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Hurricane Sam Forecast Discussion


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WTNT43 KNHC 290838
TCDAT3

Hurricane Sam Discussion Number  27
NWS National Hurricane Center Miami FL       AL182021
500 AM AST Wed Sep 29 2021

The eye of Sam has become cloud filled in infrared satellite
imagery overnight, but the ring of deep convection surrounding the
eye has cooled. There appears to be a southwest-to-northeast tilt to
Sam's circulation as some modest southwesterly shear seems to be
impinging on the hurricane.  An Air Force Reserve Hurricane
Hunter aircraft that has been investigating Sam overnight shows
that the hurricane continues to fluctuate in intensity.  The
aircraft has reported a peak flight-level wind of 124 kt and SFMR
winds of 108 kt.  Therefore the initial wind speed has been set at
115 kt, which leans toward the higher flight-level-to-surface wind
reduction.  The aircraft also reported a minimum pressure of 950 mb,
which is up a few millibars from the flight yesterday afternoon.  
The fluctuations in intensity that Sam has experienced over the 
past couple of days are fairly typical for hurricanes of this 
strength.

As mentioned in the previous advisory, Sam will be moving into a
lower vertical wind shear environment, and the expected faster
forward speed decreases the potential for upwelling beneath the
storm.  As a result, some re-intensification is forecast, but
difficult-to-predict eyewall replacement cycles could cause
additional fluctuations in intensity over the next day or two.
After 60 to 72 hours, a more pronounced weakening trend is likely
to begin as the storm moves over cooler SSTs and the vertical
shear increases. The global models indicate that Sam will begin its
transition into a powerful extratropical cyclone late in the
period, but that transition will likely not be completed until
after 120 h.

Sam is moving northwestward at about 8 kt. The track guidance
remains in excellent agreement through 96 hours as Sam should move
northwestward and then northward around the western portion of the
subtropical ridge over the eastern Atlantic during the next few
days. By Saturday, Sam is forecast to turn northeastward between
the aforementioned ridge and a deep-layer trough over the
northeastern U.S. and Nova Scotia.  By late in the period, there
is an increase in spread in the guidance which is related to how 
Sam interacts with the deep-layer trough.  For now, the NHC track
forecast at days 4 and 5 lies between the TVCA multi-model 
consensus and the HFIP corrected consensus aid.  Overall, little
change to the previous NHC track forecast was required for this
advisory.


Key Messages:

1. Large swells generated by Sam are affecting the Leeward Islands
and will spread to portions of the Virgin Islands, Puerto Rico,
Hispaniola, the Bahamas, and Bermuda by Thursday or Friday.
Significant swells will likely reach the east coast of the United
States and Atlantic Canada by the weekend.  These swells will likely
cause life-threatening surf and rip current conditions, and
beachgoers and other interests along these coasts are urged to
follow the advice of lifeguards and local officials through the
upcoming weekend.


FORECAST POSITIONS AND MAX WINDS

INIT  29/0900Z 18.9N  56.2W  115 KT 130 MPH
 12H  29/1800Z 19.7N  57.3W  120 KT 140 MPH
 24H  30/0600Z 21.0N  58.9W  120 KT 140 MPH
 36H  30/1800Z 22.8N  60.4W  120 KT 140 MPH
 48H  01/0600Z 25.2N  61.5W  115 KT 130 MPH
 60H  01/1800Z 27.9N  62.0W  115 KT 130 MPH
 72H  02/0600Z 30.8N  61.5W  110 KT 125 MPH
 96H  03/0600Z 35.8N  57.5W   95 KT 110 MPH
120H  04/0600Z 40.8N  52.0W   80 KT  90 MPH

$$
Forecaster Brown



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