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Tropical Storm Isaias Forecast Discussion Number 26
2020-08-03 16:57:19| National Hurricane Center (Atlantic)
Issued at 1100 AM EDT Mon Aug 03 2020 000 WTNT44 KNHC 031457 TCDAT4 Tropical Storm Isaias Discussion Number 26 NWS National Hurricane Center Miami FL AL092020 1100 AM EDT Mon Aug 03 2020 Isaias continues to undergo strong bursting and then weakening convective phases, with the cyclone currently in the latter mode. Satellite and radar data indicate that Isaias' convective organization has become a little disheveled since the previous advisory, but this is to be expected since the tropical cyclone has been undergoing bursting periods about every 8 hours or so beginning overnight around 0600-0900Z. Thus, the system is due for another re-organization phase shortly if it holds true to form. That may already be underway based on recent radar and recon data showing a slight eastward shift in the center position. The highest 700-mb flight-level wind observed has been 63 kt, which reduces to about a 57-kt surface wind. Jacksonville Doppler radar velocities north and north-northwest of the center have been around 60 kt at 9,000 ft, which equals about 54 kt surface winds. The initial intensity will be held at 60 kt, perhaps a little generously, for this advisory despite the recent rise in the central pressure to around 998 mb. Isaias is still moving northward but a little faster at 360/11 kt. The track forecast reasoning remains essentially the same as the previous few advisories, with Isaias expected to gradually turn toward the north-northeast later today and begin to accelerate by this evening. The latest NHC model guidance is more tightly packed about the previous advisory track, so only minor tweaks were required. The new NHC track forecast lies very close to a blend of the multi-model consensus aids TVCA, GFEX, TVCX, and NOAA-HCCA. Current westerly vertical wind shear of 20-25 kt is expected to decrease somewhat during the next 12 h and also become more southwesterly, which will be in better alignment with Isaias' forward motion vector. The decrease in the shear should result in less tilt to the cyclone, allowing Isaias to strengthen and regain hurricane status just before making landfall, and most of the intensity guidance shows a 60-65 kt system at that time. After landfall, Isaias is forecast to only slowly weaken due to interaction with an unusually strong winter-type jetstream that will be possessing winds of 100-120 kt. Expected strong baroclinic forcing will keep Isaias' circulation intact and also produce very strong wind gusts along the Mid-Atlantic states 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 in the Storm Surge Warning area. Residents in these areas should follow advice given by local emergency officials. 2. Isaias is forecast to regain hurricane strength before it reaches the coast of northeastern South Carolina and southern North Carolina, and hurricane conditions are expected in the Hurricane Warning area by this evening. Preparations should be rushed to completion. 3. Isaias is expected to bring widespread sustained tropical storm force winds and wind gusts to hurricane force to the mid-Atlantic coast Tuesday, which could cause tree damage and power outages. Tropical storm conditions are expected to reach southern New England on late Tuesday and are possible along the northern New England coast Tuesday night and early Wednesday. 4. Heavy rainfall will result in flash and urban flooding, some of which may be significant in the eastern Carolinas and the mid-Atlantic, through Tuesday night near the path of Isaias up the East Coast of the United States. Widespread minor to moderate river flooding is possible across portions of the Carolinas and the Mid-Atlantic. FORECAST POSITIONS AND MAX WINDS INIT 03/1500Z 30.7N 80.1W 60 KT 70 MPH 12H 04/0000Z 33.1N 79.2W 65 KT 75 MPH 24H 04/1200Z 37.2N 77.0W 55 KT 65 MPH...INLAND 36H 05/0000Z 42.0N 73.8W 50 KT 60 MPH...INLAND 48H 05/1200Z 46.7N 70.2W 35 KT 40 MPH...INLAND 60H 06/0000Z 50.2N 67.2W 30 KT 35 MPH...POST-TROP/EXTRATROP 72H 06/1200Z 53.5N 64.2W 25 KT 30 MPH...POST-TROP/EXTRATROP 96H 07/1200Z...DISSIPATED $$ Forecaster Stewart
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