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Tropical Storm Isaias Discussion Number 26
NWS National Hurricane Center Miami FL AL092020
1100 AM EDT Mon Aug 03 2020
Isaias continues to undergo strong bursting and then weakening
convective phases, with the cyclone currently in the latter mode.
Satellite and radar data indicate that Isaias' convective
organization has become a little disheveled since the previous
advisory, but this is to be expected since the tropical cyclone
has been undergoing bursting periods about every 8 hours or so
beginning overnight around 0600-0900Z. Thus, the system is due for
another re-organization phase shortly if it holds true to form.
That may already be underway based on recent radar and recon data
showing a slight eastward shift in the center position. The highest
700-mb flight-level wind observed has been 63 kt, which reduces to
about a 57-kt surface wind. Jacksonville Doppler radar velocities
north and north-northwest of the center have been around 60 kt at
9,000 ft, which equals about 54 kt surface winds. The initial
intensity will be held at 60 kt, perhaps a little generously, for
this advisory despite the recent rise in the central pressure to
around 998 mb.
Isaias is still moving northward but a little faster at 360/11 kt.
The track forecast reasoning remains essentially the same as the
previous few advisories, with Isaias expected to gradually turn
toward the north-northeast later today and begin to accelerate by
this evening. The latest NHC model guidance is more tightly packed
about the previous advisory track, so only minor tweaks were
required. The new NHC track forecast lies very close to a blend of
the multi-model consensus aids TVCA, GFEX, TVCX, and NOAA-HCCA.
Current westerly vertical wind shear of 20-25 kt is expected to
decrease somewhat during the next 12 h and also become more
southwesterly, which will be in better alignment with Isaias'
forward motion vector. The decrease in the shear should result in
less tilt to the cyclone, allowing Isaias to strengthen and regain
hurricane status just before making landfall, and most of the
intensity guidance shows a 60-65 kt system at that time. After
landfall, Isaias is forecast to only slowly weaken due to
interaction with an unusually strong winter-type jetstream that will
be possessing winds of 100-120 kt. Expected strong baroclinic
forcing will keep Isaias' circulation intact and also produce very
strong wind gusts along the Mid-Atlantic states 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.
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 in the
Storm Surge Warning area. Residents in these areas should follow
advice given by local emergency officials.
2. Isaias is forecast to regain hurricane strength before it reaches
the coast of northeastern South Carolina and southern North
Carolina, and hurricane conditions are expected in the Hurricane
Warning area by this evening. Preparations should be rushed to
3. Isaias is expected to bring widespread sustained tropical storm
force winds and wind gusts to hurricane force to the mid-Atlantic
coast Tuesday, which could cause tree damage and power outages.
Tropical storm conditions are expected to reach southern New England
on late Tuesday and are possible along the northern New England
coast Tuesday night and early Wednesday.
4. Heavy rainfall will result in flash and urban flooding, some of
which may be significant in the eastern Carolinas and the
mid-Atlantic, through Tuesday night near the path of Isaias up the
East Coast of the United States. Widespread minor to moderate river
flooding is possible across portions of the Carolinas and the
FORECAST POSITIONS AND MAX WINDS
INIT 03/1500Z 30.7N 80.1W 60 KT 70 MPH
12H 04/0000Z 33.1N 79.2W 65 KT 75 MPH
24H 04/1200Z 37.2N 77.0W 55 KT 65 MPH...INLAND
36H 05/0000Z 42.0N 73.8W 50 KT 60 MPH...INLAND
48H 05/1200Z 46.7N 70.2W 35 KT 40 MPH...INLAND
60H 06/0000Z 50.2N 67.2W 30 KT 35 MPH...POST-TROP/EXTRATROP
72H 06/1200Z 53.5N 64.2W 25 KT 30 MPH...POST-TROP/EXTRATROP