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Hurricane Sam Forecast Discussion

WTNT43 KNHC 251441

Hurricane Sam Discussion Number  12
NWS National Hurricane Center Miami FL       AL182021
1100 AM AST Sat Sep 25 2021

Sam has continued to intensify this morning and now has a complete 
ring of deep convection with cloud tops colder than -70 degrees C 
surrounding a mostly clear, small eye. The latest subjective Dvorak 
intensity estimates from TAFB and SAB are both 102 kt, while the 
combined objective Dvorak intensity estimates from the UW-CIMSS 
SATCON and ADT is 110 kt. Based on this data, the initial intensity 
has been increased to 105 kt for this advisory, making Sam a major 
hurricane. The cyclone remains compact, with hurricane-force winds 
extending only 20 n mi from the center. 

The hurricane wobbled to the west over the past few hours. However, 
the 12-hour motion is west-northwest at 9 kt. There are no changes 
to the forecast track reasoning. The ridge the north of Sam 
responsible for its current motion is forecast to shift to the east 
in a couple of days as a deep-layer trough establishes itself over 
the western Atlantic. Sam is forecast to turn northwestward around 
the southwestern periphery of the ridge in about 48 h, and gradually 
increase its forward speed thereafter. The model guidance continues 
to be in very good agreement with this scenario, and the latest NHC 
track forecast is unchanged from the previous one. It should be 
noted that the westernmost guidance continues to be the ECMWF 
ensemble mean, whose members have had a low bias in the intensity of 
Sam, which is likely contributing to the westward track bias. Based 
on the NHC forecast, and all of the other guidance, Sam is expected 
to still be well to the east or northeast of the northern Leeward 
Islands through day 5.  

Environmental conditions support further strengthening in the short 
term, and Sam is expected to become a category 4 hurricane by 
Sunday. In a few days, some southwesterly shear is forecast to 
develop as the cyclone begins to round the periphery of the 
subtropical ridge. Although this should cause Sam to weaken, it is 
still expected to remain a powerful hurricane throughout the 5-day 
forecast period. The latest NHC intensity forecast is near the 
previous one, which remains slightly above the various consensus 
solutions through 72 h, and then closely follows the consensus 


INIT  25/1500Z 12.9N  47.6W  105 KT 120 MPH
 12H  26/0000Z 13.3N  48.7W  110 KT 125 MPH
 24H  26/1200Z 13.7N  49.8W  115 KT 130 MPH
 36H  27/0000Z 14.3N  50.8W  120 KT 140 MPH
 48H  27/1200Z 15.0N  51.9W  120 KT 140 MPH
 60H  28/0000Z 15.8N  53.0W  115 KT 130 MPH
 72H  28/1200Z 16.6N  54.1W  115 KT 130 MPH
 96H  29/1200Z 18.4N  56.5W  110 KT 125 MPH
120H  30/1200Z 20.7N  59.8W  110 KT 125 MPH

Forecaster Latto

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