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| #1010989 (Received by flhurricane at: 5:05 PM 03.Aug.2020) TCDAT4 Tropical Storm Isaias Discussion Number 27 NWS National Hurricane Center Miami FL AL092020 500 PM EDT Mon Aug 03 2020 Isaias is undergoing its daily reorganization phase, with a ragged eye-like feature now showing up in NOAA Doppler weather radars from Charleston, Wilmington, and Jacksonville. The last Air Force Reserve reconnaissance flight reported a peak SFMR-derived surface wind speed of 62 kt in the southwest quadrant around 1630Z while Isaias was experiencing a center reformation further to the east. Since then, Doppler velocities of 70-75 kt have been observed between 10,000-15,000 ft in the northern semicircle, but within reflectivity regions less than 35 dBZ, which suggests that a the reduction factor is likely less than the 90-percent value typically used. Thus the intensity is being held at 60 kt, which means that Isaias is very near hurricane status. Another Air Force Reserve reconnaissance will be investigating the cyclone around 0000Z this evening. Isaias is now moving north-northeastward 015/14 kt. The new NHC model guidance remains in excellent agreement on Isaias continuing to gradually accelerate north-northeastward to northeastward for the next 36 hours ahead of a powerful deep-layer trough and associated cold front. The cyclone should make landfall later this evening near the South Carolina-North Carolina border, and then accelerate north-northeastward at 25-30 kt across eastern North Carolina early Tuesday, eastern Virginia and the Delmarva peninsula Tuesday afternoon, and into New England Tuesday night into early Wednesday. The new NHC track forecast is just an update and extension of the previous advisory track, and lies very close to a blend of the tightly packed multi-model consensus aids TVCA, GFEX, TVCX, and NOAA-HCCA. Satellite animation and special 1800Z upper-air soundings indicate that the vertical shear across Isaias has weakened and has also become more southwesterly, which better aligns with the forecast track. Given this and the improved structure of the system, Isaias is still expected to strengthen and regain hurricane status before making landfall, and most of the intensity guidance shows a 60-65 kt system at that time. It should be emphasized that there is little difference between a strong tropical storm or a category 1 hurricane in terms of impacts. After landfall, Isaias is forecast to only slowly weaken due to interaction with an unusually strong 100-120 kt jetstream. The expected strong baroclinic forcing will keep Isaias' circulation intact and also produce very strong wind gusts along the mid-Atlantic coast tomorrow. As a result, the gust factors at 24-48 h have been increased above the standard 20 percent in the Forecast/Advisory (TCMAT4). The cyclone is forecast to be absorbed by a larger extratropical low over Canada in 3-4 days. Key Messages: 1. There is the danger of life-threatening storm surge inundation along portions of the immediate coastline and adjacent waterways of northeastern South Carolina and 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 be at or near hurricane strength when it reaches the coast of northeastern South Carolina and southern North Carolina in a few hours, and hurricane conditions are expected in the Hurricane Warning area this evening. 3. 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, on Tuesday, which could cause tree damage and power outages. Tropical storm conditions are expected to spread across New England late Tuesday into early Wednesday. 4. Heavy rainfall along the East Coast near the path of Isaias will result in flash and urban flooding, some of which may be significant in the eastern Carolinas, mid-Atlantic and northeast through Wednesday. Widespread minor to moderate river flooding is possible across portions of the Carolinas and the Mid-Atlantic. Quick- responding rivers in the northeast will also be susceptible to minor river flooding. FORECAST POSITIONS AND MAX WINDS INIT 03/2100Z 32.0N 79.4W 60 KT 70 MPH 12H 04/0600Z 34.7N 78.1W 60 KT 70 MPH...INLAND 24H 04/1800Z 39.3N 75.3W 55 KT 65 MPH...INLAND 36H 05/0600Z 44.5N 71.8W 45 KT 50 MPH...INLAND 48H 05/1800Z 48.8N 68.6W 30 KT 35 MPH...POST-TROP/EXTRATROP 60H 06/0600Z 52.0N 67.1W 25 KT 30 MPH...POST-TROP/EXTRATROP 72H 06/1800Z 54.3N 63.4W 25 KT 30 MPH...POST-TROP/EXTRATROP 96H 07/1800Z...DISSIPATED $$ Forecaster Stewart |
