Atlantic Basin Tropical Cyclones

Hurricane Sam Forecast Discussion


499 
WTNT43 KNHC 030845
TCDAT3

Hurricane Sam Discussion Number  43
NWS National Hurricane Center Miami FL       AL182021
500 AM AST Sun Oct 03 2021

Sam has continued to weaken since the previous advisory, with cloud 
tops in the central dense overcast (CDO) having warmed to -60 to -65 
deg C near the center. However, the upper-level outflow pattern 
remains impressive  The initial intensity has been lowered to 90 kt 
based on an  average of the current data-T number of 4.5/77 kt and 
current intensity (CI) estimate of 5.5/102 kt from TAFB, and a 0530 
UTC UW-CIMSS SATCON intensity estimate of 89 kt. Further weakening 
during the is expected as Sam moves over progressively cooler water 
and southwesterly vertical wind shear of 20-25 kt continues. By 
18-24 hours, Sam will be moving north of the north wall of the 
Gulfstream, resulting in extratropical transition due to the cyclone 
moving over sub-20-deg-C sea-surface temperatures. However, 
favorable interaction with a strong mid-tropospheric trough will 
result in Sam becoming a large and powerful extratropical low with 
hurricane-force winds in 36-48 hours. Thereafter, Sam's wind field 
is expected gradually spin down over the far north Atlantic when 
baroclinic forcing will cease.  he new intensity forecast is similar 
to but slightly lower than the previous intensity forecast.

The initial motion is northeastward, or 045/15 kt. The latest NHC 
model guidance is tightly clustered around the previous track 
forecast, and thus, no significant changes were made to the previous 
advisory track.  A faster northeastward motion is expected during 
the next couple of days, followed by a slow down in the forward 
motion as Sam becomes entangled with an upper-level cutoff low. The 
new track forecast is essentially down the middle of the tightly 
packed simple and corrected-consensus models.

Recent ASCAT-A surface wind data indicate that no significant 
adjustments to the 34- and 50-kt wind radii were needed, but that 
the size of the 64-kt wind radii had to be decreased somewhat.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward Islands,
the Greater Antilles, the Bahamas, Bermuda, the eastern United
States and Atlantic Canada for the next couple of days.  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  03/0900Z 37.3N  55.9W   90 KT 105 MPH
 12H  03/1800Z 38.6N  53.6W   85 KT 100 MPH
 24H  04/0600Z 41.1N  49.4W   75 KT  85 MPH
 36H  04/1800Z 44.9N  44.1W   70 KT  80 MPH...POST-TROP/EXTRATROP
 48H  05/0600Z 49.5N  40.6W   65 KT  75 MPH...POST-TROP/EXTRATROP
 60H  05/1800Z 51.4N  39.0W   60 KT  70 MPH...POST-TROP/EXTRATROP
 72H  06/0600Z 52.0N  36.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
 96H  07/0600Z 57.0N  26.9W   40 KT  45 MPH...POST-TROP/EXTRATROP
120H  08/0600Z 60.5N  29.5W   35 KT  40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart




Source link