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Post-Tropical Cyclone Sam Forecast Discussion


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780 
WTNT43 KNHC 050838
TCDAT3

Post-Tropical Cyclone Sam Discussion Number  51
NWS National Hurricane Center Miami FL       AL182021
900 AM GMT Tue Oct 05 2021

Sam's low-level circulation center has become exposed on the south 
side of an east-to-west oriented cloud band containing cloud tops 
that have warmed to around -50 deg C. The cyclone's overall cloud 
pattern in satellite imagery resembles that of an occluded 
extratropical low pressure system, and surface analyses from the 
NOAA Ocean Prediction Center indicate that Sam's inner-core is now 
interacting with a frontal system.  Furthermore, the system is 
located over sea-surface temperatures of around 14 deg C. Although 
the system likely still has a warm-core thermal structure in the 
mid- and upper-levels of the troposphere based on earlier 04/2321Z 
AMSU-A/-B data, the low-level circulation now is comprised of an 
extensive field of cold-air stratocumulus clouds. Based on these 
data, Sam has made the transition to an extratropical cyclone over 
the far north Atlantic.  The initial wind speed has been lowered to 
70 kt based on a standard decay rate.

The initial motion estimate is northward or 010/20 kt. 
Extratropical-Sam has been moving north-northeastward at 35 kt 
since the previous advisory, but the latest global and regional 
models are all in very good agreement that the system will slow 
down significantly during the next 6-12 hours as it interacts with 
an approaching baroclinic mid- to upper-level trough/low. By late 
Tuesday, the post-tropical cyclone should turn east-northeastward, 
and then make a counter-clockwise loop southwest of Iceland on 
Wednesday. By Thursday, the large cyclone is expected to move 
east-northeastward or eastward and pass very near the southern coast 
of Iceland on days 4 and 5 as a weakening extratropical low pressure 
system. The new NHC track forecast is very similar to the previous 
advisory track, and lies close to the tightly packed simple- and 
corrected-consensus track models.

Extratropical-Sam is expected to maintain hurricane-force winds for 
the next 12-18 hours, followed by a slow weakening trend as the 
cyclone loses its baroclinic forcing.  However, Post-Tropical-Sam 
is still expected to pass close to Iceland in the 96-120-h period as 
an expansive low pressure system producing gale-force winds over a 
very large area.

This is the last advisory issued by the National Hurricane Center on 
Sam.  Additional information on this system can be found in High 
Seas Forecasts issued by the National Weather Service, under AWIPS 
header NFDHSFAT1, WMO header FZNT01 KWBC, and online at 
ocean.weather.gov/shtml/NFDHSFAT1.php.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward
Islands, the Greater Antilles, portions of the Bahamas, and
southeastern Newfoundland through midweek.  These swells
could cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.


FORECAST POSITIONS AND MAX WINDS

INIT  05/0900Z 51.0N  39.4W   70 KT  80 MPH...POST-TROP/EXTRATROP
 12H  05/1800Z 52.1N  38.9W   65 KT  75 MPH...POST-TROP/EXTRATROP
 24H  06/0600Z 52.8N  35.1W   60 KT  70 MPH...POST-TROP/EXTRATROP
 36H  06/1800Z 54.3N  30.0W   55 KT  65 MPH...POST-TROP/EXTRATROP
 48H  07/0600Z 57.9N  24.6W   50 KT  60 MPH...POST-TROP/EXTRATROP
 60H  07/1800Z 61.8N  23.9W   45 KT  50 MPH...POST-TROP/EXTRATROP
 72H  08/0600Z 62.7N  27.4W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  09/0600Z 61.7N  26.2W   40 KT  45 MPH...POST-TROP/EXTRATROP
120H  10/0600Z 63.6N  15.0W   40 KT  45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart