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Hurricane Sam Forecast Discussion

WTNT43 KNHC 040234

Hurricane Sam Discussion Number  46
NWS National Hurricane Center Miami FL       AL182021
1100 PM AST Sun Oct 03 2021

Sam still had a surprise up its sleeve tonight, with its eye 
becoming warmer on satellite images along with a stronger eyewall.  
Microwave data from earlier today indicated that Sam has been 
undergoing a concentric eyewall cycle, and it appears that the inner 
eyewall has become better defined.  Regardless, it is not every day 
you see a hurricane with that clear of an eye near 40N, and the  
intensity estimates the evening range from 90-100 kt. Given the 
concentric eyewalls, the initial wind speed is conservatively 
raised to 90 kt, but it could be higher. 

High-resolution NOAA OISST data indicate that Sam is moving near a 
warm eddy along the north wall of the Gulf Stream.  This favorable 
factor, in addition to low shear, should keep Sam's weakening to a 
minimum in the near term.  Later on, although the shear increases 
rapidly and SSTs fall quickly, Sam is expected to transition into a 
powerful extratropical cyclone late Monday or early Tuesday due to a 
mid-latitude trough interaction well east of Newfoundland.  This 
should cause extratropical Sam to maintain hurricane-force winds 
until early Wednesday.  A slow weakening is expected thereafter as 
it slowly spins down as an occluded low.  No significant changes 
were made to the previous forecast.

Sam is moving faster, now 050/17 kt. A northeastward motion and 
continued acceleration is forecast through Monday as the cyclone 
gets caught in southwesterly flow ahead of the mid-latitude trough. 
The tropical cyclone is expected to be the main surface low center 
as it merges with the trough in a couple days, hooking briefly to 
the left. Thereafter, the system should resume a northeastward 
motion and gradually turn northward and even westward at long range 
as it moves around another trough.  Model guidance is close to the 
previous cycle, even with the loopy track, and the new NHC forecast 
is basically just an update of the last advisory.

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.


INIT  04/0300Z 39.3N  51.2W   90 KT 105 MPH
 12H  04/1200Z 41.6N  47.3W   85 KT 100 MPH
 24H  05/0000Z 46.5N  42.1W   75 KT  85 MPH
 36H  05/1200Z 50.4N  40.3W   70 KT  80 MPH...POST-TROP/EXTRATROP
 48H  06/0000Z 50.8N  38.3W   65 KT  75 MPH...POST-TROP/EXTRATROP
 60H  06/1200Z 51.6N  33.3W   55 KT  65 MPH...POST-TROP/EXTRATROP
 72H  07/0000Z 54.5N  28.2W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  08/0000Z 61.0N  28.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  09/0000Z 61.5N  32.0W   40 KT  45 MPH...POST-TROP/EXTRATROP

Forecaster Blake

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