Tropical Storm Sam Discussion Number 4
NWS National Hurricane Center Miami FL AL182021
1100 AM AST Thu Sep 23 2021

Overnight and this morning, the satellite structure of the tropical
cyclone has been improving, with prominent curved bands, and obvious
low-level cloud motions that indicate the circulation is becoming
established within the convection. In particular, an SSMIS microwave
pass at 0653 UTC indicated a substantial improvement in the
convective structure, with a well-defined curved band wrapping
three-quarters of the way around the center in both the 91- and
37-GHz channels. ASCAT-B wind retrievals at 1234 UTC also indicated
a tight, well-defined circulation had formed, with peak winds of
44-kt on the north side of the vortex. Subjective Dvorak satellite
intensity estimates are now T3.5/55-kt from SAB and T2.5/35-kt from
TAFB. In addition, the latest objective intensity estimates from
UW-CIMSS ADT and SATCON were at 35-kt and 43-kt respectively. Given
the recent scatterometer data, the intensity has been set to 45-kt
for this advisory. Thus, Tropical Depression 18 has been upgraded to
Tropical Storm Sam. It is noteworthy that this is the 2nd earliest
formation of the 18th named storm in the Atlantic basin, moving
ahead of the 2005 hurricane season, and only trailing last year.

Sam continues to move to the west-northwest with an estimated motion
at 280/14 kt. A prominent mid-level ridge remains entrenched to the
north of the cyclone, and this feature should continue to steer Sam
to the west-northwest over the next 2-3 days. Over this period, the
ridge orientation actually shifts more westward ahead of the cyclone
as a deep-layer trough digs in well northeast of Sam. This gradual
change in the synoptic pattern should also result in a slowdown in
Sam's forward motion over the next 72 hours. While the track
guidance remains in good agreement over this period, larger spread
begins to take shape beyond the day 3 forecast. Similar to
yesterday, the ECMWF model and its ensemble mean flanks the
southwest side of the guidance envelop, while the GFS is on the
northeast side by day 5. These differences appear to be partially
related to the mid-level ridge intensity on the western end, which
could begin to be eroded by a mid-latitude trough off the eastern
United States in 120-h. In addition, differences in both the size
and intensity of Sam may also play a role in its ultimate track
evolution. For this advisory, the latest NHC track forecast remains
close to the consensus aids TCVN and TCVE, which represents a slight
shift westward and a bit faster motion compared to the previous
forecast. Because Sam is forecast to slow down, it remains too early
to determine what impacts, direct or indirect, could be felt by the
Lesser Antilles by this cyclone.

The previously mentioned SSMIS microwave imagery also indicated that
the low-level center has become better aligned with the mid-level
vortex, perhaps a bit earlier than anticipated given the
scatterometer data from last night. This improvement in structure,
combined with favorable low vertical wind shear under 10 kt and warm
28-29 C sea-surface temperatures, argues for significant
intensification. In fact, the GFS-SHIPS rapid intensification index
now give Sam a 39 percent chance of a 65-kt increase over the next
72 hours, which is more than 7 times the climatological value. Thus,
the NHC intensity forecast has been raised quite a bit from the
prior one, especially in the short-term. The latest forecast now
makes Sam a hurricane in 36 hours, and a major hurricane in 72
hours. This intensity forecast is just a shade under the latest HCCA
consensus aid. Thereafter, a more gradual intensification rate is
forecast. By this period it is possible that Sam could undergo hard
to predict inner-core changes such as eyewall replacement cycles. In
addition, the ECMWF-SHIPS suggests that the shear may also increase
a little in days 4-5 which also argues for a bit slower rate of
intensification for this period.


FORECAST POSITIONS AND MAX WINDS

INIT 23/1500Z 10.9N 38.1W 45 KT 50 MPH
12H 24/0000Z 11.3N 40.3W 50 KT 60 MPH
24H 24/1200Z 11.9N 42.9W 60 KT 70 MPH
36H 25/0000Z 12.3N 45.0W 70 KT 80 MPH
48H 25/1200Z 12.7N 46.7W 80 KT 90 MPH
60H 26/0000Z 13.2N 48.2W 90 KT 105 MPH
72H 26/1200Z 13.7N 49.4W 100 KT 115 MPH
96H 27/1200Z 15.3N 52.1W 105 KT 120 MPH
120H 28/1200Z 17.4N 55.2W 110 KT 125 MPH