[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 281433
TCDAT3

Hurricane Sam Discussion Number  24
NWS National Hurricane Center Miami FL       AL182021
1100 AM AST Tue Sep 28 2021

Satellite images indicate that Sam has been generally steady in
strength during the past several hours, and it remains a powerful
category 4 hurricane on the Saffir-Simpson Hurricane Wind Scale.
The storm has a well-defined eye, and the convective pattern
surrounding the center is fairly symmetric. A 1043 UTC SSMIS pass
showed a closed and symmetric eyewall, with no indication of a
concentric eyewall.  The latest Dvorak classifications from TAFB
and SAB were 5.5/102 kt and the CIMSS automated technique is
currently 6.2/120 kt.  Based on these satellite estimates and the
overnight reconnaissance data, the initial intensity is held at
115 kt.  Another Air Force Hurricane Hunter aircraft is scheduled to
investigate Sam this afternoon.

The major hurricane is moving slowly northwestward, with the initial
motion estimated to be 305/7 kt.  A mid-level ridge situated to the
north-northeast of Sam is expected to shift eastward as a deep-layer
trough digs southeastward over the western Atlantic.  This change in
the large-scale pattern should cause Sam to speed up and gradually
turn toward the north in 2 or 3 days and then to the northeast in 4
to 5 days. The models are in fairly good agreement, and the new
forecast track is essentially an update of the previous one.  Based
on the forecast, Sam should pass well east of the northern Leeward
Island through Wednesday and east of Bermuda Friday night and
Saturday.

Sam is expected to remain in favorable environmental conditions for
it to maintain its intensity, or perhaps strengthen slightly, during
the next day or two.  However, after that time, a gradual increase
in southerly shear should promote a slow weakening trend, and Sam
will likely be beginning its transition to an extratropical cyclone
toward the end of the forecast period.  The NHC intensity forecast
is also an update of the previous one and in line with the various
consensus aids.  Sam is expected to remain a major hurricane during
the next several days.

The initial wind radii have been adjusted based on a 1114 UTC
ASCAT-A pass.

Key Messages:

1. Large swells generated by Sam are affecting the Leeward Islands
and will spread to portions of the Virgin Islands, Puerto Rico,
Hispaniola, the Bahamas, and Bermuda by Thursday or Friday.
Significant swells will likely reach the east coast of the United
States and Atlantic Canada by the weekend.  These swells will likely
cause life-threatening surf and rip current conditions, and
beachgoers and other interests along these coasts are urged to
follow the advice of lifeguards and local officials through the
upcoming weekend.


FORECAST POSITIONS AND MAX WINDS

INIT  28/1500Z 17.5N  54.3W  115 KT 130 MPH
 12H  29/0000Z 18.2N  55.1W  120 KT 140 MPH
 24H  29/1200Z 19.2N  56.5W  115 KT 130 MPH
 36H  30/0000Z 20.5N  58.1W  115 KT 130 MPH
 48H  30/1200Z 22.0N  59.6W  110 KT 125 MPH
 60H  01/0000Z 24.0N  60.9W  110 KT 125 MPH
 72H  01/1200Z 26.6N  61.5W  110 KT 125 MPH
 96H  02/1200Z 32.5N  59.7W  100 KT 115 MPH
120H  03/1200Z 39.2N  54.8W   90 KT 105 MPH

$$
Forecaster Cangialosi
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