[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT43 KNHC 280234
TCDAT3

Hurricane Sam Discussion Number  22
NWS National Hurricane Center Miami FL       AL182021
1100 PM AST Mon Sep 27 2021

The NOAA Hurricane Hunters have been flying through Sam tonight,
with radar and wind observations showing broken concentric eyewalls
at times.  The maximum 700-mb flight-level winds on the mission
were 114 kt, with peak SFMR values to 103 kt and a central pressure
around 956 mb, so the initial wind speed will remain 105 kt.  While
the overall intensity doesn't seem to have changed much in the past
several hours, the aircraft data show that the hurricane has grown
in size, and that is reflected in the initial wind radii.

The future intensity of Sam is hard to pinpoint.  Eyewall cycles
will likely play a key role in determining its second peak intensity
since the hurricane should remain in low- or moderate-shear and
warm-water environments for the next few days.  Those cycles are
basically impossible to forecast, but there is some suggestion on
satellite that a larger eye is trying to form, which could allow for
some increase in strength in the near term.  It seems likely that
Sam will be a category 3 or 4 hurricane given the conducive
environment for most of the forecast period, so the new NHC forecast
is similar to the previous one, with the most significant change
showing a larger hurricane consistent with the latest guidance.

Sam continues to move northwestward, or 315/8 kt.  The hurricane
should move in that direction for the next few days around the
southwestern portion of the subtropical high.  A turn to the north
is likely as Sam meets the western edge of the high, with a gradual
acceleration to the northeast anticipated by the weekend due to an
incoming mid-latitude trough from the west.  While there are some
speed differences in the models, especially near and after
recurvature, the NHC forecast leans toward the faster GFS solution
over the slower ECMWF, given the magnitude of the strong
southwesterly flow that Sam should encounter at long range, plus a
nod to the superior performance of late for the American model.  The
new forecast is adjusted a bit to the right and faster beyond day 3,
with no significant changes made before that time.


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  28/0300Z 16.8N  53.2W  105 KT 120 MPH
 12H  28/1200Z 17.5N  54.0W  115 KT 130 MPH
 24H  29/0000Z 18.4N  55.1W  115 KT 130 MPH
 36H  29/1200Z 19.3N  56.4W  110 KT 125 MPH
 48H  30/0000Z 20.6N  58.0W  110 KT 125 MPH
 60H  30/1200Z 22.2N  59.7W  110 KT 125 MPH
 72H  01/0000Z 24.3N  61.0W  110 KT 125 MPH
 96H  02/0000Z 29.6N  61.5W  105 KT 120 MPH
120H  03/0000Z 37.5N  56.9W  100 KT 115 MPH

$$
Forecaster Blake
This weather bulletin was brought to you by:
www.nashvilleweather.net