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Hurricane Sam Forecast Discussion


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000
WTNT43 KNHC 282040
TCDAT3

Hurricane Sam Discussion Number  25
NWS National Hurricane Center Miami FL       AL182021
500 PM AST Tue Sep 28 2021

Sam has looked a little less impressive on satellite imagery this 
afternoon, particularly on the infrared channels where the eye is 
not very well-defined.  However, observations from the Air Force 
Hurricane Hunters indicate that the hurricane is maintaining 
category 4 intensity since a blend of the latest flight-level and 
SFMR-observed surface winds support maintaining 115-kt winds.  The 
aircraft also measured a quite low central pressure of 947 mb on 
two passes through the eye.  Upper-level outflow remains well 
defined except over the southwest quadrant of the circulation, and 
several arc clouds are seen emanating from the system over the 
western semicircle, indicative of some dry mid-level air in the 
environment.  However, this dry air is apparently not significantly 
affecting the inner core of Sam, given that it has remained a 
powerful hurricane.

Center fixes yield a continued slow northwestward motion of around 
305/8 kt.  There is little change to the track forecast reasoning 
from the previous advisories.  Sam is likely to move around the 
southwestern periphery of a subtropical ridge for the next couple of 
days.  A turn toward the north, ahead of a large mid-tropospheric 
trough moving off the United States east coast, is forecast around 
day 3.  Later in the forecast period, the hurricane is expected to 
accelerate northeastward within the flow on the eastern side of the 
trough.  There is good agreement among most of the guidance models 
on this general track, and the official forecast is very close 
to the model consensus.

Since Sam will remain in a low-shear environment and over a warm 
ocean surface for the next few days, some re-intensification is 
possible, and the system is likely to remain a major hurricane 
for the next 4 days or so.  The official intensity forecast is not 
much different from the latest NOAA corrected consensus forecast, 
HCCA.


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  28/2100Z 17.9N  55.0W  115 KT 130 MPH
 12H  29/0600Z 18.6N  55.9W  120 KT 140 MPH
 24H  29/1800Z 19.7N  57.4W  115 KT 130 MPH
 36H  30/0600Z 21.0N  59.1W  115 KT 130 MPH
 48H  30/1800Z 22.8N  60.6W  110 KT 125 MPH
 60H  01/0600Z 25.0N  61.6W  110 KT 125 MPH
 72H  01/1800Z 27.7N  62.0W  110 KT 125 MPH
 96H  02/1800Z 33.5N  59.0W  100 KT 115 MPH
120H  03/1800Z 40.0N  53.5W   90 KT 105 MPH

$$
Forecaster Pasch