Atlantic Hurricane and Tropical Cyclone Track Updates

Hurricane Sam Forecast Discussion

WTNT43 KNHC 020843

Hurricane Sam Discussion Number  39
NWS National Hurricane Center Miami FL       AL182021
500 AM AST Sat Oct 02 2021

The Air Force Reserve Hurricane Hunters conducted their final
mission into Sam a few hours ago and found that the hurricane is
still of category 4 intensity, but the maximum winds have decreased
some.  The crew reported a peak 700-mb flight-level wind of 130 kt
(equating to surface winds of 115-120 kt) and unflagged SFMR winds
of 110-115 kt.  Based on these wind data, Sam's intensity is now
estimated to be 115 kt.  This value is at the upper end of the 
latest satellite estimates, which range from 100-115 kt.  The 
hurricane's central pressure has also risen to 945 mb.

Sam is still moving toward the north-northeast with a motion of
020/15 kt.  The hurricane is entering the area between a deep-layer
ridge to the east and a large mid- to upper-level low over Atlantic
Canada, and this flow should cause Sam to turn toward the northeast
by tonight and then maintain that general heading for much of the
forecast period.  The storm is also expected to accelerate,
reaching a peak forward speed of more than 25 kt in 60-72 hours.  
The track guidance is tightly clustered through day 3, but there is
significantly more spread on days 4 and 5 due to uncertainty on
exactly how Sam will interact with the aforementioned
mid-/upper-level low.  The NHC forecast has been placed near a
blend of the GFS-ECMWF mean and the HCCA consensus aid, which
necessitated a northward shift from the previous forecast only on
days 4 and 5.

Deep-layer southerly shear of 15-20 kt appears to be contributing
to Sam's current weakening.  Continued shear and cooler waters
along Sam's path should lead to additional weakening in the coming
days, although not at a rapid rate due to some baroclinic forcing.
Global models suggest that Sam will begin extratropical transition
in about 48 hours, with that process completing by 72 hours.  The
NHC intensity forecast closely follows the HCCA and IVCN consensus
aids during Sam's tropical phase, but then transitions to a blend of
the GFS and ECMWF global model guidance on days 3 through 5 during
its extratropical phase.  Sam is expected to continue producing
hurricane-force winds through at least day 3, with more significant
weakening occurring on days 4 and 5 once it is a vertically stacked
occluded low.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward Islands
the Greater Antilles, the Bahamas, and Bermuda during the next
couple of days.  Swells are expected to reach the United States
east coast and Atlantic Canada today.  These swells could cause
life-threatening surf and rip current conditions.  Please consult
products from your local weather office.

2. Although the Tropical Storm Warning for Bermuda has been
discontinued, a few wind gusts to tropical storm force will still
be possible on the island during the morning.


INIT  02/0900Z 32.8N  60.5W  115 KT 130 MPH
 12H  02/1800Z 34.6N  59.3W  100 KT 115 MPH
 24H  03/0600Z 36.6N  57.1W   95 KT 110 MPH
 36H  03/1800Z 38.4N  54.2W   90 KT 105 MPH
 48H  04/0600Z 40.7N  50.0W   85 KT 100 MPH
 60H  04/1800Z 44.4N  44.8W   80 KT  90 MPH
 72H  05/0600Z 48.9N  40.7W   75 KT  85 MPH...POST-TROP/EXTRATROP
 96H  06/0600Z 53.4N  34.7W   55 KT  65 MPH...POST-TROP/EXTRATROP
120H  07/0600Z 58.7N  26.4W   45 KT  50 MPH...POST-TROP/EXTRATROP

Forecaster Berg

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