Official Discussion issued by the National Hurricane Center

Isaias (AL092020) DATA RELEASED: 8/3/2020 9:00:00 PM UTC

Copy of official data

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