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Tropical Storm Isaias Forecast Discussion Number 29
2020-08-04 10:51:33| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Tue Aug 04 2020 000 WTNT44 KNHC 040851 TCDAT4 Tropical Storm Isaias Discussion Number 29 NWS National Hurricane Center Miami FL AL092020 500 AM EDT Tue Aug 04 2020 Isaias made landfall near Ocean Isle Beach, North Carolina, a few minutes after the previous advisory package was issued. Since that time, the center of Isaias has been moving quickly north-northeastward across eastern North Carolina. Now that the inner core of Isaias has moved inland, the peak surface winds have decreased despite Doppler radar data still showing an area of 75-85 kt winds aloft. The latest surface observations indicated that strong winds are occuring over the North Carolina Sounds and Outer Banks. Based on a blend of the available data, the initial wind speed is set at 60 kt for this advisory. As the tropical storm moves northward near the Mid-Atlantic coast, interaction with a strong jet stream is likely to result in a slower-than-typical weakening rate. The global model guidance indicates 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 has been increased above the standard 20 percent in the Forecast/Advisory (TCMAT4). Isaias is forecast to weaken more quickly tonight as it moves into eastern Canada and becomes post-tropical. The post-tropical cyclone is forecast to be absorbed by a larger extratropical low in 2-3 days, if not sooner. Isaias is moving north-northeastward or 020/24 kt. The cyclone is forecast to continue accelerating north-northeastward today as it is embedded in strong south-southwesterly flow ahead of a deep-layer trough over the Great Lakes region. After 24 h, the post-tropical cyclone is expected to decelerate before it is absorbed by the extratropical low. The updated NHC track forecast is near the center of the tightly clustered guidance envelope. In addition to the storm surge and wind threats, 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 is also a risk of tornadoes from southeast Virginia to New Jersey through midday. The risk of tornadoes will spread northward into southeastern New York this afternoon and across New England by tonight. Key Messages: 1. There is the danger of life-threatening storm surge inundation along portions of the immediate coastline and adjacent waterways of the North Carolina coast, including portions of Pamlico and Albemarle Sounds. Residents in these areas should follow advice given by local emergency officials. 2. Isaias is expected to bring widespread sustained tropical storm force winds and wind gusts to hurricane force to 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. 3. Heavy rainfall along the East Coast, near the path of Isaias, will result in flash flooding, some of which may be significant in North Carolina through the Mid-Atlantic and Northeast through tonight. Potentially life-threatening urban flooding is possible in D.C., Baltimore, and elsewhere along and just west of the I-95 corridor today. Widespread minor to moderate river flooding is likely across portions of the Carolinas and the Mid-Atlantic. Quick-responding rivers in Northeast will also be susceptible to minor river flooding. 4. Tornadoes have already occurred over portions of northeastern North Carolina and southeastern Virginia overnight. The threat of tornadoes will spread northward along the mid-Atlantic coast today and then across New England by tonight. FORECAST POSITIONS AND MAX WINDS INIT 04/0900Z 36.3N 77.5W 60 KT 70 MPH...INLAND 12H 04/1800Z 40.3N 75.0W 55 KT 65 MPH...INLAND 24H 05/0600Z 45.8N 72.1W 35 KT 40 MPH...POST-TROP/INLAND 36H 05/1800Z 50.3N 70.3W 30 KT 35 MPH...POST-TROP/EXTRATROP 48H 06/0600Z 53.2N 69.5W 20 KT 25 MPH...POST-TROP/EXTRATROP 60H 06/1800Z...DISSIPATED $$ Forecaster Brown/Berg
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