Atlantic Basin Tropical Cyclones

Hurricane Sam Forecast Discussion


634 
WTNT43 KNHC 032035
TCDAT3

Hurricane Sam Discussion Number  45
NWS National Hurricane Center Miami FL       AL182021
500 PM AST Sun Oct 03 2021

Sam is a resilient hurricane despite having just crossed the 26 
degrees C isotherm. An eye remains readily apparent in satellite 
imagery, while microwave images show a symmetrical concentric 
eyewall structure. Recent scatterometer data revealed that the 
tropical-storm-force wind field continues to expand, with these 
winds now extending 200 n mi or greater in all quadrants. A blend 
of the latest Dvorak intensity estimates from TAFB, SAB, and the 
UW-CIMMS ADT supports maintaining an initial advisory intensity of 
85 kt. 

Gradual weakening is expected over the next 36 h as Sam moves over 
progressively cooler waters. However, the wind field is forecast to 
continue expanding. On Monday, the cyclone is expected to interact 
with a strong mid-level trough moving off the coast of Newfoundland 
which should cause Sam to transition into a powerful extratropical 
cyclone. With a lack of baroclinic forcing once Sam becomes 
vertically stacked with the trough in 48 h, the extratropical 
cyclone is expected to steadily weaken over the northern Atlantic 
through late this week. The latest NHC intensity forecast is little 
changed from the previous one, and is near the various model 
consensus solutions.

Sam has begun to accelerate and is now moving 060/14 kt. A 
northwestward motion and continued acceleration is forecast through 
Monday as the cyclone gets caught in southwesterly flow ahead of the 
mid-level trough. Just before Sam merges with the trough in a couple 
of days, the cyclone should slow down as it pivots around the 
eastern flank of the trough. Thereafter, the merged system should 
continue northeastward then northward in the flow around a larger 
mid- to upper-level trough over the far northern Atlantic. The model 
guidance is in very good agreement on this track scenario, and the 
latest NHC track forecast remains near the previous one.

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  03/2100Z 38.6N  52.8W   85 KT 100 MPH
 12H  04/0600Z 40.3N  49.7W   80 KT  90 MPH
 24H  04/1800Z 44.2N  44.3W   75 KT  85 MPH
 36H  05/0600Z 49.0N  40.4W   70 KT  80 MPH...POST-TROP/EXTRATROP
 48H  05/1800Z 50.8N  39.5W   65 KT  75 MPH...POST-TROP/EXTRATROP
 60H  06/0600Z 51.5N  35.7W   55 KT  65 MPH...POST-TROP/EXTRATROP
 72H  06/1800Z 53.2N  31.2W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  07/1800Z 59.4N  27.2W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  08/1800Z 61.2N  27.5W   40 KT  45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Latto




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