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Hurricane Sam Forecast Discussion

WTNT43 KNHC 250234

Hurricane Sam Discussion Number  10
NWS National Hurricane Center Miami FL       AL182021
1100 PM AST Fri Sep 24 2021

Sam developed a 10-n mi wide eye in infrared satellite images a 
few hours ago, surrounded by a ring of cloud tops as cold as about 
-70 degrees Celsius.  The core of the hurricane remains compact.  
In fact, recent Synthetic Aperture Radar (SAR) data indicated that 
the radius of maximum winds is only about 6 n mi.  Dvorak estimates 
were T5.0/90 kt from TAFB and T4.5/77 kt from SAB at 0000 UTC, and 
given the additional clearing of the eye since then, Sam's intensity 
is raised to 85 kt.

Sam has been moving toward the west-northwest during the past 6 to 
12 hours with a motion of 285/12 kt.  The track forecast reasoning 
remains the same, and the model guidance is packed more tightly 
than normal through day 5.  A blocking ridge lying to the north and 
northwest is expected to cause Sam to slow down over the weekend 
and maintain a motion toward the west or west-northwest.  On days 3 
through 5, the ridge is expected to shift eastward, with a 
deep-layer trough developing over the western Atlantic.  This 
should allow Sam to turn northwestward toward the end of the 
forecast period.  The NHC track forecast has not budged much and 
remains close to the TVCA consensus aid.  Based on this forecast, 
and nearly all of the reliable guidance, Sam is expected to still 
be well to the east or northeast of the northern Leeward Islands 
through day 5.

Deep-layer shear is expected to be less than 10 kt for the next day 
or two, and Sam will be heading toward waters of higher oceanic 
heat content.  These conditions favor continued strengthening, and 
the hurricane is forecast to become a major hurricane on Saturday 
and possibly reach a peak intensity in 2 to 3 days.  For this 
period, the NHC forecast is close to the HCCA consensus aid and 
is near the high end of the guidance envelope.  As is usually the 
case with hurricanes this small and strong, fluctuations in 
intensity due to eyewall replacement cycles will be possible.  On 
days 3 through 5, some increase in shear as Sam approaches the 
deep-layer western Atlantic trough is possible, and modest weakening 
is shown at those times.  


INIT  25/0300Z 12.4N  45.9W   85 KT 100 MPH
 12H  25/1200Z 12.7N  47.4W  100 KT 115 MPH
 24H  26/0000Z 13.1N  48.8W  110 KT 125 MPH
 36H  26/1200Z 13.5N  50.0W  115 KT 130 MPH
 48H  27/0000Z 14.1N  51.2W  120 KT 140 MPH
 60H  27/1200Z 14.8N  52.4W  120 KT 140 MPH
 72H  28/0000Z 15.7N  53.5W  115 KT 130 MPH
 96H  29/0000Z 17.4N  55.8W  110 KT 125 MPH
120H  30/0000Z 19.5N  58.8W  110 KT 125 MPH

Forecaster Berg

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