Atlantic Basin Tropical Cyclones

Hurricane Sam Forecast Discussion


042 
WTNT43 KNHC 010253
TCDAT3

Hurricane Sam Discussion Number  34
NWS National Hurricane Center Miami FL       AL182021
1100 PM AST Thu Sep 30 2021

Sam's satellite structure on infrared remains quite impressive this 
evening, with a circular eye staying quite warm (+10-20C) in 
comparison to the ring of cold cloud tops associated with the 
eyewall (-60 to -70C). The most recent microwave imagery was a SSMIS 
pass at 1957 UTC, which indicated the eyewall remains quite intense, 
if just a little asymmetric with a stronger western semicircle. This 
eyewall asymmetry is consistent with a touch of light southerly 
shear affecting the cyclone as diagnosed by ECMWF-SHIPS. However, 
this shear does not appear to be having a significant impact on the 
hurricane's intensity. While there have not been any recent aircraft 
data to determine Sam's intensity since the last mission departed 
the hurricane at around 2000 UTC, the satellite intensity estimates 
have not changed much this evening. Thus, the initial intensity has 
been held at 125-kt this advisory. Another Air Force Reserve 
Reconnaissance Mission will be flying into Sam overnight to get a 
better sense of the current intensity.

Following Sam's eye tonight, the hurricane is beginning to make a 
gradual turn northward at a faster motion, with the latest estimate 
now north-northwest at 345/14 kt. The track guidance in the 
short-term remains in good agreement that Sam should continue to 
turn to the north, north-northeast, and then northeastward as it is 
steered around the western edge of a large subtropical ridge over 
the central Atlantic. This track should take the hurricane east of 
Bermuda. While there is a large deep-layer trough located north of 
Sam, this feature will not initially capture the cyclone, as this 
trough first de-amplifies and merges with a larger deep-layer 
trough located over the far eastern Provinces of Canada. In fact, 
Sam's forward motion to the northeast in 48-72 hours may even slow a 
tad as it remains just south of the stronger upper-level 
westerlies. However, by 96-120 hours, the deep-layer low over Canada 
is expected to dig southeastward into North Atlantic, and will 
likely capture Sam as the hurricane rapidly accelerates ahead of 
this amplifying trough. It is in the 72-120 hour time-frame that a 
lot of the track guidance diverges, mostly related to the timing and 
degree of Sam's interaction of with this trough ejecting out of 
Canada. For now, the official NHC track forecast has elected to 
stay close to the track consensus TVCN, which has preformed well 
with this hurricane so far. This latest track is just a bit 
northeast and faster than the previous one after 72 hours.  

Sam still has another 24 hours under warm 28-29 C sea-surface 
temperatures while the southerly vertical wind shear is expected to 
remain light. Thus, the hurricane is expected to maintain Category 4 
intensity over the next day or so, with short-term intensity changes 
likely to be controlled by inner core dynamics (such as eyewall 
replacement cycles). Thereafter, sea surface temperatures decrease 
markedly as southerly shear is also expected to gradually increase. 
This combination should lead to steady weakening. After 72 hours, 
the rate of weakening has actually been slowed a tad, since the 
ECMWF, GFS, and Canadian models all show Sam receiving significant 
baroclinic support as it undergoes a dynamic extratropical 
transition while the hurricane phases with the large baroclinic 
trough emerging off Canada. Simulated satellite imagery from these 
models suggest that Sam's transition to an extratropical cyclone 
should take place between 96-120 hours, with the resulting cyclone 
producing a large wind footprint across the North Atlantic basin.  


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
tomorrow, 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 is in 
effect for the island.


FORECAST POSITIONS AND MAX WINDS

INIT  01/0300Z 25.0N  61.2W  125 KT 145 MPH
 12H  01/1200Z 27.2N  61.8W  125 KT 145 MPH
 24H  02/0000Z 30.2N  61.6W  115 KT 130 MPH
 36H  02/1200Z 32.9N  60.5W  105 KT 120 MPH
 48H  03/0000Z 35.0N  58.9W   95 KT 110 MPH
 60H  03/1200Z 36.6N  56.8W   85 KT 100 MPH
 72H  04/0000Z 38.7N  53.8W   80 KT  90 MPH
 96H  05/0000Z 44.6N  47.2W   75 KT  85 MPH
120H  06/0000Z 48.9N  42.5W   70 KT  80 MPH...POST-TROP/EXTRATROP

$$
Forecaster Papin




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