[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT43 KNHC 010253
TCDAT3

Hurricane Sam Discussion Number  34
NWS National Hurricane Center Miami FL       AL182021
1100 PM AST Thu Sep 30 2021

Sam's satellite structure on infrared remains quite impressive this
evening, with a circular eye staying quite warm (+10-20C) in
comparison to the ring of cold cloud tops associated with the
eyewall (-60 to -70C). The most recent microwave imagery was a SSMIS
pass at 1957 UTC, which indicated the eyewall remains quite intense,
if just a little asymmetric with a stronger western semicircle. This
eyewall asymmetry is consistent with a touch of light southerly
shear affecting the cyclone as diagnosed by ECMWF-SHIPS. However,
this shear does not appear to be having a significant impact on the
hurricane's intensity. While there have not been any recent aircraft
data to determine Sam's intensity since the last mission departed
the hurricane at around 2000 UTC, the satellite intensity estimates
have not changed much this evening. Thus, the initial intensity has
been held at 125-kt this advisory. Another Air Force Reserve
Reconnaissance Mission will be flying into Sam overnight to get a
better sense of the current intensity.

Following Sam's eye tonight, the hurricane is beginning to make a
gradual turn northward at a faster motion, with the latest estimate
now north-northwest at 345/14 kt. The track guidance in the
short-term remains in good agreement that Sam should continue to
turn to the north, north-northeast, and then northeastward as it is
steered around the western edge of a large subtropical ridge over
the central Atlantic. This track should take the hurricane east of
Bermuda. While there is a large deep-layer trough located north of
Sam, this feature will not initially capture the cyclone, as this
trough first de-amplifies and merges with a larger deep-layer
trough located over the far eastern Provinces of Canada. In fact,
Sam's forward motion to the northeast in 48-72 hours may even slow a
tad as it remains just south of the stronger upper-level
westerlies. However, by 96-120 hours, the deep-layer low over Canada
is expected to dig southeastward into North Atlantic, and will
likely capture Sam as the hurricane rapidly accelerates ahead of
this amplifying trough. It is in the 72-120 hour time-frame that a
lot of the track guidance diverges, mostly related to the timing and
degree of Sam's interaction of with this trough ejecting out of
Canada. For now, the official NHC track forecast has elected to
stay close to the track consensus TVCN, which has preformed well
with this hurricane so far. This latest track is just a bit
northeast and faster than the previous one after 72 hours.

Sam still has another 24 hours under warm 28-29 C sea-surface
temperatures while the southerly vertical wind shear is expected to
remain light. Thus, the hurricane is expected to maintain Category 4
intensity over the next day or so, with short-term intensity changes
likely to be controlled by inner core dynamics (such as eyewall
replacement cycles). Thereafter, sea surface temperatures decrease
markedly as southerly shear is also expected to gradually increase.
This combination should lead to steady weakening. After 72 hours,
the rate of weakening has actually been slowed a tad, since the
ECMWF, GFS, and Canadian models all show Sam receiving significant
baroclinic support as it undergoes a dynamic extratropical
transition while the hurricane phases with the large baroclinic
trough emerging off Canada. Simulated satellite imagery from these
models suggest that Sam's transition to an extratropical cyclone
should take place between 96-120 hours, with the resulting cyclone
producing a large wind footprint across the North Atlantic basin.


Key Messages:

1. Swells generated by Sam will impact the Northern Leeward Islands
and the Greater Antilles, including Puerto Rico, during the next
few days.  Swells are expected to reach Bermuda and the Bahamas by
tomorrow, and then spread to the United States east coast by this
weekend.  These swells could cause life-threatening surf and rip
current conditions.  Please consult products from your local
weather office.

2. Tropical storm conditions are expected on Bermuda beginning
Friday night or early Saturday, and a Tropical Storm Warning is in
effect for the island.


FORECAST POSITIONS AND MAX WINDS

INIT  01/0300Z 25.0N  61.2W  125 KT 145 MPH
 12H  01/1200Z 27.2N  61.8W  125 KT 145 MPH
 24H  02/0000Z 30.2N  61.6W  115 KT 130 MPH
 36H  02/1200Z 32.9N  60.5W  105 KT 120 MPH
 48H  03/0000Z 35.0N  58.9W   95 KT 110 MPH
 60H  03/1200Z 36.6N  56.8W   85 KT 100 MPH
 72H  04/0000Z 38.7N  53.8W   80 KT  90 MPH
 96H  05/0000Z 44.6N  47.2W   75 KT  85 MPH
120H  06/0000Z 48.9N  42.5W   70 KT  80 MPH...POST-TROP/EXTRATROP

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