Atlantic Hurricane Feed

Hurricane Sam Forecast Discussion

WTNT43 KNHC 280234

Hurricane Sam Discussion Number  22
NWS National Hurricane Center Miami FL       AL182021
1100 PM AST Mon Sep 27 2021

The NOAA Hurricane Hunters have been flying through Sam tonight, 
with radar and wind observations showing broken concentric eyewalls 
at times.  The maximum 700-mb flight-level winds on the mission 
were 114 kt, with peak SFMR values to 103 kt and a central pressure 
around 956 mb, so the initial wind speed will remain 105 kt.  While 
the overall intensity doesn't seem to have changed much in the past 
several hours, the aircraft data show that the hurricane has grown 
in size, and that is reflected in the initial wind radii.

The future intensity of Sam is hard to pinpoint.  Eyewall cycles 
will likely play a key role in determining its second peak intensity 
since the hurricane should remain in low- or moderate-shear and 
warm-water environments for the next few days.  Those cycles are 
basically impossible to forecast, but there is some suggestion on 
satellite that a larger eye is trying to form, which could allow for 
some increase in strength in the near term.  It seems likely that 
Sam will be a category 3 or 4 hurricane given the conducive 
environment for most of the forecast period, so the new NHC forecast 
is similar to the previous one, with the most significant change 
showing a larger hurricane consistent with the latest guidance.  

Sam continues to move northwestward, or 315/8 kt.  The hurricane 
should move in that direction for the next few days around the 
southwestern portion of the subtropical high.  A turn to the north 
is likely as Sam meets the western edge of the high, with a gradual 
acceleration to the northeast anticipated by the weekend due to an 
incoming mid-latitude trough from the west.  While there are some 
speed differences in the models, especially near and after 
recurvature, the NHC forecast leans toward the faster GFS solution 
over the slower ECMWF, given the magnitude of the strong 
southwesterly flow that Sam should encounter at long range, plus a 
nod to the superior performance of late for the American model.  The 
new forecast is adjusted a bit to the right and faster beyond day 3, 
with no significant changes made before that time.

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.


INIT  28/0300Z 16.8N  53.2W  105 KT 120 MPH
 12H  28/1200Z 17.5N  54.0W  115 KT 130 MPH
 24H  29/0000Z 18.4N  55.1W  115 KT 130 MPH
 36H  29/1200Z 19.3N  56.4W  110 KT 125 MPH
 48H  30/0000Z 20.6N  58.0W  110 KT 125 MPH
 60H  30/1200Z 22.2N  59.7W  110 KT 125 MPH
 72H  01/0000Z 24.3N  61.0W  110 KT 125 MPH
 96H  02/0000Z 29.6N  61.5W  105 KT 120 MPH
120H  03/0000Z 37.5N  56.9W  100 KT 115 MPH

Forecaster Blake

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