Atlantic Basin Tropical Cyclones

Hurricane Sam Forecast Discussion


000
WTNT43 KNHC 301455
TCDAT3

Hurricane Sam Discussion Number  32
NWS National Hurricane Center Miami FL       AL182021
1100 AM AST Thu Sep 30 2021

Sam continues to look quite impressive on satellite images, with a 
20-25 n mi wide eye surrounded by very deep convection.  The 
system's convective banding features and upper-level anticyclonic 
outflow also remain well-defined, with particularly strong outflow 
to the northwest and north.  Subjective and objective Dvorak 
intensity estimates have not changed significantly since earlier 
this morning, so the advisory intensity will remain at 125 kt.  
Interestingly, aircraft observations show that Sam has been a 
little stronger than indicated by satellite-derived intensity 
estimates, which underscores the value of aerial reconnaissance 
of tropical cyclones.  Another Air Force Hurricane Hunter aircraft 
is scheduled to investigate the hurricane later today.

Sam should continue to move over waters of high oceanic heat 
content and within a low-shear atmospheric environment for the next 
24 hours or so.  Thus, it could still strengthen a little today and 
tonight. By 48 hours, decreasing SSTs and increasing shear should 
lead to a gradual weakening trend.  Around day 5, global model 
fields suggest that Sam will be merging with a baroclinic zone and 
transition into a large and powerful extratropical cyclone over the 
north Atlantic.  The official intensity forecast is in good 
agreement with the HCCA corrected consensus model prediction.

The hurricane is moving a little faster toward the northwest, or at 
about 320/11 kt.  During the next couple of days, a gradual turn 
toward the north is likely as Sam rounds the western end of a large 
subtropical anticyclone over the eastern and central Atlantic. 
Thereafter, Sam is forecast to move generally northeastward in the 
flow between the anticyclone and a trough near Atlantic Canada.  
The NHC forecast is very similar to a consensus of the typically 
reliable GFS and ECMWF predictions, with a little bit of smoothing 
around days 3-4.

Although the core of Sam is forecast to pass east of Bermuda early
Saturday, the tropical-storm-force wind field is forecast to expand
and could pass very close to the island beginning late Friday night
or early Saturday. Therefore, a Tropical Storm Watch is in effect 
for Bermuda.


Key Messages:

1. Swells generated by Sam will impact the Northern Leeward Islands
and the Greater Antilles, including Puerto Rico, during the next
few days.  Swells are expected to reach Bermuda and the Bahamas by
Friday, 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.

2. Tropical storm conditions are possible on Bermuda beginning
Friday night or early Saturday, and a Tropical Storm Watch has
been issued for that island.


FORECAST POSITIONS AND MAX WINDS

INIT  30/1500Z 22.6N  60.0W  125 KT 145 MPH
 12H  01/0000Z 24.4N  60.8W  130 KT 150 MPH
 24H  01/1200Z 27.2N  61.7W  125 KT 145 MPH
 36H  02/0000Z 30.2N  61.6W  115 KT 130 MPH
 48H  02/1200Z 32.7N  60.8W  105 KT 120 MPH
 60H  03/0000Z 34.9N  59.2W   95 KT 110 MPH
 72H  03/1200Z 36.9N  57.0W   90 KT 105 MPH
 96H  04/1200Z 40.0N  51.0W   80 KT  90 MPH
120H  05/1200Z 44.5N  45.5W   65 KT  75 MPH...POST-TROP/EXTRATROP

$$
Forecaster Pasch



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