[Tropical] Tropical Cyclone Discussion

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WTNT45 KNHC 011438
TCDAT5

Post-Tropical Cyclone Idalia Discussion Number  24
NWS National Hurricane Center Miami FL       AL102023
1100 AM AST Fri Sep 01 2023

Idalia is not producing deep convection and remains an extratropical
cyclone, with the strongest winds located along the northwestern
side of the attached frontal boundary.  The initial intensity
remains 50 kt based on continuity from the previous advisory.
Scatterometer should hit the area of strongest winds later this
morning and give us a better idea on the intensity.

The cyclone may be finally slowing down a bit, and the initial
motion estimate is east-southeastward (110 degrees) at 15 kt.
Additional deceleration is expected during the next 36 hours while
the center turns eastward and moves near or to the south of
Bermuda.  A shortwave trough is forecast to move off the coast of
New England by early Sunday (about 48 hours), and that feature
should turn Idalia toward the northeast at a faster speed by day 3.
Even then, there is disagreement among the models whether this
trough will pull Idalia quickly northward (e.g., the ECMWF) or if
it will leave Idalia behind (e.g, the GFS).  The NHC track forecast
therefore shows a slower north or north-northeast motion on days 4
and 5, leaning toward the various consensus aids.

There remains significant uncertainty on what type of cyclone
Idalia will be during the next 3 days.  On one hand, simulated
satellite imagery from the GFS and ECMWF models shows deep
convection redeveloping near Idalia's center on Saturday once the
cyclone reaches warmer waters.  On the other hand, global model
fields show the thickness gradient near the front weakening, but
they don't necessarily show Idalia shedding all of its frontal
features.  To maintain continuity from previous forecasts, the NHC
forecast shows Idalia regaining tropical storm status in about 24
hours while it is near Bermuda, and some reintensification is
possible after that time.  Model fields show the temperature
gradient tightening again as Idalia goes through a possible
occlusion in 3-4 days, and the forecast therefore shows an
extratropical cyclone again on day 4.

Based on the updated forecast, the Bermuda Weather Service has
issued a Tropical Storm Warning for the island.


KEY MESSAGES:

1. Tropical storm conditions are expected in Bermuda by Saturday.
Heavy rainfall will likely cause areas of flash and urban flooding
on the island.


FORECAST POSITIONS AND MAX WINDS

INIT  01/1500Z 31.9N  67.9W   50 KT  60 MPH...POST-TROP/EXTRATROP
 12H  02/0000Z 31.3N  66.3W   45 KT  50 MPH...POST-TROP/EXTRATROP
 24H  02/1200Z 30.9N  64.9W   40 KT  45 MPH...TROPICAL CYCLONE
 36H  03/0000Z 31.1N  63.7W   45 KT  50 MPH
 48H  03/1200Z 32.0N  61.8W   50 KT  60 MPH
 60H  04/0000Z 33.5N  59.9W   50 KT  60 MPH
 72H  04/1200Z 35.1N  58.2W   45 KT  50 MPH
 96H  05/1200Z 38.3N  57.2W   40 KT  45 MPH...POST-TROP/EXTRATROP
120H  06/1200Z 40.1N  57.1W   35 KT  40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Berg
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