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Hurricane Sam Forecast Discussion


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WTNT43 KNHC 290234
TCDAT3

Hurricane Sam Discussion Number  26
NWS National Hurricane Center Miami FL       AL182021
1100 PM AST Tue Sep 28 2021

Satellite images suggest that Sam has strengthened.  The eyewall of 
the hurricane has become more intense, with a warmer eye noted 
during the past several hours since the last reconnaissance aircraft 
departed.  The initial wind speed is nudged upward to 120 kt, a 
little above what the previous aircraft mission from this afternoon 
supported.  Another Air Force Reserve Hurricane Hunter plane is 
scheduled to fly through Sam in a few hours for a better look at the 
intensity.

Sam is forecast to move into an area with somewhat lower wind shear 
and higher ocean heat content by late Wednesday.  In addition, the 
hurricane will likely be moving a little faster, which decreases the 
upwelling potential under the storm.  These factors lead me to 
believe Sam isn't quite done intensifying, and could reach another 
peak sometime late Wednesday or Thursday.  The one fly-in-the- 
ointment to this forecast is another eyewall replacement cycle 
potentially beginning, although recent microwave data doesn't give 
that impression.  Thus, the new forecast is raised from the last 
one during the first couple of days, on the higher side of the 
guidance.  A more consistent weakening trend is expected late week, 
due to cooler SSTs and increasing shear, and no significant 
changes were made to the official forecast at long range.  

The hurricane continues moving northwestward at about 8 kt.  Models 
are locked into this track continuing for the next day or two at a 
faster pace around the southwestern portion of a strengthening 
Atlantic subtropical ridge.  Thereafter, Sam should turn northward 
on Friday and northeastward this weekend, likely a fair distance 
east of Bermuda, due to a large mid-latitude trough moving off of 
the United States east coast.  Model guidance remains in close 
agreement on almost all of the forecast, and the only significant 
change from the last advisory is an eastward adjustment on Day 5 due 
to guidance suggesting that Sam gets shunted more eastward by the 
trough, rather than captured by it.  


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  29/0300Z 18.4N  55.6W  120 KT 140 MPH
 12H  29/1200Z 19.1N  56.7W  120 KT 140 MPH
 24H  30/0000Z 20.2N  58.2W  125 KT 145 MPH
 36H  30/1200Z 21.8N  59.9W  125 KT 145 MPH
 48H  01/0000Z 23.9N  61.2W  120 KT 140 MPH
 60H  01/1200Z 26.4N  61.9W  115 KT 130 MPH
 72H  02/0000Z 29.1N  61.8W  110 KT 125 MPH
 96H  03/0000Z 35.0N  58.0W  100 KT 115 MPH
120H  04/0000Z 40.0N  52.0W   85 KT 100 MPH

$$
Forecaster Blake