Hurricane Sam Forecast Discussion Number 18

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September 26, 2021

Issued at 1100 PM AST Sun Sep 26 2021

WTNT43 KNHC 270249

Hurricane Sam Discussion Number 18
NWS National Hurricane Center Miami FL

1100 PM AST Sun Sep 26 2021

A NOAA Hurricane Hunter aircraft investigating Sam this afternoon
and evening found that the major hurricane likely peaked in
intensity at around 135 kt with a central pressure of about 929 mb
between 1900-2200 UTC when the eye contracted down to about 7 nmi in
diameter. Since that time, however, wind and radar data from the
aircraft reveal that Sam has been undergoing an eyewall replacement
cycle (ERC), including significant erosion of the eyewall convection
in the eastern semicircle, which has resulted in the central
pressure increasing by at least 14 mb in only a few hours. The eye
diameter has also doubled in size now. The ERC is also confirmed by
the ragged inner-core convection and cloud-filled eye that have
developed in satellite imagery. The advisory intensity of 125 kt is
based on a blend of the available reliable SFMR surface wind data
and flight-level wind data, and also using the pressure-wind
relationship for a 943-mb central pressure.

The initial motion estimate is now 315/06 kt. A strong subtropical
ridge to the north and northeast of Sam is forecast by the global
and regional models to remain entrenched across the central and
eastern Atlantic for the next several days, resulting in the
hurricane moving northwestward for the 72 hours. Thereafter, a mid-
to upper-level trough is forecast to dig southward just off the U.S.
east coast, creating a deep southerly flow regime that will act to
accelerate Sam northward on days 4 and 5. The new NHC track
forecast is similar the previous advisory, and lies close to the
tightly packed consensus models through 72 hours. In the 96-120 hour
period, the models diverge significantly on when and where Sam will
begin moving out toward the north, and the NHC track forecast
favors the more eastward GFS solution since that model has been
performing quite well with Sam thus far.

Now that Sam is undergoing an ERC, fluctuations in intensity could
occur for the next day or two while the vertical wind shear remains
low at least than 10 kt. Thereafter, a slow weakening trend is
expected due to a modest increase in the wind shear from the
southwest and occasional intrusions of drier mid-level air. The NHC
intensity forecast is a little lower than the previous advisory,
mainly due to the lower initial intensity, and closely follows the
HCCA and IVCN consensus model intensity forecasts.


INIT 27/0300Z 14.7N 50.8W 125 KT 145 MPH
12H 27/1200Z 15.3N 51.6W 125 KT 145 MPH
24H 28/0000Z 16.2N 52.8W 120 KT 140 MPH
36H 28/1200Z 17.0N 53.9W 120 KT 140 MPH
48H 29/0000Z 17.8N 55.0W 120 KT 140 MPH
60H 29/1200Z 18.7N 56.3W 115 KT 130 MPH
72H 30/0000Z 20.0N 58.0W 115 KT 130 MPH
96H 01/0000Z 23.4N 61.0W 110 KT 125 MPH
120H 02/0000Z 28.4N 62.6W 105 KT 120 MPH

Forecaster Stewart

Click here to read more at The National Hurricane Center

News date : 09/26/2021    Category : Hurricane, Weather

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