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Tropical Storm Isaias Discussion Number 21
NWS National Hurricane Center Miami FL AL092020
500 AM EDT Sun Aug 02 2020
The burst of deep convection that developed to the northeast of the
center last evening has not resulted in any significant improvement
in the structure of Isaias according to recent reconnaissance
aircraft data and radar imagery. The tropical cyclone is still
being adversely affected by about 25 kt of 850-200 mb vertical wind
shear. An Air Force Reserve reconnaissance aircraft that has
provided several center fixes overnight has found peak 850-mb
flight-level winds of 67 kt, and peak believable SFMR winds of 51
kt. These data, along with an earlier ASCAT overpass, indicate
that Isaias' intensity is 55 kt. In addition, both the aircraft
and ASCAT data suggest that the wind field over the western portion
of the circulation is somewhat smaller than previous estimated.
Since Isaias has not shown any signs of re-organizing overnight, it
appears that the window of opportunity for it to re-strengthen is
closing. The SHIPS guidance and global model fields do not show
the shear abating during the next 24-36 hours while Isaias moves
near the Florida coast. As a result, the new NHC intensity forecast
shows little change in strength during this time. Later in the
period, gradual weakening should occur as the storm moves northward
along the east coast of the United States. The NHC intensity
forecast is in good agreement with the HFIP corrected consensus
model and the latest multi-model intensity consensus.
Recent aircraft fixes and radar data show that Isaias is moving
northwestward or 325/8 kt. A slow northwestward to
north-northwestward motion around the western periphery of a
deep-layer ridge centered near Bermuda should take the center of
Isaias very near the east coast of Florida through Monday. After
that, the tropical cyclone should begin to accelerate
north-northeastward to northeastward as a trough slides eastward
into the eastern United States. The dynamical models are tightly
clustered, and the NHC track forecast lies between the typically
reliable GFS and ECMWF models.
The reduction in the initial and predicted intensity of Isaias has
necessitated changes in warnings along the east coast of Florida.
The Hurricane Warning along the east-central coast of Florida has
been replaced with a Tropical Storm Warning.
1. Tropical storm conditions will continue across portions of the
northwestern Bahamas through today.
2. Tropical storm conditions are expected to spread northward along
the east coast of Florida within the warning area through early
Monday. These conditions are expected to spread northward along the
coast of Georgia and South Carolina within the warning area on
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 across portions of the northwestern Bahamas, and
flash and urban flooding, especially in low-lying and poorly
drained areas, along the East Coast of the United States. Minor to
isolated moderate river flooding is possible across portions of the
Carolinas and Mid-Atlantic this week.
5. Tropical storm watches are in effect for portions of the coast
of South and North Carolina. Additional watches and warnings will
likely be issued later today as Isaias is expected to move
northward near or over the southeast and mid-Atlantic coasts over
the next couple of days.
FORECAST POSITIONS AND MAX WINDS
INIT 02/0900Z 26.3N 79.5W 55 KT 65 MPH
12H 02/1800Z 27.4N 80.1W 55 KT 65 MPH
24H 03/0600Z 28.8N 80.6W 55 KT 65 MPH
36H 03/1800Z 30.7N 80.6W 55 KT 65 MPH
48H 04/0600Z 33.7N 79.1W 55 KT 65 MPH...INLAND
60H 04/1800Z 37.8N 76.4W 50 KT 60 MPH...INLAND
72H 05/0600Z 42.9N 72.4W 40 KT 45 MPH...INLAND
96H 06/0600Z 50.0N 65.0W 30 KT 35 MPH...POST-TROP/INLAND