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Hurricane Sam Forecast Discussion


813 
WTNT43 KNHC 050232
TCDAT3

Hurricane Sam Discussion Number  50
NWS National Hurricane Center Miami FL       AL182021
300 AM GMT Tue Oct 05 2021

Sam's extratropical transition is well underway.  The central core
convection has decreased over the past several hours, with the
telltale elongated cloud pattern forming on the northwestern side
of the circulation.  Still, AMSU-microwave data just before 00Z was 
very clear it is still has a healthy warm core, so advisories will
continue.  The initial wind speed is decreased to 75 kt, a bit
above the satellite estimates.

An approaching mid-latitude trough should cause Sam to quickly
transition into a powerful extratropical cyclone within the next
6-12 hours while maintaining hurricane-force winds for about the
next day.  A slow weakening is expected after that time as Sam loses
its baroclinic forcing.  The only significant change from the
last advisory is that models are in better agreement that Sam will
become absorbed by another extratropical low in about 3 days,
rather than be the dominant center, so the 96 h forecast is now
dissipated.

Sam is moving north-northeastward or 030/25 kt.  The system is
forecast to slow down as it merges with the mid-latitude trough. By
late Tuesday, the post-tropical cyclone should turn east-
northeastward before it moves northward and northwestward around the
eastern portion of another trough/cut-off low over the north
Atlantic.  The latest NHC forecast is very close to the last one and
the model consensus.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward
Islands, the Greater Antilles, portions of the Bahamas, and
southeastern Newfoundland through midweek.  These swells
could cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.


FORECAST POSITIONS AND MAX WINDS

INIT  05/0300Z 47.7N  40.2W   75 KT  85 MPH
 12H  05/1200Z 50.6N  39.3W   70 KT  80 MPH...POST-TROP/EXTRATROP
 24H  06/0000Z 51.0N  38.2W   65 KT  75 MPH...POST-TROP/EXTRATROP
 36H  06/1200Z 51.5N  33.3W   55 KT  65 MPH...POST-TROP/EXTRATROP
 48H  07/0000Z 54.0N  28.1W   50 KT  60 MPH...POST-TROP/EXTRATROP
 60H  07/1200Z 58.0N  22.9W   45 KT  50 MPH...POST-TROP/EXTRATROP
 72H  08/0000Z 61.5N  25.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  09/0000Z...DISSIPATED

$$
Forecaster Blake




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