[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 240831
TCDAT3

Hurricane Sam Discussion Number   7
NWS National Hurricane Center Miami FL       AL182021
500 AM AST Fri Sep 24 2021

Sam continues to rapidly intensify.  Satellite images show a small,
but well-developed inner core and pronounced curved bands that wrap
most of the way around the center.  There are some dry slots,
however, between the core and bands.  The latest satellite intensity
estimates range from 55 to 77 kt, and based on that data and the
continued improvement in the cyclone's structure, the initial wind
speed is increased to 65 kt.  This makes Sam a hurricane, the
seventh one of the 2021 Atlantic hurricane season. It should be
noted that Sam has a compact wind field, with hurricane-force and
tropical-storm-force winds estimated to only extend outward up to 15
and 50 n mi from the center, respectively.

Sam is moving westward at about 13 kt, and this general motion
should continue for another 12 to 24 hours as it moves in the flow
on the south side of a mid-level ridge.  After that time, a decrease
in forward speed and a turn to the west-northwest are expected, and
the official forecast has Sam moving at a slow pace of only 6-8 kt
during the 48-96-hour time period.  By the middle of next week, the
ridge is forecast to slide eastward as a trough moves over the
western Atlantic. In response, the hurricane will likely turn
northwestward as it approaches the northern Leeward Islands.  The
models have changed little this cycle with the GFS still on the
northern side of the guidance envelope and the ECMWF on the
southern side.  The NHC track forecast is essentially an update of
the previous one and lies between the GFS and ECMWF models, and near
the consensus aids.

The large scale environmental conditions all appear favorable for
continued rapid intensification during the next day or so as the
hurricane is expected to remain over warm 29 deg C waters and in
very low wind shear conditions.  All of the SHIPS rapid
intensification indices are well above the climatological means, and
the NHC intensity forecast calls for Sam to become a major hurricane
by early Saturday.  Beyond a couple of days, the environment is
likely to become a little less ideal, and most of the models show
Sam leveling off in strength, and so does the official forecast.
This intensity prediction lies near a blend of the FSSE, HCCA, and
IVCN consensus models.  Regardless of the details, Sam is expected
to be a significant hurricane during the next several days.


FORECAST POSITIONS AND MAX WINDS

INIT  24/0900Z 11.5N  42.2W   65 KT  75 MPH
 12H  24/1800Z 11.7N  44.0W   80 KT  90 MPH
 24H  25/0600Z 12.1N  45.9W   95 KT 110 MPH
 36H  25/1800Z 12.5N  47.4W  105 KT 120 MPH
 48H  26/0600Z 12.9N  48.7W  115 KT 130 MPH
 60H  26/1800Z 13.4N  50.0W  115 KT 130 MPH
 72H  27/0600Z 14.1N  51.1W  110 KT 125 MPH
 96H  28/0600Z 15.9N  53.8W  110 KT 125 MPH
120H  29/0600Z 17.8N  56.6W  110 KT 125 MPH

$$
Forecaster Cangialosi
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