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Hurricane Sam Forecast Discussion

WTNT43 KNHC 292051

Hurricane Sam Discussion Number  29
NWS National Hurricane Center Miami FL       AL182021
500 PM AST Wed Sep 29 2021

Sam continues to exhibit a well-organized cloud pattern on satellite 
imagery, although there is some asymmetry of the coldest-topped 
convection around the center.  The hurricane is also maintaining 
numerous banding features with well-defined upper-level 
anticyclonic outflow.  Observations from an Air Force Hurricane 
Hunter plane showed peak 700 mb fight-level winds of 130 kt in the 
northeast quadrant which, using the standard reduction, corresponds 
to maximum surface winds of 117 kt.  However, the maximum 
SFMR-observed surface winds were 103 kt.  Based on these data, with 
more emphasis on the flight-level winds, the current intensity 
estimate is held at 115 kt.  The aircraft also measured a minimum 
central pressure of 945  mb by dropsonde.

Since the hurricane is forecast to remain in an environment of low 
vertical shear and to remain over warm waters through 48 hours, Sam 
could restrengthen a little more during the next day or so.  Since 
the Hurricane Hunters reported a concentric eyewall, 
short-term strengthening may be curtailed until the eyewall 
replacement cycle completes.  Later in the forecast period, gradual 
weakening is likely due to cooler SSTs and increasing shear.  The 
official intensity forecast is in very good agreement with the 
latest corrected consensus prediction.  Sam will likely be close to 
extratropical transition around day 5.

The initial motion is still northwestward, or 310/8 kt.  During 
the next few days, Sam is likely to move northwestward and then 
northward around the western periphery of a large subtropical high 
pressure system centered over the eastern Atlantic.  A turn to the 
north-northeast and northeast is expected later in the forecast 
period as Sam begins to encounter the flow on the southern side of 
a large mid-tropospheric trough moving through Atlantic Canada.  
The track guidance models remain in excellent agreement through 72 
hours, including the time that Sam passes by Bermuda.  At days 4-5 
there is some divergence of the model guidance, apparently due to 
how Sam may interact with the trough.   Most notably the ECMWF is a 
lot slower than the other global models suggesting that the trough 
will at least partially bypass it in that case.  Not much change 
has been made to the previous official forecast, however, and the 
NHC track remains very close to the TVCN dynamical model consensus. 

Key Messages:

1. Swells generated by Sam will impact the Lesser Antilles during 
the next several days.  Swells are expected to reach Bermuda
and the Bahamas in a day or so, 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.


INIT  29/2100Z 20.2N  57.6W  115 KT 130 MPH
 12H  30/0600Z 21.2N  58.9W  120 KT 140 MPH
 24H  30/1800Z 23.1N  60.4W  120 KT 140 MPH
 36H  01/0600Z 25.6N  61.4W  120 KT 140 MPH
 48H  01/1800Z 28.3N  61.9W  115 KT 130 MPH
 60H  02/0600Z 31.0N  61.3W  110 KT 125 MPH
 72H  02/1800Z 33.6N  59.9W  100 KT 115 MPH
 96H  03/1800Z 37.5N  55.0W   85 KT 100 MPH
120H  04/1800Z 42.0N  48.5W   70 KT  80 MPH

Forecaster Pasch

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