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Hurricane Sam Forecast Discussion


041 
WTNT43 KNHC 041443
TCDAT3

Hurricane Sam Discussion Number  48
NWS National Hurricane Center Miami FL       AL182021
1100 AM AST Mon Oct 04 2021

Sam remains a formidable high-latitude hurricane this morning. An 
earlier 1102 UTC GMI microwave overpass continued to depict 
concentric eyewalls with a fairly well-defined inner-eye that was 
open over the southwestern quadrant. Although there has been a 
generally warming of the cloud tops in infrared satellite imagery, 
the eye remains apparent and the system is still fairly symmetric. 
A blend of the subjective Dvorak T- and CI-numbers from both SAB 
and TAFB yield an intensity of 85 kt, which is used as the initial 
intensity for this advisory. 

Now that Sam has moved north of the north wall of the Gulf Stream 
it will be moving over progressively colder waters and into much 
higher vertical wind shear conditions by this evening, which should 
cause some gradual weakening.  However, the interaction of the 
cyclone with a strong upper-level trough is likely to lead to the
rapid transition of Sam into a powerful extratropical cyclone later 
this evening.  The post-tropical cyclone is expected to maintain 
hurricane-force winds until late Tuesday.  Gradual weakening should 
occur after that time as the baroclinic forcing decreases, and the 
system is forecast to spin down slowly over the north Atlantic later 
this week. 

Sam continues to accelerate northeastward and is now moving at 
045/28 kt.  A faster northeastward motion is forecast through 
tonight as Sam moves within the strong southwesterly flow ahead of 
a mid-latitude trough. The post-tropical cyclone is forecast to 
slow down on Tuesday as it interacts with the trough/cut-off low.  
After that time, the system is forecast to turn east-northeastward, 
and then rotate cyclonically around the eastern flank of yet 
another cut-off low later in the week.  The dynamical model 
guidance remains in generally good agreement on the overall track 
forecast scenario, albeit for some forward speed variations.  The 
latest official forecast is similar to the previous advisory and 
lies near the TVCA multi-model consensus in deference to the 
aforementioned along-track differences. 

Key Messages:

1. Swells generated by Sam will impact the northern Leeward
Islands, the Greater Antilles, portions of the Bahamas, and 
southeastern Newfoundland for the next day or so.  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  04/1500Z 42.6N  45.7W   85 KT 100 MPH
 12H  05/0000Z 46.5N  41.9W   75 KT  85 MPH
 24H  05/1200Z 50.0N  40.5W   70 KT  80 MPH...POST-TROP/EXTRATROP
 36H  06/0000Z 50.3N  38.3W   60 KT  70 MPH...POST-TROP/EXTRATROP
 48H  06/1200Z 51.4N  33.2W   55 KT  65 MPH...POST-TROP/EXTRATROP
 60H  07/0000Z 54.3N  27.7W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  07/1200Z 58.3N  24.8W   50 KT  60 MPH...POST-TROP/EXTRATROP
 96H  08/1200Z 61.0N  29.7W   50 KT  60 MPH...POST-TROP/EXTRATROP
120H  09/1200Z 60.0N  29.8W   40 KT  45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Brown




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