241
WTNT45 KNHC 310843
TCDAT5

Tropical Storm Idalia Discussion Number 19
NWS National Hurricane Center Miami FL AL102023
500 AM EDT Thu Aug 31 2023

Surface observations from around 0600 UTC showed that the center of
Idalia moved offshore of the coast of northeastern South Carolina.
Strong southwesterly vertical wind shear has displaced the deep
convective bands and heavy rainfall well north and northeast of the
center over eastern North Carolina and the adjacent Atlantic. A
partial ASCAT-B overpass over the southeastern portion of the
circulation revealed winds of 40-45 kt and given that instrument's
typical undersampling, the initial wind speed is maintained at
50 kt for this advisory.

Idalia has turned east-northeastward between a mid-level
anticyclone over the Bahamas and a mid-tropospheric trough moving
eastward over the northeastern United States. The anticyclone is
forecast to retrograde over Florida during the next couple of days,
causing Idalia to turn east-southeastward tonight and Friday.
After that time, steering currents are predicted to weaken and the
cyclone is expected to move very slowly to the southwest of
Bermuda during the weekend. By early next week, another
mid-latitude trough moving off the northeast U.S. coast should
cause Idalia to begin moving northeastward, however there is still
significant uncertainty in the details of the track forecast
later in the period.

Little change in strength is predicted today, but strong
upper-level winds and drier mid-level air are likely to cause some
gradual weakening by the weekend. Simulated satellite imagery from
the GFS and ECMWF models suggest that the system could lose
organized deep convection as soon as tonight, and Idalia could
degenerate into a post-tropical cyclone in a day or so. However,
the ECMWF does show a return of deep convection over the weekend so
the official forecast continues to retain the system as a tropical
storm throughout the foreast period.


KEY MESSAGES:

1. Areas of flash, urban, and moderate river flooding, with
considerable impacts, will continue across coastal North Carolina
through today.

2. Coastal flooding is expected within the Storm Surge Watch
area in North Carolina today. Residents in these areas should
follow any advice given by local officials.

3. Tropical storm conditions are expected in northeastern South
Carolina and portions of eastern North Carolina today.


FORECAST POSITIONS AND MAX WINDS

INIT 31/0900Z 33.6N 78.0W 50 KT 60 MPH
12H 31/1800Z 34.0N 75.1W 50 KT 60 MPH
24H 01/0600Z 33.5N 71.7W 45 KT 50 MPH
36H 01/1800Z 32.4N 69.1W 45 KT 50 MPH
48H 02/0600Z 31.5N 68.0W 40 KT 45 MPH
60H 02/1800Z 31.2N 67.5W 40 KT 45 MPH
72H 03/0600Z 31.4N 66.6W 40 KT 45 MPH
96H 04/0600Z 33.1N 63.8W 40 KT 45 MPH
120H 05/0600Z 35.6N 61.4W 45 KT 50 MPH

$$
Forecaster Brown


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