Atlantic Basin Tropical Cyclones

Hurricane Sam Forecast Discussion


191 
WTNT43 KNHC 042031
TCDAT3

Hurricane Sam Discussion Number  49
NWS National Hurricane Center Miami FL       AL182021
900 PM GMT Mon Oct 04 2021

Sam's cloud pattern has finally begun to degrade as it moves over
the cooler waters of the north Atlantic. The cloud tops have
warmed and the eye is no longer discernible in infrared imagery,
but banding features still remain well defined in all quadrants.
Dvorak T- and CI-numbers from both SAB and TAFB have continued to
decline and a blend of the latest subjective and objective
estimates yields an initial intensity estimate of 80 kt for this 
advisory.  Scatterometer data that arrived just after the release 
of the previous advisory showed that Sam's wind field has continued 
to expand and the latest radii estimates are based on those data.

Sam should gradually weaken this evening as it moves over
progressively colder waters and into an area of strong southwesterly
upper-level winds.  However, an approaching mid-latitude trough
should result in the system's quick transition into a powerful
extratropical cyclone overnight.  The post-tropical cyclone is 
forecast to maintain hurricane-force winds for until late Tuesday, 
but gradual weakening should occur after that time as the system 
occludes and baroclinic forcing decreases. It is not entirely 
clear as to whether Post-Tropical Sam will remain the dominant low 
(GFS solution) or if it will be absorbed by another extratropical 
low (ECMWF solution) later in the week.  As a compromise, the new 
official forecast calls for absorbed by day 5 which is between 
the solutions from those typically reliable global models. 

Sam is moving briskly northeastward or 035/31 kt.  The cyclone is
currently caught in strong southwesterly flow ahead of the
aforementioned mid-latitude trough and a rapid northeastward motion
is expected overnight.  After that time, the post-tropical cyclone
is forecast to slow down as it merges with the cut-off low.  By 
late Tuesday, the post-tropical cyclone should turn east- 
northeastward before it rotates cyclonically around the eastern 
portion of another trough/cut off low over the north Atlantic.  The 
latest NHC forecast is a blend of the GFS and ECMWF models and 
their respective ensemble means.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward
Islands, the Greater Antilles, portions of the Bahamas, and
southeastern Newfoundland through early Tuesday.  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  04/2100Z 45.1N  42.5W   80 KT  90 MPH
 12H  05/0600Z 49.0N  39.8W   70 KT  80 MPH...POST-TROP/EXTRATROP
 24H  05/1800Z 50.9N  39.9W   65 KT  75 MPH...POST-TROP/EXTRATROP
 36H  06/0600Z 50.5N  36.0W   55 KT  65 MPH...POST-TROP/EXTRATROP
 48H  06/1800Z 52.5N  30.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
 60H  07/0600Z 56.5N  25.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
 72H  07/1800Z 60.0N  25.5W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  08/1800Z 60.0N  32.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  09/1800Z...DISSIPATED

$$
Forecaster Brown




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