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Tropical Storm Sam Forecast Discussion


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WTNT43 KNHC 232059
TCDAT3

Tropical Storm Sam Discussion Number   5
NWS National Hurricane Center Miami FL       AL182021
500 PM AST Thu Sep 23 2021

Sam could be in the beginning stages of developing a small 
inner-core this afternoon as a central dense overcast is beginning 
to take shape. On geostationary visible satellite imagery, the storm 
has occasionally exhibited a clear area, though this feature has 
been tilted a bit southeast of the estimated low-level center 
position. In addition, overshooting convective cloud tops have been 
recirculating cyclonically around this feature, suggesting 
convective symmetrization that may help shield the low-level vortex 
from dry-air intrusion as seen earlier on the northwest side of Sam. 
Unfortunately, I have not received a high resolution microwave pass 
over Sam in 9-12 hours, so there remains some uncertainty on the 
structure underneath the cirrus canopy. The latest subjective Dvorak 
estimates from SAB and TAFB were 55-kt and 45-kt respectively. 
Taking a blend of these intensity estimates yields 50-kt for this 
advisory.

Sam's estimated motion is nearly identical to this morning, moving 
north of due west at 280/14 kt. A large and anomalously strong 
mid-level ridge is located poleward of Sam and should maintain the 
current west to west-northwest heading over the next 2-3 days. 
However, this ridge-axis is forecast to build in to the northwest 
ahead of Sam, which is likely to gradually slow down this forward 
motion over the next few days. In fact, Sam is forecast to move 
unusually slow for a tropical cyclone in the deep tropics east of 
the Lesser Antilles. Afterwards, a weakness in the ridge to the 
northwest is forecast to gradually develop as a broad deep-layer 
trough parks itself just offshore of the eastern United States by 
early next week. This could potentially allow Sam to gain a bit more 
latitude at a somewhat faster forward motion as the ridge axis 
shifts back east. The track guidance this cycle has slowed down a 
bit compared to this morning and is also a bit further south in the 
early portion of the forecast. The NHC official track will follow 
suit this advisory, though is not quite as slow as the GFS model. It 
is interesting to note that both the GFS and ECMWF ensemble guidance 
this afternoon shows a significant spread in track solutions by day 
5 in the cross-track direction, so this track forecast is a bit more 
uncertain than usual.

With the apparent improvement in inner-core structure inferred
on satellite today, Sam appears poised to rapidly intensify in the
short-term. In fact, the latest GFS-SHIPS guidance is now explicitly
forecasting rapid intensification over the next 24-48 hours. This
seems feasible given that the guidance is forecasting 5 kt or less
vertical wind shear over the next 60 hours as the cyclone traverses
over warm 28-29 C sea-surface temperatures. Thus, the intensity
guidance in the short-term has been raised once again, now showing a
50-kt increase in intensity over the next 48 hours taking Sam to
major hurricane intensity by Saturday afternoon. Remarkably, this
rapid intensification is still below the latest HCCA and LGEM
guidance over this time span. After this period, Sam is likely to
start undergoing inner core fluctuations, related to eyewall
replacement cycles that are challenging to forecast far in advance.
A modest increase in southwesterly vertical wind shear could also
occur in the day 3-5 period. Thus, the NHC intensity forecast still
tops off Sam as 110-kt hurricane by the end of the forecast period,
which is on the high end of the intensity guidance envelope.


FORECAST POSITIONS AND MAX WINDS

INIT  23/2100Z 11.1N  39.7W   50 KT  60 MPH
 12H  24/0600Z 11.5N  41.9W   60 KT  70 MPH
 24H  24/1800Z 11.9N  44.2W   75 KT  85 MPH
 36H  25/0600Z 12.3N  46.0W   90 KT 105 MPH
 48H  25/1800Z 12.6N  47.4W  100 KT 115 MPH
 60H  26/0600Z 13.1N  48.7W  105 KT 120 MPH
 72H  26/1800Z 13.7N  50.0W  110 KT 125 MPH
 96H  27/1800Z 15.2N  52.5W  110 KT 125 MPH
120H  28/1800Z 17.5N  55.5W  110 KT 125 MPH

$$
Forecaster Papin



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