Atlantic Basin Tropical Cyclones

Hurricane Sam Forecast Discussion


000
WTNT43 KNHC 022037
TCDAT3

Hurricane Sam Discussion Number  41
NWS National Hurricane Center Miami FL       AL182021
500 PM AST Sat Oct 02 2021

The hurricane is gradually losing strength.  Satellite images show 
that the eye has become more cloud filled, with Sam's ring of deep 
convection becoming less intense and two main towers in the 
southeast and northwest quadrants.  Still, the tropical cyclone is 
quite a sight even on full-disk images with a large distinctive 
cloud pattern. The latest intensity estimates have decreased, so 
the initial wind speed is set to 110 kt, with higher uncertainty 
than average in this figure due to recent recon/satellite 
differences. 

Sam wisely has avoided much shear during its 10-day journey across 
the Atlantic, and it could remain a major hurricane for another 
day or so.  However, in 36 hours or so, the system should move 
north of the Gulf Stream, which would normally cause a dramatic 
weakening.  But in this case, a favorable mid-latitude trough 
interaction should cause Sam to transition into a large and 
powerful extratropical low by 60 hours.  After that, it loses its 
baroclinic forcing and should gradually spin down over the far North 
Atlantic southwest of Iceland.  Guidance is in fairly solid 
agreement on this scenario, and the new intensity forecast is 
mostly just an update of the previous one, remaining close to the 
consensus.

The system continues moving northeastward at about 15 kt.   Sam 
should move faster to the northeast by Monday as it encounters 
stronger wind flow between a deep-layer ridge to the east and a 
large mid- to upper-level low over Atlantic Canada.  The track will 
weave leftward a bit at longer ranges due to the upper trough 
pulling the extratropical system north-northeastward at times, and 
eventually another trough has a similar tug by day 5. There are no 
significant changes to the previous forecast with guidance in close 
agreement. 

Key Messages:

1. Swells generated by Sam will impact the northern Leeward Islands,
the Greater Antilles, the Bahamas, Bermuda, the eastern United
States 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  02/2100Z 35.4N  58.2W  110 KT 125 MPH
 12H  03/0600Z 36.8N  56.5W  100 KT 115 MPH
 24H  03/1800Z 38.6N  53.6W   95 KT 110 MPH
 36H  04/0600Z 40.8N  49.5W   90 KT 105 MPH
 48H  04/1800Z 44.7N  44.0W   85 KT 100 MPH
 60H  05/0600Z 49.3N  40.2W   75 KT  85 MPH...POST-TROP/EXTRATROP
 72H  05/1800Z 51.6N  38.5W   65 KT  75 MPH...POST-TROP/EXTRATROP
 96H  06/1800Z 53.0N  32.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
120H  07/1800Z 60.0N  28.0W   40 KT  45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Blake



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