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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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