subtropical ridge axis to its south, and the cyclone should gradually get caught up in the mid-latitude westerlies and turn northeastward during the next 12 hours or so. A northeastward motion along with a steadily increasing forward speed is expected through Friday. The track guidance is tightly packed around the previous advisory track, so the new NHC forecast track is essentially just an extension of the previous advisory, and lies near the track consensus models HCCA and TVCN. On the forecast track, Ernesto is expected to approach Ireland as an extratropical gale area on Saturday. Ernesto is expected to remain over sea-surface temperatures (SST) of 25 deg C or warmer for the next 18-24 hours, along with low vertical shear conditions of less than 10 kt. These conditions should allow for some slight strengthening during that short time window. By 36 hours, Ernesto is forecast to be moving over SSTs of 20 deg C and colder and into a more stable airmass, resulting in a degeneration of the inner-core convection. As a result, Ernesto is expected to become a post-tropical cyclone by 36 hours, and become an extratropical gale area after merging with a frontal zone near Ireland and the United Kingdom in about 3 days. FORECAST POSITIONS AND MAX WINDS INIT 16/0300Z 39.7N 45.1W 35 KT 40 MPH 12H 16/1200Z 41.3N 43.4W 40 KT 45 MPH 24H 17/0000Z 43.7N 40.0W 40 KT 45 MPH 36H 17/1200Z 46.3N 34.9W 40 KT 45 MPH...POST-TROPICAL 48H 18/0000Z 49.0N 27.9W 40 KT 45 MPH...POST-TROP/EXTRATROP 72H 19/0000Z 54.0N 12.1W 40 KT 45 MPH...POST-TROP/EXTRATROP 96H 20/0000Z...DISSIPATED $$ Forecaster Stewart
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