[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT45 KNHC 310253
TCDAT5

Tropical Storm Idalia Discussion Number  18
NWS National Hurricane Center Miami FL       AL102023
1100 PM EDT Wed Aug 30 2023

Idalia continues to have an asymmetric presentation on radar and
satellite imagery, in the presence of strong upper-level
southwesterly winds.  Surface observations indicate that the
circulation is somewhat elongated from southwest to northeast as the
system interacts with a coastal front, and the system is producing
very heavy precipitation over portions of South and North Carolina.
Assuming some slow weakening has taken place since this afternoon,
the intensity is set at 50 kt, although this may still be generous.

The initial motion estimate remains around 035/18 kt.  Idalia
continues to move between the northwestern periphery of a mid-level
anticyclone near the Bahamas and an eastward-moving mid-tropospheric
trough over the northeastern United States.  The dynamical guidance
indicates that the anticyclone will retrograde westward, and result
in Idalia moving eastward to east-southeastward on its northern side
for the next several days.  Although it seems likely that the
cyclone will move rather slowly in the 2- to 5-day time frame, the
details of its track are uncertain due to significant track model
differences in the latter part of the forecast period.  The official
forecast is similar to the previous NHC track and roughly in the
middle of the guidance.

Since Idalia is now fairly close to the coast with a lot of its
circulation over water, not much additional weakening is
anticipated.  However, the system's interaction with the coastal
front, along with the ongoing strong upper-level flow, suggest that
it will not be able to strengthen even after moving over the western
Atlantic waters.  This is also indicated by much of the intensity
guidance models.  As noted earlier, the system could lose organized
deep convection in a few days and degenerate into a post-tropical
cyclone, as suggested by the GFS model-simulated satellite imagery.
For now, however, the official forecast again retains the system as
a tropical storm for the entire forecast period.


KEY MESSAGES:

1. Storm surge is expected along the southeastern U.S. coast within
the Storm Surge Warning area through tonight.  Coastal flooding is
also expected within the Storm Surge Watch area in North Carolina on
Thursday.  Residents in these areas should follow any advice given
by local officials.

2. Tropical-storm-force winds will affect portions of the
southeastern U.S. coast through Thursday.

3. Areas of flash, urban, and moderate river flooding, with
considerable impacts, are expected from eastern South Carolina
through eastern North Carolina into Thursday.



FORECAST POSITIONS AND MAX WINDS

INIT  31/0300Z 33.0N  80.0W   50 KT  60 MPH...INLAND
 12H  31/1200Z 33.9N  77.4W   50 KT  60 MPH...OVER WATER
 24H  01/0000Z 33.9N  73.8W   50 KT  60 MPH
 36H  01/1200Z 33.2N  70.9W   45 KT  50 MPH
 48H  02/0000Z 32.3N  69.0W   45 KT  50 MPH
 60H  02/1200Z 31.6N  68.2W   45 KT  50 MPH
 72H  03/0000Z 31.4N  67.6W   45 KT  50 MPH
 96H  04/0000Z 32.0N  65.4W   45 KT  50 MPH
120H  05/0000Z 33.5N  63.0W   45 KT  50 MPH

$$
Forecaster Pasch
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