Atlantic Hurricane Feed

Hurricane Sam Forecast Discussion

WTNT43 KNHC 040842

Hurricane Sam Discussion Number  47
NWS National Hurricane Center Miami FL       AL182021
500 AM AST Mon Oct 04 2021

A 0255Z GPM microwave pass indicated that Sam had continued to go 
through an eyewall replacement cycle, consisting of a 20-nmi-wide 
inner eye and an 85-nmi-wide outer eye. The inner eye was rapidly 
eroding on the northwest side compared to microwave data from just a 
few hours earlier. Additionally, late-arriving ASCAT surface wind 
data from 0000-0100Z indicated that the inner-core wind field had 
contracted or weakened, while the outer wind field (34-kt radii) had 
continued to expand in all quadrants. Satellite intensity estimates 
have decreased to around 80 kt. However, the advisory intensity has 
been maintained at 90 kt based on the 10-kt increase in forward 
speed, which should offset any weakening of the tangential winds. 
Gradual weakening is expected now that Sam has passed north of the 
warm Gulf Stream and is moving over sub-25-deg-C sea-surface 
temperatures (SST), with much colder SSTs lying ahead of the 
hurricane. Vertical wind shear is forecast to increase to more than 
30 kt by 18 h, which should cause further weakening. By 24 h and 
beyond, interaction with a strong upper-level trough/low should 
cause Sam to transition into a powerful extratropical cyclone by 
early Tuesday to the east of Newfoundland.  This baroclinic 
interaction is expected to cause extratropical-Sam to maintain 
hurricane-force winds until early Wednesday.  Slow weakening is 
forecast thereafter as the very large cyclone slowly spins down as 
an occluded low pressure system.  No significant changes were made 
to the previous intensity forecast.

Sam has continued to accelerate and is now moving at 045/26 kt. A 
northeastward motion along with continued acceleration is forecast 
through today as Sam moves into stronger southwesterly flow ahead of 
an approaching mid-latitude trough. The large cyclone is expected to 
remain the primary surface low center as it merges with the trough 
in a couple days. A gradual turn toward the north and northwest is 
expected on days 4 and 5 when extratropical-Sam moves around another 
high-latitude trough.  The new NHC forecast track is essentially 
just an update of the previous advisory track, and lies close to 
the tightly packed consensus track models.

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  04/0900Z 40.8N  48.7W   90 KT 105 MPH
 12H  04/1800Z 43.7N  45.0W   80 KT  90 MPH
 24H  05/0600Z 47.8N  41.5W   75 KT  85 MPH...POST-TROP/EXTRATROP
 36H  05/1800Z 50.1N  39.3W   70 KT  80 MPH...POST-TROP/EXTRATROP
 48H  06/0600Z 51.0N  35.8W   60 KT  70 MPH...POST-TROP/EXTRATROP
 60H  06/1800Z 52.8N  31.2W   55 KT  65 MPH...POST-TROP/EXTRATROP
 72H  07/0600Z 56.0N  28.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
 96H  08/0600Z 60.6N  29.3W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  09/0600Z 61.5N  30.0W   40 KT  45 MPH...POST-TROP/EXTRATROP

Forecaster Stewart

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