[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 280844
TCDAT3

Hurricane Sam Discussion Number  23
NWS National Hurricane Center Miami FL       AL182021
500 AM AST Tue Sep 28 2021

The Air Force Reserve Hurricane Hunters that investigated Sam early
this morning found evidence that the hurricane has strengthened.
Peak 700-mb flight-level winds of 126 kt were reported in the
northeastern quadrant, which after the standard adjustment would
support an intensity of 110-115 kt. Satellite data also indicate
that Sam's structure has improved overnight. Sam's inner core
appears to have consolidated into a single, primary eyewall in
recent GMI and AMSR2 microwave imagery, and the latest GOES-16
infrared imagery shows a colder ring of convective cloud tops
developing around the eye of Sam. This has brought the latest
UW-CIMSS ADT and SATCON estimates up to around 115 kt. Based on the
flight-level wind data and ongoing satellite trends, the initial
intensity is raised to 115 kt for this advisory. The minimum central
pressure of 953 mb is based on a center dropsonde of 954 mb with
12-kt surface winds.

In the short-term, internal dynamic processes that are difficult to
predict could cause some fluctuations in Sam's intensity. If Sam's
eyewall is able to contract today, some additional strengthening
could occur, and this possibility is reflected in the latest NHC
intensity forecast. Thereafter, the warm sea-surface temperatures
and weak to moderate vertical wind shear along Sam's forecast track
suggest it should remain a major hurricane for the next several
days. Thus, the NHC forecast only shows gradual weakening through
72-96 h, in agreement with the consensus aids IVCN and HCCA. By days
4-5, increasing southerly wind shear along with decreasing SSTs
should increase Sam's rate of weakening as it recurves deeper into
the mid-latitudes.

Aircraft and microwave data indicate that Sam is slightly tilted in
the vertical, with the low-level center displaced just a bit to the
south of the mid-level eye. Based on recent aircraft fixes, Sam's
initial motion is northwestward, or 310/8 kt. This motion is
expected to continue for the next few days as Sam moves around the
southwestern periphery of the subtropical ridge. A mid- to
upper-level trough is forecast to dig southward over the western
Atlantic later this week, which should steer Sam toward the north by
Friday. Then, Sam is expected to accelerate north-northeastward
within the deep-layer southerly flow ahead of the trough this
weekend. The official NHC track forecast is very similar to the
previous one, with just a slight adjustment to the left based on the
latest track guidance consensus aids. The along-track spread in the
guidance noticeably increases as Sam recurves over the western
Atlantic, with the ECMWF much slower than the GFS. Once again, the
NHC forecast trends closer to the faster GFS solution at longer
ranges, given its better overall performance this season.


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  28/0900Z 17.2N  53.9W  115 KT 130 MPH
 12H  28/1800Z 17.9N  54.7W  120 KT 140 MPH
 24H  29/0600Z 18.8N  55.9W  115 KT 130 MPH
 36H  29/1800Z 19.9N  57.4W  115 KT 130 MPH
 48H  30/0600Z 21.3N  59.0W  110 KT 125 MPH
 60H  30/1800Z 23.2N  60.5W  110 KT 125 MPH
 72H  01/0600Z 25.6N  61.5W  110 KT 125 MPH
 96H  02/0600Z 31.4N  60.7W  105 KT 120 MPH
120H  03/0600Z 39.0N  56.0W   95 KT 110 MPH

$$
Forecaster Reinhart
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