Atlantic Basin Tropical Cyclones

Hurricane Sam Forecast Discussion


000
WTNT43 KNHC 302057
TCDAT3

Hurricane Sam Discussion Number  33
NWS National Hurricane Center Miami FL       AL182021
500 PM AST Thu Sep 30 2021

Sam remains a formidable hurricane, with a well-organized 
cloud pattern on satellite imagery.  The 25 n mi wide eye is 
surrounded by a fairly symmetric pattern of deep convective cloud 
tops, and the upper-level outflow pattern is very well defined.  An 
Air Force Reserve Unit Hurricane Hunter aircraft investigated the 
system and found peak 700-mb flight level winds of 133 kt, and a
highest unflagged SFMR-observed surface wind of 125 kt.  Based on 
these observations, the advisory intensity is kept at 125 kt.  
Measurements from the aircraft also showed that the central 
pressure remains quite low, near 938 mb.

Sam will continue to traverse waters of high oceanic heat content 
for another 12-18 hours, and the shear should remain fairly low for 
the next few days.  Therefore, some intensification could still 
occur overnight.  In any event, the system is expected to maintain 
major hurricane status for the next 36-48 hours.  Over the weekend, 
a gradual weakening trend should commence due to cooler SSTs.  
However, Sam will likely remain a powerful cyclone with hurricane- 
force winds until the end of the forecast period.  By 120 hours, 
simulated satellite imagery from the ECMWF and GFS models forecast 
the system's cloud pattern to resemble that of an extratropical low, 
and this is reflected in the official forecast, which shows 
extratropical status at that time.  The official intensity forecast 
remains close to the HCCA corrected consensus model solution.

Center fixes continue to show a gradual increase in forward speed, 
and the hurricane is moving northwestward, or about 320/12 kt.  
During the next 24-36 hours, Sam should turn toward the north as it 
moves around the western periphery of a large subtropical high over 
the eastern and central Atlantic.  Then, the cyclone is expected  to 
move generally northeastward in the flow between the anticyclone and 
a trough near Atlantic Canada.  There is fairly close agreement in 
the track models through 60 hours or so.  However, there are some 
significant differences in the model-predicted track of Sam at 
higher latitudes, probably due to variations in how the system 
interacts with the trough in the various guidance.  The NHC track 
forecast follows the latest model consensus, TVCN.  

Although the core of Sam is predicted 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 Warning has been 
issued 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 expected on Bermuda beginning
Friday night or early Saturday, and a Tropical Storm Warning has
been issued for that island.


FORECAST POSITIONS AND MAX WINDS

INIT  30/2100Z 23.6N  60.9W  125 KT 145 MPH
 12H  01/0600Z 25.6N  61.5W  130 KT 150 MPH
 24H  01/1800Z 28.5N  61.9W  125 KT 145 MPH
 36H  02/0600Z 31.3N  61.4W  110 KT 125 MPH
 48H  02/1800Z 33.8N  60.1W  100 KT 115 MPH
 60H  03/0600Z 35.8N  58.3W   90 KT 105 MPH
 72H  03/1800Z 37.4N  56.2W   85 KT 100 MPH
 96H  04/1800Z 42.5N  50.0W   75 KT  85 MPH
120H  05/1800Z 47.5N  46.0W   65 KT  75 MPH...POST-TROP/EXTRATROP

$$
Forecaster Pasch



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