Official Discussion issued by the National Hurricane Center

Isaias (AL092020) DATA RELEASED: 8/4/2020 9:00:00 AM UTC

Copy of official data

Tropical Storm Isaias Discussion Number  29
NWS National Hurricane Center Miami FL       AL092020
500 AM EDT Tue Aug 04 2020

Isaias made landfall near Ocean Isle Beach, North Carolina, a few
minutes after the previous advisory package was issued.  Since
that time, the center of Isaias has been moving quickly
north-northeastward across eastern North Carolina.  Now that the
inner core of Isaias has moved inland, the peak surface winds have
decreased despite Doppler radar data still showing an area of
75-85 kt winds aloft.  The latest surface observations indicated
that strong winds are occuring over the North Carolina Sounds and
Outer Banks.  Based on a blend of the available data, the initial
wind speed is set at 60 kt for this advisory.

As the tropical storm moves northward near the Mid-Atlantic coast,
interaction with a strong jet stream is likely to result in a
slower-than-typical weakening rate. The global model guidance
indicates that Isaias is likely to produce widespread
tropical-storm conditions, with hurricane-force wind gusts possible
along the mid-Atlantic coast through this afternoon. As a result,
the gust factor at 12 h has been increased above the standard 20
percent in the Forecast/Advisory (TCMAT4). Isaias is forecast to
weaken more quickly tonight as it moves into eastern Canada and
becomes post-tropical. The post-tropical cyclone is forecast to be
absorbed by a larger extratropical low in 2-3 days, if not sooner.

Isaias is moving north-northeastward or 020/24 kt.  The cyclone is
forecast to continue accelerating north-northeastward today as it
is embedded in strong south-southwesterly flow ahead of a deep-layer
trough over the Great Lakes region. After 24 h, the post-tropical
cyclone is expected to decelerate before it is absorbed by the
extratropical low.  The updated NHC track forecast is near the
center of the tightly clustered guidance envelope.

In addition to the storm surge and wind threats, Isaias is expected
to produce heavy rainfall along and just west of the I-95 corridor 
today, and the Weather Prediction Center has placed a portion of 
this area in a high risk for life-threatening flash flooding.  
There is also a risk of tornadoes from southeast Virginia to New 
Jersey through midday.  The risk of tornadoes will spread northward 
into southeastern New York this afternoon and across New England by

Key Messages:

1. There is the danger of life-threatening storm surge inundation
along portions of the immediate coastline and adjacent waterways of
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 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, through
this afternoon, which could cause tree damage and power outages.
Tropical storm conditions are expected to spread across New England

3.  Heavy rainfall along the East Coast, near the path of Isaias,
will result in flash flooding, some of which may be significant in
North Carolina through the Mid-Atlantic and Northeast through
tonight. Potentially life-threatening urban flooding is possible in 
D.C., Baltimore, and elsewhere along and just west of the I-95 
corridor today. Widespread minor to moderate river flooding is 
likely across portions of the Carolinas and the Mid-Atlantic. 
Quick-responding rivers in Northeast will also be susceptible to 
minor river flooding.

4.  Tornadoes have already occurred over portions of northeastern 
North Carolina and southeastern Virginia overnight.  The threat of 
tornadoes will spread northward along the mid-Atlantic coast today 
and then across New England by tonight.


INIT  04/0900Z 36.3N  77.5W   60 KT  70 MPH...INLAND
12H  04/1800Z 40.3N  75.0W   55 KT  65 MPH...INLAND
24H  05/0600Z 45.8N  72.1W   35 KT  40 MPH...POST-TROP/INLAND
36H  05/1800Z 50.3N  70.3W   30 KT  35 MPH...POST-TROP/EXTRATROP
48H  06/0600Z 53.2N  69.5W   20 KT  25 MPH...POST-TROP/EXTRATROP
60H  06/1800Z...DISSIPATED

Forecaster Brown/Berg