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Hurricane Sam Forecast Discussion

WTNT43 KNHC 241455

Hurricane Sam Discussion Number   8
NWS National Hurricane Center Miami FL       AL182021
1100 AM AST Fri Sep 24 2021

After rapidly intensifying from a tropical depression to a hurricane 
over the past 24 hours, Sam is taking a momentary pause from 
intensification. The structure on visible satellite imagery remains 
quite healthy, with tightly coiled outer bands and a small but very 
cold central dense overcast. However, microwave passes at 0957 UTC 
and 1211 UTC on the 89-91 GHz channels show an erosion of the 
inner-core structure on the west side, which could be related to the 
dry-air slots mentioned in the previous advisory entraining into the 
core, temporarily disrupting the structure underneath the cirrus. 
The latest subjective Dvorak intensity estimates from TAFB and SAB 
were T4.0/4.0-65 kt and T4.0/4.5-75 kt while the latest UW-CIMSS ADT 
and SATCON objective estimates were both at 59 kt. The current 
intensity is being maintained at 65 kt for this advisory, taking a 
blend of these subjective and objective estimates. Sam remains a 
very small tropical cyclone, with the latest scatterometer wind data 
indicating that tropical-storm and hurricane-force winds have have 
not expanded much from last night. 

Sam continues to move just north of due west, but is beginning to 
gradually slow down at 280/12 kt. Over the next 24-36 hours, Sam is 
expected to maintain a general west to west-northwest heading but 
will slow down gradually as the strongest mid-level ridging becomes 
oriented more to the northwest of the hurricane. This ridge 
repositioning is thanks in part to a strong deep-layer trough well 
northeast of Sam that is digging in south of the Azores. Towards the 
end of the forecast, this trough will cutoff and gradually decay, 
allow the mid-level ridging to build-in back east as another 
deep-layer trough becomes established just off the eastern United 
States coastline. In general, the track guidance is in fairly good 
agreement on this general solution. However, differences remain in 
the details between how strong the ridge will be immediately to the 
north of Sam at the end of the 5-day forecast. The deterministic GFS 
and Canadian models show weaker ridging that erodes sooner, allowing 
a bit more poleward motion, while the ECMWF has stronger ridging 
that places its track on the equatorward side of the guidance 
envelope. The consensus track aid TCVN blends the aforementioned 
model guidance and has changed little this cycle, and the latest NHC 
track forecast is largely an update of the previous advisory. 

While the shear remains very low over Sam as diagnosed by the GFS- 
and ECMWF-based SHIPS guidance, the 700-500 mb layer mean relative 
humidity is a bit on the low side, and could possibly explain why 
Sam was susceptible to the dry-air intrusion that disrupted the core 
structure this morning. However, the vertical wind shear is expected 
to remain very low for the next 48-60 hours, and the hurricane 
should be able to easily mix out the dry air over the inner-core. 
The wind field also remains very compact, so once the inner-core 
closes back off, rapid intensification is expected to resume. The 
latest NHC intensity forecast still shows Sam becoming a major 
hurricane by tomorrow and currently has a peak intensity as a 
category 4 hurricane in 48-72 hours. Thereafter, a bit more vertical 
wind shear, and the likelihood of eyewall replacement cycles are 
likely to lead to fluctuations in the intensity that are difficult 
to predict ahead of time. The current intensity forecast remains 
close to the HCCA and IVCN consensus guidance.  Regardless of the 
details, Sam is expected to be a significant hurricane through the 
forecast period. 


INIT  24/1500Z 11.8N  43.7W   65 KT  75 MPH
 12H  25/0000Z 12.0N  45.4W   75 KT  85 MPH
 24H  25/1200Z 12.3N  47.1W   90 KT 105 MPH
 36H  26/0000Z 12.7N  48.4W  105 KT 120 MPH
 48H  26/1200Z 13.1N  49.7W  115 KT 130 MPH
 60H  27/0000Z 13.7N  50.9W  115 KT 130 MPH
 72H  27/1200Z 14.6N  52.1W  115 KT 130 MPH
 96H  28/1200Z 16.4N  54.8W  110 KT 125 MPH
120H  29/1200Z 18.2N  57.7W  110 KT 125 MPH

Forecaster Papin

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