Atlantic Basin Tropical Cyclones

Hurricane Sam Forecast Discussion


113 
WTNT43 KNHC 300856
TCDAT3

Hurricane Sam Discussion Number  31
NWS National Hurricane Center Miami FL       AL182021
500 AM AST Thu Sep 30 2021

Sam is a very impressive hurricane in infrared satellite imagery
this morning. The 25-nm-wide eye remains very distinct and is
surrounded by a ring of convection with cloud tops around -65C.
The outflow is also well established in all quadrants. An Air Force
Reserve Hurricane Hunter aircraft that performed three eyewall
penetrations overnight has reported peak 700-mb flight-level winds
of 139 kt in the northeastern, southeastern, and eastern eyewall, 
and peak SFMR surface winds of 119 kt.  These data still support an
initial wind speed of 125 kt.  Sam's minimum pressure has fallen a 
few millibars since the NOAA aircraft mission last evening.  The 
latest estimated pressure from dropsonde data gathered by the Air 
Force is 937 mb. NOAA buoy 41044 has recently reported a peak 
one-minute wind of 68 kt with a gust to 86 kt in the northeastern 
eyewall of Sam.  The buoy has also reported peak seas of 40 ft. 

Sam could still strengthen a little today as it continues to move
over a warm ocean eddy and remains in low vertical wind shear
conditions.  After that time, difficult-to-predict eyewall cycles 
however, could cause some fluctuations in intensity. After 48 
hours, gradually decreasing SSTs and ocean heat content are likely 
to cause a more definitive weakening trend, with a faster rate of 
weakening likely after day 3. Sam is forecast to complete its 
extratropical transition by day 5, and it is predicted by the 
global model guidance to become a large and powerful extratropical 
low over the north Atlantic. 

Sam is moving northwestward or 320/10 kt. The track forecast 
reasoning is again unchanged from the previous advisory. Sam will 
move northwestward, and then northward around the western portion 
of a subtropical ridge during the next 48 hours.  After that time, 
Sam is predicted to turn northeastward between the ridge and a 
large mid-latitude trough over the northeastern United States. As 
that trough lifts northeastward by day 3, Sam is forecast to 
continue on a northeastward heading, but it is not likely to 
accelerate as much as a typical recurving tropical cyclone over 
the north-central Atlantic.  The dynamical model guidance remains 
in excellent agreement through 72 hours, but there is increasing 
spread after that time.  The latest runs of the GFS and ECMWF 
models have come into somewhat better agreement at days 3 through 
5, and the NHC track forecast is near the consensus of those 
typically reliable models. 

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, the Bermuda Weather Service has issued 
a Tropical Storm Watch 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  30/0900Z 21.5N  59.1W  125 KT 145 MPH
 12H  30/1800Z 23.0N  60.2W  130 KT 150 MPH
 24H  01/0600Z 25.6N  61.4W  125 KT 145 MPH
 36H  01/1800Z 28.4N  61.8W  120 KT 140 MPH
 48H  02/0600Z 31.1N  61.3W  115 KT 130 MPH
 60H  02/1800Z 33.5N  59.9W  105 KT 120 MPH
 72H  03/0600Z 35.5N  58.0W   95 KT 110 MPH
 96H  04/0600Z 38.7N  52.6W   80 KT  90 MPH
120H  05/0600Z 44.3N  45.0W   65 KT  75 MPH...POST-TROP/EXTRATROP

$$
Forecaster Brown




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