Official Discussion issued by the National Hurricane Center

Isaias (AL092020) DATA RELEASED: 7/31/2020 3:00:00 PM UTC

Copy of official data

Hurricane Isaias Discussion Number  14
NWS National Hurricane Center Miami FL       AL092020
1100 AM EDT Fri Jul 31 2020

After a brief convective hiatus earlier this morning, a small burst 
of deep convection with cloud tops as cold as -80C to -85C have 
developed over the previously exposed low-level center. Dry 
downslope air coming off of Hispaniola wrapped into the center, 
eroding the inner-core convection somewhat. However, a surge of 
moist southwesterly inflow moving through the Windward Passage now 
appears to be fueling the recent increase in convection. The 
upper-level outflow has improved somewhat in the western  
semicircle but still remains restricted due to some modest 
southwesterly vertical wind shear. The initial intensity has been 
lowered to 65 kt based on 700-mb flight-level winds of near 70 kt 
and an increase in the central pressure of 992 mb, which is based on 
a dropsonde report of 995 mb with 26-kt surface winds.

Isaias is continuing on a steady northwestward motion of 305/14 kt. 
This motion is expected to continue for the next 36 hours or so as 
the hurricane rounds the southwestern periphery of the 
Bermuda-Azores High. A gradual turn to the north-northwest and 
north is expected by 48-60 hours due to a break in the ridge 
developing in response to a central U.S. shortwave trough digging 
into the southeastern United States. The timing and strength of this 
trough will determine how far west Isaias moves before the hurricane 
turns northward. The 06Z UKMET and and GFS have shifted westward, 
closer to the Florida coast, which is similar to the westward shift 
seen in the 00Z ECMWF model fields. By 72 hours, the hurricane is 
forecast to begin accelerating northeastward, possibly passing over 
eastern North Carolina by day 4 and across eastern New England on 
day 5. As a result of the westward shift in the latest model 
guidance, the new NHC forecast track has also been shifted farther 
west closer to the southeastern U.S. coast, and lies a little to the 
west of the consensus models TVCA and NOAA-HCCA. The westward shift 
in the track forecast has required the issuance of a Hurricane Watch 
for portions of the Florida east coast.

Although some slight weakening has occurred, radar data from the 
aircraft and the Bahamas radar indicate about a 60-percent 
eyewall has formed in the northeastern semicircle, which is an 
indication that the cyclone is trying to reorganize. As a result, 
strengthening is still expected during the next day or so, 
especially tonight and Saturday morning during the convective 
maximum period when the hurricane will be moving over the Gulf 
Stream where SSTs are 30C or warmer and while the vertical remains 
reasonably low. Increasing southwesterly shear could cause a gradual 
decrease in intensity over the weekend. The new official intensity 
forecast is similar to the previous advisory, and is a little above 
the available model guidance out of respect for continuity. 

Key Messages:

1. Isaias will produce heavy rains and potentially life-threatening 
flash flooding and mudslides across the Dominican Republic, northern 
Haiti, Turks and Caicos, and the Bahamas. Heavy rains associated 
with Isaias may begin to affect south and east-central Florida late 
Friday night, and the eastern Carolinas by early next week, 
potentially resulting in isolated flash and urban flooding, 
especially in low-lying and poorly drained areas. Isolated minor 
river flooding is possible in the Carolinas early next week.

2. Hurricane conditions and dangerous storm surge are expected in 
portions of the Bahamas today and Saturday, and Hurricane Warnings 
are in effect. Preparations to protect life and property should be 
rushed to completion.

3. Tropical storm conditions are expected with hurricane conditions 
possible along portions of the Florida east coast beginning 
Saturday, and a Tropical Storm Warning and Hurricane Watch are in 
effect.  Storm surge watches or warnings could be needed for part of 
this area this afternoon.

4. There is a risk of impacts from winds, heavy rainfall, and storm 
surge beginning late this weekend along the northeastern Florida 
coast and spreading northward along the remainder of the U.S. east 
coast through early next week.  Interests along the entire U.S. east 
coast should monitor the progress of Isaias and updates to the 


INIT  31/1500Z 21.7N  74.5W   65 KT  75 MPH
12H  01/0000Z 23.0N  76.1W   70 KT  80 MPH
24H  01/1200Z 24.8N  78.0W   80 KT  90 MPH
36H  02/0000Z 26.3N  79.2W   80 KT  90 MPH
48H  02/1200Z 27.7N  79.8W   75 KT  85 MPH
60H  03/0000Z 29.2N  80.0W   70 KT  80 MPH
72H  03/1200Z 31.6N  79.5W   65 KT  75 MPH
96H  04/1200Z 36.4N  76.2W   55 KT  65 MPH
120H  05/1200Z 43.3N  69.0W   55 KT  65 MPH...POST-TROP/EXTRATROP

Forecaster Stewart