Hurricane Isaias Forecast Discussion

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533 
WTNT44 KNHC 011500
TCDAT4

Hurricane Isaias Discussion Number  18
NWS National Hurricane Center Miami FL       AL092020
1100 AM EDT Sat Aug 01 2020

Data from an Air Force Reserve reconnaissance aircraft around 1410 
UTC indicated that the center of the eye of Isaias was located 
along the southern coast of northern Andros Island about 15 nmi 
south-southwest of Andros Town. The eye appearance in Bahamas and  
aircraft radar data, along with the visual reports from the flight 
crew, has gone from nearly closed a few hours ago to open in the 
southwestern quadrant more recently. However, the diameter of the 
eye has been has been holding steady between 20-22 nmi, an 
indication that Isaias has been able to fight off some modest 
southwesterly wind shear. The initial intensity has been lowered 
to 70 kt based on maximum 700-mb flight-level winds of 77 kt, which 
equals about 70-kt surface wind speed, and the central pressure 
fluctuating between 987-990 mb.

The initial motion estimate remains 315/10 kt. The latest NHC model 
guidance is now tightly packed about the previous 18-h worth of 
official track forecasts, and as a result, no significant changes 
were made to the previous advisory track. The global and regional 
models have come into much better agreement compared to 24 hours ago 
in taking Isaias northwestward slowly for the next 36 h or so, and 
moving the center near or keeping it just offshore the east-central 
Florida coast. By 48 hours, the hurricane is expected to turn 
northward around the western portion of the Bermuda-Azores ridge 
that will slowly be eroded by an approaching mid-level shortwave 
trough currently situated over the central United States. By 60 h 
and beyond, the aforementioned trough is expected to gradually 
accelerate Isaias northeastward near or along the coast from 
South Carolina to New England. The new NHC track forecast is 
basically an extension of the previous one, and lies very close to 
an average of the simple consensus model TVCA, and the corrected- 
consensus models NOAA-HCCA and FSSE.

In the near term, Isaias could weaken a little bit this afternoon 
while passing over northern Andros Island. However, the still 
impressive vertical structure of the cyclone should allow for some 
re-strengthening after the center moves back over the warm Gulf 
Stream by this evening. The current SHIPS analyzed westerly 
vertical wind shear of 25 kt could be too high due to the model 
incorporating some of the storm's outflow. The models forecast the 
shear to weaken somewhat over the next 36 h while Isaias is moving 
over the Gulf Stream, and the 06Z UKMET shows Isaias moving 
underneath a 200-mb anticyclone, which would normally favor some 
strengthening. Given all of these factors, the official forecast 
maintains a steady intensity through Monday night. Slow weakening 
is forecast when Isais encounters more significant southwesterly 
vertical wind shear ahead of a strong upper-level trough that will 
be approaching the U.S. east coast on days 3-5.

Key Messages:

1. Hurricane conditions and dangerous storm surge will continue in
portions of the northwest Bahamas today and tonight. 

2. Hurricane conditions are expected along portions of the Florida 
east coast by late tonight and Sunday. Preparations to protect life 
and property should be rushed to completion.

3. Dangerous storm surge is possible along the Florida east coast 
from Jupiter Inlet to Ponte Vedra Beach where water rises of 2 to 4 
feet above ground level are possible along the immediate coastline 
and adjacent waterways. Residents there should follow advice given 
by local emergency officials.

4. Isaias will produce heavy rains and potentially life-threatening 
flash flooding in the Bahamas, and flash urban flooding, especially 
in low-lying and poorly drained areas, in eastern Florida, and from 
the Carolinas to the mid-Atlantic and Northeast U.S.  Minor river 
flooding is possible across portions of the Carolinas and Virginia 
early next week.

5. Tropical storm watches and warnings are in effect for portions 
of the northeast Florida and southern Georgia coasts. Additional 
watches and warnings will likely be issued later today and Sunday 
along the U.S. east coast as the risk of wind, heavy rainfall, and 
storm surge impacts continues to increase.

FORECAST POSITIONS AND MAX WINDS

INIT  01/1500Z 24.7N  77.9W   70 KT  80 MPH
 12H  02/0000Z 25.7N  78.8W   70 KT  80 MPH
 24H  02/1200Z 27.1N  79.7W   70 KT  80 MPH
 36H  03/0000Z 28.5N  80.3W   70 KT  80 MPH
 48H  03/1200Z 30.1N  80.4W   65 KT  75 MPH
 60H  04/0000Z 32.6N  79.4W   60 KT  70 MPH
 72H  04/1200Z 36.0N  76.9W   50 KT  60 MPH...INLAND
 96H  05/1200Z 43.3N  69.7W   45 KT  50 MPH...OVER WATER
120H  06/1200Z 49.0N  57.9W   40 KT  45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart




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