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Hurricane Sam Forecast Discussion

WTNT43 KNHC 012034

Hurricane Sam Discussion Number  37
NWS National Hurricane Center Miami FL       AL182021
500 PM AST Fri Oct 01 2021

Sam remains a powerful hurricane.  The eye is still very distinct
and there are several mesovorticies evident in satellite imagery.
In addition, microwave images show a closed and symmetric eyewall.
Data from the Air Force Hurricane Hunters indicate that Sam has been
maintaining its category 4 intensity.  The aircraft measured peak
flight-level winds at 700 mb of 152 kt, which corresponds to a
135-140 kt surface wind. However, the peak SFMR winds are around 115
kt.  Dropsonde data from the aircraft show a sharp decrease in winds
around the 925 mb level, which could mean that the strongest winds
are not reaching the surface.  The minimum pressure in the eye
remains very low though at 937 mb.  Based on all of this data, the
initial intensity is held at 130 kt, but there is some uncertainty
in that estimate.

The major hurricane continues northward and the latest initial 
motion estimate is 355/17 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 on the 
faster side and the GFS on the slower side of the guidance envelope. 
No major changes were made to the NHC track forecast, and it remains 
near the consensus aids.

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 shortly
after 72 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
the Greater Antilles, the Bahamas, and Bermuda during the next
couple of days.  Swells are expected to reach 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.


INIT  01/2100Z 30.0N  61.8W  130 KT 150 MPH
 12H  02/0600Z 32.3N  61.0W  120 KT 140 MPH
 24H  02/1800Z 34.8N  59.4W  110 KT 125 MPH
 36H  03/0600Z 36.7N  57.3W  100 KT 115 MPH
 48H  03/1800Z 38.6N  54.2W   95 KT 110 MPH
 60H  04/0600Z 41.1N  50.1W   90 KT 105 MPH
 72H  04/1800Z 44.9N  45.6W   85 KT 100 MPH
 96H  05/1800Z 50.5N  40.5W   70 KT  80 MPH...POST-TROP/EXTRATROP
120H  06/1800Z 52.6N  30.7W   50 KT  60 MPH...POST-TROP/EXTRATROP

Forecaster Cangialosi/Nepaul

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