[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT43 KNHC 270848
TCDAT3

Hurricane Sam Discussion Number  19
NWS National Hurricane Center Miami FL       AL182021
500 AM AST Mon Sep 27 2021

The satellite presentation of Sam deteriorated overnight, as GOES-16
infrared imagery showed periodic disruptions to Sam's inner core
convection. The eye has not been readily apparent in conventional
satellite imagery for much of the night, although very recent
imagery suggests an eye could be re-emerging. Data from the
reconnaissance aircraft last night indicated that an eyewall
replacement cycle (ERC) was underway, and some dry mid-level air
impinging on the western side of the circulation could also be
negatively impacting Sam. No recent high-resolution microwave data
is available, but an earlier scatterometer pass revealed that the
tropical-storm-force wind radii had expanded just a bit in the
eastern semicircle of the hurricane. The initial intensity is
lowered to 115 kt for this advisory based on a blend of the
objective and subjective Dvorak current intensity estimates. A NOAA
hurricane hunter aircraft is scheduled to investigate Sam later this
morning, which will provide crucial data to assess changes in Sam's
structure and intensity.

The initial motion is estimated to be northwestward, or 315/7 kt. A
subtropical ridge to the north and northeast of Sam will remain the
primary steering mechanism over the next several days, and Sam is
expected to maintain a northwestward heading through midweek. Then,
a mid- to upper-level trough is forecast to dig southward over the
western Atlantic late this week. The deep-layer southerly flow ahead
of this feature should cause Sam to move faster toward the
north-northwest by 96 h, followed by a northward acceleration
thereafter. The NHC forecast track is shifted slightly to the right
of the previous one through the first 48-60 h of the forecast
period, which accounts for the more northwestward initial motion
observed during the past 12 h. Otherwise, the NHC track is virtually
unchanged as the guidance has remained fairly consistent.

Sam's intensity has likely peaked now that an ERC has commenced, but
environmental conditions should allow Sam to persist as a major
hurricane for the next several days. Sea-surface temperatures are
expected to be 28 deg C or greater along Sam's track through 120 h,
and the vertical wind shear is forecast to remain weak (< 10 kt)
through at least midweek. Thus, the official NHC intensity forecast
only shows very gradual weakening during the next several days,
generally following the HCCA and IVCN consensus aids. Given the
lower initial intensity of Sam, the latest NHC forecast is once
again lower than the previous one. By 120 h, southerly shear
associated with the deep-layer trough is forecast to increase over
Sam, which should induce more weakening by this weekend.


FORECAST POSITIONS AND MAX WINDS

INIT  27/0900Z 15.2N  51.4W  115 KT 130 MPH
 12H  27/1800Z 15.9N  52.2W  115 KT 130 MPH
 24H  28/0600Z 16.8N  53.3W  115 KT 130 MPH
 36H  28/1800Z 17.7N  54.4W  115 KT 130 MPH
 48H  29/0600Z 18.6N  55.6W  110 KT 125 MPH
 60H  29/1800Z 19.6N  57.1W  110 KT 125 MPH
 72H  30/0600Z 21.0N  58.8W  110 KT 125 MPH
 96H  01/0600Z 24.9N  61.6W  110 KT 125 MPH
120H  02/0600Z 30.5N  62.0W  105 KT 120 MPH

$$
Forecaster Reinhart
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