Atlantic Basin Tropical Cyclones

Hurricane Sam Forecast Discussion


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WTNT43 KNHC 010846
TCDAT3

Hurricane Sam Discussion Number  35
NWS National Hurricane Center Miami FL       AL182021
500 AM AST Fri Oct 01 2021

An Air Force Reserve Hurricane Hunter aircraft investigated Sam a 
few hours ago and found that the already-intense hurricane had 
strengthened a bit since yesterday afternoon's reconnaissance 
flight.  The plane measured a peak 700-mb flight-level wind of 147 
kt in the northeastern eyewall, and using a typical 90-percent 
reduction from that altitude, the intensity is estimated to be 130 
kt.  Dropsondes released in the eye also indicated that the central 
pressure had fallen to 934 mb.  This drop in pressure is likely due 
to a combination of the strengthening and an expansion of the 
tropical-storm- and hurricane-force wind fields, which was 
confirmed by the aircraft and scatterometer data from last evening.

Sam is moving a little faster toward the north-northwest (345/15 
kt) as it curves around the western periphery of a mid-tropospheric 
ridge located over the eastern and central subtropical Atlantic.  
The hurricane is expected to turn northward later today and then 
northeastward by Saturday night--passing well to the east of 
Bermuda--when it begins to move between the ridge and a mid- to 
upper-level low located over the northeastern United States and 
Atlantic Canada.  Interaction and a possible merger with the 
aforementioned low during extratropical transition is likely to 
cause a sudden north-northeastward acceleration to the southeast of 
Newfoundland on day 4, followed by a slower motion toward the 
east-northeast on day 5.  The track models are very tightly 
clustered for much of the 5-day forecast period, yielding high 
confidence in the NHC track forecast.  The new NHC forecast is 
relatively unchanged from the previous forecast, and closely 
follows the GFEX and TVCA consensus aids.

Although fluctuations in intensity will still be possible, a 
general weakening trend is anticipated in the coming days as Sam 
moves over a gradually cooler ocean, reaching sub-26C waters by day 
3.  However, the weakening is not expected to be rapid due to 
continued upper-level divergence and only light-to-moderate 
deep-layer shear, and Sam could remain a major hurricane at least 
through Saturday night.  Global models now indicate that Sam should 
complete extratropical transition by day 4, and baroclinic forcing 
is likely to keep the system as a potent hurricane-force 
post-tropical cyclone through the end of the forecast period.  The 
NHC intensity forecast is essentially an update of the previous 
forecast, lying close to the HCCA and IVCN consensus aids.


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 today, and then spread to the United States east coast and 
Atlantic Canada 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
tonight or early Saturday, and a Tropical Storm Warning is in effect 
for the island.


FORECAST POSITIONS AND MAX WINDS

INIT  01/0900Z 26.6N  61.7W  130 KT 150 MPH
 12H  01/1800Z 28.8N  62.0W  125 KT 145 MPH
 24H  02/0600Z 31.7N  61.2W  115 KT 130 MPH
 36H  02/1800Z 34.2N  59.7W  105 KT 120 MPH
 48H  03/0600Z 36.0N  57.8W   95 KT 110 MPH
 60H  03/1800Z 37.7N  55.0W   90 KT 105 MPH
 72H  04/0600Z 40.1N  51.4W   85 KT 100 MPH
 96H  05/0600Z 47.7N  44.8W   75 KT  85 MPH...POST-TROP/EXTRATROP
120H  06/0600Z 50.1N  38.1W   65 KT  75 MPH...POST-TROP/EXTRATROP

$$
Forecaster Berg



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