Atlantic Basin Tropical Cyclones

Hurricane Sam Forecast Discussion


000
WTNT43 KNHC 031447
TCDAT3

Hurricane Sam Discussion Number  44
NWS National Hurricane Center Miami FL       AL182021
1100 AM AST Sun Oct 03 2021

Sam's cloud pattern remains quite symmetric for a hurricane moving
into the higher latitudes. The eye has become more apparent in both
visible and infrared satellite imagery over the past few hours. A
very timely 1155 UTC GMI microwave image also reveals a well-defined
inner eye that was surrounded by an outer ring of convection at a
much larger radius.  A blend of the T- and CI-numbers from the
lastest subjective Dvorak classifications yields an initial
intensity of 85 kt, this is also supported by earlier UW/CIMSS 
SATCON estimates.

The vertical shear over Sam is expected to remain low for the next
6-12 hours and only gradual weakening is anticipated while the
system moves over gradually decreasing SSTs.  Later tonight, Sam is
expected to move over much cooler waters north of the north wall of
the Gulf Stream and into an area of increasing southwesterly shear,
which should result in continued weakening.  However, a strong
mid-tropospheric trough approaching Sam from the west on Monday is
expected to result in a fairly quick transition of Sam to a large
and powerful extratropical low.  The low is forecast to gradually
weaken over the far north Atlantic during the middle to latter
portions of the week as the baroclinic forcing decreases. The
latest NHC intensity forecast is similar to the previous advisory
and is a blend of the various intensity aids during the early
portion of the period, but leans toward the global model guidance
during the post-tropical phase.

Sam is moving northeastward or 050/14 kt. The cyclone should
continue to move northeastward in the flow between a strong
subtropical ridge over the eastern Atlantic and mid-latitude
trough over Atlantic Canada.  As this trough approaches Sam from
the west, a much faster northeastward motion is expected by Monday
and Monday night. Around midweek, the post-tropical cyclone is
forecast to slow down as it rotates around a large cut-off low
over the north Atlantic.  After that time, the low is forecast to
resume a northeastward motion before it slows once again to the
southwest of Iceland. Given the future complex interactions of the
post-tropical low with the cut-off low and trough over the north
Atlantic, the track guidance is in surprisingly good agreement, and
the NHC forecast is near a blend of the ECMWF and GFS global
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.


FORECAST POSITIONS AND MAX WINDS

INIT  03/1500Z 37.7N  54.6W   85 KT 100 MPH
 12H  04/0000Z 39.3N  51.9W   80 KT  90 MPH
 24H  04/1200Z 42.4N  47.0W   75 KT  85 MPH
 36H  05/0000Z 47.0N  41.7W   70 KT  80 MPH...POST-TROP/EXTRATROP
 48H  05/1200Z 50.9N  39.8W   65 KT  75 MPH...POST-TROP/EXTRATROP
 60H  06/0000Z 50.9N  38.5W   60 KT  70 MPH...POST-TROP/EXTRATROP
 72H  06/1200Z 51.5N  33.5W   50 KT  60 MPH...POST-TROP/EXTRATROP
 96H  07/1200Z 59.2N  27.5W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  08/1200Z 60.5N  31.0W   40 KT  45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Brown



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