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Tropical Storm Isaias Forecast Discussion Number 30
2020-08-04 16:59:12| National Hurricane Center (Atlantic)
Issued at 1100 AM EDT Tue Aug 04 2020 000 WTNT44 KNHC 041459 TCDAT4 Tropical Storm Isaias Discussion Number 30 NWS National Hurricane Center Miami FL AL092020 1100 AM EDT Tue Aug 04 2020 Doppler radar data from Dover, Delaware (KDOX), and Ft. Dix, New Jersey (KDIX) this morning continue to indicate 70-80 kt winds between 4000-7000 ft over the ocean just offshore the Delmarva peninsula and the southeastern coast of New Jersey, which would normally correspond to 65-70 kt surface winds. However, these velocity values have been collocated with only 15-25 dBZ reflectivity echoes, so the typical 80-85 percent reduction factors likely don't apply. Sustained observed surface winds of 50-55 kt have been reported, so the initial intensity will be held at 60 kt, but that is only for winds over the ocean and near the coast. Isaias is moving north-northeastward or 020/30 kt. The cyclone will continue to accelerate north-northeastward today and tonight within strong southwesterly flow ahead of a deep-layer trough and associated cold front approaching the mid-Atlantic and New England areas from the west. By 24 h, the Isaias is expected to slow down and degenerate into a post-tropical cyclone over Maine before it is absorbed by a larger extratropical low located over southeastern Canada. The new NHC track forecast is just an extension of the previous advisory, and lies very close to the tightly clustered consensus track models. As Isaias moves north-northeastward through the Mid-Atlantic coast, interaction with a strong upper-level jet maximum is forecast to maintain the tropical storm's intensity longer than what typically would be expected for inland decaying tropical cyclone. The global models continue to indicate that Isaias is likely to produce widespread tropical-storm conditions, with hurricane-force wind gusts possible along the mid-Atlantic coast through this afternoon. As a result, the gust factor at 12 h remains above the standard 20-percent value in the Forecast/Advisory Product (TCMAT4). In addition to the wind threat, Isaias is expected to produce heavy rainfall along and just west of the I-95 corridor today, and the Weather Prediction Center has placed a portion of this area in a high risk for life-threatening flash flooding. There also remains a significant risk of tornadoes across the mid-Atlantic coast and southeastern New York this afternoon, possibly spreading into southern New England tonight. Key Messages: 1. Widespread sustained tropical storm force winds and gusts to hurricane force are expected along the mid-Atlantic coast, including portions of the Chesapeake Bay region, through this afternoon, which could cause tree damage and power outages. Tropical storm conditions are expected to spread across New England tonight. 2. Heavy rainfall along the East Coast, near the path of Isaias, will result in flash flooding, some of which may be significant in the mid-Atlantic and Northeast through tonight. Potentially life-threatening urban flooding remains possible in Philadelphia and elsewhere along and just west of the I-95 corridor today. Scattered minor to moderate river flooding is likely across portions of the and the Mid-Atlantic. Quick-responding rivers in Northeast will also be susceptible to minor river flooding. 3. Numerous tornadoes have already occurred over portions of the mid-Atlantic coast this morning. The threat of tornadoes will continue along the mid-Atlantic coast spread into New England this afternoon. FORECAST POSITIONS AND MAX WINDS INIT 04/1500Z 39.1N 76.1W 60 KT 70 MPH...INLAND 12H 05/0000Z 43.2N 73.8W 50 KT 60 MPH...INLAND 24H 05/1200Z 48.2N 71.7W 35 KT 40 MPH...POST-TROP/INLAND 36H 06/0000Z 52.0N 71.1W 30 KT 35 MPH...POST-TROP/EXTRATROP 48H 06/1200Z 53.6N 69.5W 20 KT 25 MPH...POST-TROP/EXTRATROP 60H 07/0000Z...DISSIPATED INLAND $$ Forecaster Stewart
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