Official Discussion issued by the National Hurricane Center

Isaias (AL092020) DATA RELEASED: 8/4/2020 3:00:00 PM UTC

Copy of official data

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.

Key Messages:

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 
afternoon.

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

$$
Forecaster Stewart