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Tropical Storm Isaias Discussion Number 30
NWS National Hurricane Center Miami FL AL092020
1100 AM EDT Tue Aug 04 2020
Doppler radar data from Dover, Delaware (KDOX), and Ft. Dix, New
Jersey (KDIX) this morning continue to indicate 70-80 kt winds
between 4000-7000 ft over the ocean just offshore the Delmarva
peninsula and the southeastern coast of New Jersey, which would
normally correspond to 65-70 kt surface winds. However, these
velocity values have been collocated with only 15-25 dBZ
reflectivity echoes, so the typical 80-85 percent reduction factors
likely don't apply. Sustained observed surface winds of 50-55 kt
have been reported, so the initial intensity will be held at 60 kt,
but that is only for winds over the ocean and near the coast.
Isaias is moving north-northeastward or 020/30 kt. The cyclone will
continue to accelerate north-northeastward today and tonight within
strong southwesterly flow ahead of a deep-layer trough and
associated cold front approaching the mid-Atlantic and New England
areas from the west. By 24 h, the Isaias is expected to slow down
and degenerate into a post-tropical cyclone over Maine before it is
absorbed by a larger extratropical low located over southeastern
Canada. The new NHC track forecast is just an extension of the
previous advisory, and lies very close to the tightly clustered
consensus track models.
As Isaias moves north-northeastward through the Mid-Atlantic coast,
interaction with a strong upper-level jet maximum is forecast to
maintain the tropical storm's intensity longer than what typically
would be expected for inland decaying tropical cyclone. The global
models continue to indicate 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 remains above the standard
20-percent value in the Forecast/Advisory Product (TCMAT4).
In addition to the wind threat, 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 also remains a significant risk of tornadoes across the
mid-Atlantic coast and southeastern New York this afternoon,
possibly spreading into southern New England tonight.
1. Widespread sustained tropical storm force winds and gusts to
hurricane force are expected along 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 tonight.
2. Heavy rainfall along the East Coast, near the path of Isaias,
will result in flash flooding, some of which may be significant in
the mid-Atlantic and Northeast through tonight. Potentially
life-threatening urban flooding remains possible in Philadelphia
and elsewhere along and just west of the I-95 corridor today.
Scattered minor to moderate river flooding is likely across portions
of the and the Mid-Atlantic. Quick-responding rivers in Northeast
will also be susceptible to minor river flooding.
3. Numerous tornadoes have already occurred over portions of the
mid-Atlantic coast this morning. The threat of tornadoes will
continue along the mid-Atlantic coast spread into New England this
FORECAST POSITIONS AND MAX WINDS
INIT 04/1500Z 39.1N 76.1W 60 KT 70 MPH...INLAND
12H 05/0000Z 43.2N 73.8W 50 KT 60 MPH...INLAND
24H 05/1200Z 48.2N 71.7W 35 KT 40 MPH...POST-TROP/INLAND
36H 06/0000Z 52.0N 71.1W 30 KT 35 MPH...POST-TROP/EXTRATROP
48H 06/1200Z 53.6N 69.5W 20 KT 25 MPH...POST-TROP/EXTRATROP
60H 07/0000Z...DISSIPATED INLAND