Atlantic Basin Tropical Cyclones

Hurricane Sam Forecast Discussion


000
WTNT43 KNHC 250831
TCDAT3

Hurricane Sam Discussion Number  11
NWS National Hurricane Center Miami FL       AL182021
500 AM AST Sat Sep 25 2021

Sam continues to rapidly intensify.  Geostationary satellite and
microwave images show that the storm has a very small and distinct
eye surrounded by a nearly symmetric ring of cold cloud tops.  The
latest Dvorak estimates range from 90 to 102 kt, and based on that
data, the initial intensity is increased to 95 kt.  Although Sam
is nearly a major hurricane, it remains quite compact with
tropical-storm-force and hurricane-force winds estimated to only
extend 50 n mi and 15 n mi from the center, respectively.

The hurricane is moving west-northwestward at 11 kt and is currently
being steered by a mid-level ridge to its north. There has been
little change to the track forecast philosophy or model guidance.
Sam is expected to continue west-northwestward during the next two
or three days, but it is forecast to move at a notably slower pace
of 6-8 kt during most of that time period.  After that time, the
ridge is expected to shift eastward as a deep-layer trough becomes
established over the western Atlantic.  This change in the steering
pattern should cause Sam to turn northwestward and speed up some by
the middle of next week.  The models are in fairly good agreement,
and the NHC track forecast is largely an update of the previous one.
Based on this forecast, and nearly all of the reliable guidance,
Sam is expected to still be well to the east or northeast of the
northern Leeward Islands through day 5.

Continued steady or rapid strengthening is expected during the next 
day or two as the hurricane remains in near ideal conditions of very 
low wind shear and warm 29 deg C waters.  However, beyond that time 
the shear is expected to increase a little, and that will likely 
cause Sam to level off in strength or weaken slightly next week.  
The NHC intensity forecast is slightly above the models in the short 
term, given the impressive structure of the hurricane and favorable 
conditions, but falls in line with the consensus aids at the longer 
lead times.  Regardless of how strong Sam gets, nearly all of the 
models indicate that it will remain a powerful hurricane during the 
next several days.

FORECAST POSITIONS AND MAX WINDS

INIT  25/0900Z 12.8N  46.7W   95 KT 110 MPH
 12H  25/1800Z 13.1N  48.0W  110 KT 125 MPH
 24H  26/0600Z 13.5N  49.3W  115 KT 130 MPH
 36H  26/1800Z 14.0N  50.4W  120 KT 140 MPH
 48H  27/0600Z 14.7N  51.5W  120 KT 140 MPH
 60H  27/1800Z 15.4N  52.5W  115 KT 130 MPH
 72H  28/0600Z 16.3N  53.7W  115 KT 130 MPH
 96H  29/0600Z 18.0N  56.0W  110 KT 125 MPH
120H  30/0600Z 20.1N  59.0W  110 KT 125 MPH

$$
Forecaster Cangialosi



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