Tropical Storm Idalia Discussion Number  20
NWS National Hurricane Center Miami FL       AL102023
1100 AM EDT Thu Aug 31 2023

Satellite imagery, surface observations, and Air Force Reserve
Hurricane Hunter data all indicate that Idalia continues to tangle
with a frontal boundary that extends northward offshore the Outer
Banks.  In addition, organized deep convection has mostly dissipated
near the center, and it appears that Idalia is on its way to
becoming extratropical.  The initial intensity remains 50 kt based
on coastal wind reports and the reconnaissance data.

Global model fields show winds increasing on the northwestern side
of the frontal boundary during the day as Idalia is pulling away
from the coast of North Carolina, and as a result, the NHC intensity
forecast shows some strengthening in the short term.  Idalia is
expected to become fully extratropical by this evening, which is
now shown in the official forecast.  Winds should begin decreasing
tonight, and simulated satellite imagery from the GFS and ECMWF
models suggests that the circulation will continue to be devoid of
deep convection, and attached to a front, through at least 36
hours.  By 48 hours, the low will reach warmer waters, the
front is likely to weaken, and the ECMWF shows deep convection
redeveloping.  As a result, the official forecast shows Idalia
becoming a tropical storm again on Saturday.  Intensity-wise, the
guidance indicates that the peak winds are likely to fluctuate
between 40-45 kt from days 2 through 5.

Idalia is now moving eastward, or 090/17 kt, between a deep-layer
trough over the northeastern U.S. and a subtropical anticyclone
near the Bahamas.  The trough is likely to bypass Idalia in a few
days, which will cause its eastward motion to slow down to about 5
kt on days 2 and 3 as it approaches Bermuda.  On days 4 and 5, a
second trough is expected to turn Idalia to the northeast, but still
at a relatively slow forward speed.  The track guidance is in a bit
more agreement than the past few days, and the NHC forecast lies
between the TVCX and HCCA consensus aids.


KEY MESSAGES:

1. Rainfall will be diminishing by early afternoon from coastal
North Carolina into far southeast Virginia.  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 will continue across eastern North
Carolina within the Tropical Storm Warning area through this
evening.


FORECAST POSITIONS AND MAX WINDS

INIT  31/1500Z 33.6N  75.8W   50 KT  60 MPH
12H  01/0000Z 33.6N  73.2W   55 KT  65 MPH...POST-TROP/EXTRATROP
24H  01/1200Z 32.8N  70.1W   45 KT  50 MPH...POST-TROP/EXTRATROP
36H  02/0000Z 31.7N  68.1W   40 KT  45 MPH...POST-TROP/EXTRATROP
48H  02/1200Z 31.1N  67.3W   40 KT  45 MPH...TROPICAL CYCLONE
60H  03/0000Z 31.1N  66.6W   40 KT  45 MPH
72H  03/1200Z 31.8N  65.3W   45 KT  50 MPH
96H  04/1200Z 34.3N  62.0W   45 KT  50 MPH
120H  05/1200Z 37.0N  61.0W   45 KT  50 MPH

$$
Forecaster Berg