Atlantic Basin Tropical Cyclones

Hurricane Sam Forecast Discussion


000
WTNT43 KNHC 021451
TCDAT3

Hurricane Sam Discussion Number  40
NWS National Hurricane Center Miami FL       AL182021
1100 AM AST Sat Oct 02 2021

Sam has been an impressive hurricane during the past several days.  
The latest satellite imagery shows it remains a powerful system, 
with a well-defined eye and symmetric cloud pattern.  In fact, 
since the plane left overnight, the eye has actually warmed, with 
little change in the eyewall convection.  Thus the initial intensity 
will stay 115 kt, above the latest satellite classifications, but 
consistent with the earlier aircraft data and low bias of the 
Dvorak technique for much of the this storm.  Sam is now in the top 
10 for consecutive days as a category 4 hurricane or greater in 
the historical record, about the same as Matthew 2016.

The hurricane is forecast to gradually lose strength during the 
next 36 hours or so while it remains over marginally warm waters, 
but in fairly light shear.  In fact, some of the guidance decrease 
the shear overnight, which should allow Sam to keep much of its 
strength, provided it doesn't undertake an eyewall cycle.  The new 
intensity forecast is higher in the first day or so, consistent 
with the latest model solutions.  In about 2 days, Sam will cross 
the north wall of the Gulf Stream, and in 3 days is expected to 
spectacularly transition into a large hurricane-force post-tropical 
cyclone over the far north Atlantic.  No changes were made to the 
end of the intensity forecast.

Sam has turned northeastward at about 15 kt.  The hurricane should 
accelerate later this weekend due to increasing flow between a 
deep-layer ridge to the east and a large mid- to upper-level low 
over Atlantic Canada.  By midweek the system is forecast to turn 
more to the north-northeast as it becomes a spoke in a very large 
extratropical low over the far North Atlantic.  The new forecast has 
shifted somewhat eastward during the first couple of days but ends 
up very near the last advisory by day 5. 

Key Messages:

1. Swells generated by Sam will impact the northern Leeward Islands,
the Greater Antilles, the Bahamas, Bermuda, the eastern United 
States coast and Atlantic Canada for the next couple of days.  
These swells could cause life-threatening surf and rip current 
conditions.  Please consult products from your local weather office.


FORECAST POSITIONS AND MAX WINDS

INIT  02/1500Z 33.9N  59.3W  115 KT 130 MPH
 12H  03/0000Z 35.6N  57.8W  105 KT 120 MPH
 24H  03/1200Z 37.4N  55.3W  100 KT 115 MPH
 36H  04/0000Z 39.2N  52.0W   95 KT 110 MPH
 48H  04/1200Z 41.8N  47.3W   85 KT 100 MPH
 60H  05/0000Z 46.0N  42.0W   80 KT  90 MPH
 72H  05/1200Z 50.0N  38.5W   75 KT  85 MPH...POST-TROP/EXTRATROP
 96H  06/1200Z 53.0N  33.0W   55 KT  65 MPH...POST-TROP/EXTRATROP
120H  07/1200Z 59.0N  26.0W   45 KT  50 MPH...POST-TROP/EXTRATROP

$$
Forecaster Blake



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