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Hurricane Sam Forecast Discussion


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431 
WTNT43 KNHC 270249
TCDAT3

Hurricane Sam Discussion Number  18
NWS National Hurricane Center Miami FL       AL182021
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.


FORECAST POSITIONS AND MAX WINDS

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