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Hurricane Sam Forecast Discussion


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WTNT43 KNHC 011452
TCDAT3

Hurricane Sam Discussion Number  36
NWS National Hurricane Center Miami FL       AL182021
1100 AM AST Fri Oct 01 2021

Sam remains a powerful category 4 hurricane on the Saffir-Simpson 
Hurricane Wind Scale over the subtropical central Atlantic. 
Satellite images show that the hurricane still has a circular and 
quite distinct eye and closed eyewall.  However, an 0827 UTC SSMIS 
pass indicated that a dry slot was present on the eastern side of 
the circulation between the eyewall and rainbands.  The latest 
Dvorak classifications from TAFB, SAB, and CIMSS at the University 
of Wisconsin range from 102-115 kt.  However, Air Force 
reconnaissance data from earlier this morning showed that the winds 
were much higher, and in fact, the Dvorak estimates have had a low 
bias compared to the Hurricane Hunter data for the past 24-36 hours. 
Therefore, the initial intensity is held at 130 kt for now.  Another 
Air Force Hurricane Hunter aircraft is scheduled to investigate Sam 
this afternoon.  An ASCAT-A pass from around 12Z indicated that 
Sam's tropical-storm-force wind radii have expanded, and the initial 
wind radii were adjusted based on that data.  In addition, the eye 
of Sam passed about 60 n mi east of NOAA buoy 41049 earlier this 
morning.  The buoy reported a minimum pressure of 998 mb, maximum 
winds of around 45 kt, gusts to 62 kt, and maximum seas of about 22 
ft.

The major hurricane has turned to the north and is now moving 
faster.  The latest initial motion estimate is 355/18 kt.  The
large-scale pattern consists of a deep-layer ridge to the northeast
of Sam and a large low pressure system centered over Atlantic
Canada.  The steering flow between these features should cause Sam
to move northeastward at a sightly slower pace this weekend.  By
early next week, the deep-layer low is expected to retreat
northward, which should cause Sam to turn a little more to the
right.  The models are in fairly good agreement, but there are some
notable speed differences with the UKMET model being a fast outlier.
The new NHC track forecast is slightly to the right of the previous
one at 60 and 72 h, but is otherwise very similar.  This forecast
lies near the middle of the guidance envelope.

Sam is expected to slowly weaken during the next several days as it
moves over progressively cooler waters and into an environment of
stronger shear.  The hurricane will likely begin extratropical
transition in a couple of days when it crosses the 26 degree C
isotherm, and this transition is expected to be completed by 96
hours.  All of the model guidance shows steady weakening through the
forecast period, and so does the NHC forecast.  This prediction is
close to the IVCN, IVDR, and HCCA models in the short term and near
the GFS model during the extratropical portion of the forecast.
Even though Sam is likely to weaken, it is still expected to be a
significant storm over the next several days.


Key Messages:

1. Swells generated by Sam will impact the northern Leeward Islands 
and the Greater Antilles, including Puerto Rico, during the next 
couple of days.  Swells are expected to reach Bermuda and the 
Bahamas this afternoon, and then spread to the United States east 
coast and Atlantic Canada on Saturday.  These swells could cause 
life-threatening surf and rip current conditions.  Please consult 
products from your local weather office.

2. Tropical storm conditions are possible on Bermuda beginning
tonight or early Saturday, and a Tropical Storm Warning is in effect
for the island.


FORECAST POSITIONS AND MAX WINDS

INIT  01/1500Z 28.4N  61.8W  130 KT 150 MPH
 12H  02/0000Z 30.7N  61.5W  125 KT 145 MPH
 24H  02/1200Z 33.4N  60.2W  110 KT 125 MPH
 36H  03/0000Z 35.6N  58.4W  100 KT 115 MPH
 48H  03/1200Z 37.2N  56.0W   95 KT 110 MPH
 60H  04/0000Z 38.7N  52.9W   90 KT 105 MPH
 72H  04/1200Z 41.9N  48.6W   85 KT 100 MPH
 96H  05/1200Z 49.0N  42.2W   75 KT  85 MPH...POST-TROP/EXTRATROP
120H  06/1200Z 51.0N  34.3W   60 KT  70 MPH...POST-TROP/EXTRATROP

$$
Forecaster Cangialosi/Nepaul



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