[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 291441
TCDAT3

Hurricane Sam Discussion Number  28
NWS National Hurricane Center Miami FL       AL182021
1100 AM AST Wed Sep 29 2021

Sam has a well-organized cloud pattern on satellite imagery, with a
90 n mi-wide Central Dense Overcast surrounded by numerous
convective banding features.  Upper-level outflow has now become
better established to the southeast of the hurricane.  A NOAA
Hurricane Hunter mission from a couple of hours ago reported a 20 n
mi-wide diameter circular eye.  The current intensity is held at 115
kt for this advisory, which is a little higher than the maximum
winds observed in the earlier aircraft mission.  However, since the
estimated minimum central pressure of 949 is a little lower than it
was earlier this morning and Sam's intensity has been fluctuating,
it is considered prudent not to lower the winds at this time.  An
Air Force Hurricane Hunter aircraft is scheduled to investigate the
hurricane soon and will provide another intensity estimate.

Sam continues its slow northwestward trek with the current motion
remaining near 310/8 kt.  Sam should move northwestward and then
northward around the western portion of the subtropical ridge over
the eastern Atlantic during the next few days, and turn toward
the north-northeast and northeast later in the forecast period.
The track guidance models are in very good agreement through 72
hours or so, with some increase in model spread thereafter.  This
increase in spread is largely due to how Sam will interact with a
strong mid-latitude trough moving through Atlantic Canada.  Some
of the guidance suggests that this trough could partially bypass
the hurricane around the end of the period and beyond, but this
is not yet clear.  The current NHC forecast track is not much
different from the previous one and is in good agreement with the
dynamical model consensus, TVCA.

The hurricane will continue to move over warm waters and in an
environment of low vertical shear and diffluent upper-level flow
for the next 1-2 days.  Thus, it would not be surprising to soon
see some restrengthening.  The official intensity forecast is
essentially a blend of the latest Decay-SHIPS and LGEM predictions,
and is very similar to the previous NHC forecast.  Sam is likely to
remain a major hurricane into this weekend.


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  29/1500Z 19.4N  57.0W  115 KT 130 MPH
 12H  30/0000Z 20.3N  58.1W  120 KT 140 MPH
 24H  30/1200Z 21.8N  59.6W  120 KT 140 MPH
 36H  01/0000Z 23.9N  61.0W  120 KT 140 MPH
 48H  01/1200Z 26.6N  61.8W  115 KT 130 MPH
 60H  02/0000Z 29.4N  61.8W  110 KT 125 MPH
 72H  02/1200Z 32.0N  60.9W  105 KT 120 MPH
 96H  03/1200Z 36.2N  57.3W   90 KT 105 MPH
120H  04/1200Z 39.5N  53.0W   75 KT  85 MPH

$$
Forecaster Pasch

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