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Hurricane Sam Forecast Discussion

WTNT43 KNHC 252039

Hurricane Sam Discussion Number  13
NWS National Hurricane Center Miami FL       AL182021
500 PM AST Sat Sep 25 2021

After a brief hiccup, Sam has resumed rapid strengthening and now
shows a 12-15 n mi wide well-defined eye embedded in a central 
dense overcast with cloud tops colder than -70C. Subjective 
satellite intensity estimates from TAFB and SAB were 115 kt at 18Z, 
and objective intensity estimates have also been near 115 kt.  
Since the organization has increased some since 18Z, the initial 
intensity is increased to 120 kt.  A NOAA Hurricane Hunter aircraft 
is currently enroute to Sam to provide a better estimate of the 
intensity.  The hurricane has good cirrus outflow in all quadrants. 
However, a large arc cloud to the northwest of the central core 
suggests dry air is present in that area.

The initial motion is still west-northwestward or 290/9.  There are
again no changes to the forecast track reasoning.  The weak
subtropical ridge currently to the north of Sam is forecast to shift
eastward as a large deep-layer mid-latitude trough develops over the
western Atlantic.  This evolution should cause Sam to continue
west-northwestward with some decrease in forward speed during the
next couple of days, followed by a turn toward the northwest.  Some
increase in forward speed is likely by 120 h as Sam encounters
stronger steering flow.  The track guidance has shifted a little to
the north and east since the previous advisory, and the new forecast
track is also nudged in that direction, with the new track lying
between the HCCA corrected consensus model and the other consensus
models.  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.

The current strengthening is expected to continue for the next
6-12 h, and based on this the new intensity forecast now calls for a
peak intensity of 130 kt.  After the peak, Sam is expected to be in
an environment of warm sea surface temperatures and light wind
shear for at least the next 3-4 days.  This suggests the intensity
will be controlled mainly by eyewall replacement cycles and
possible intrusions of dry air, which would cause some short-term
fluctuations in intensity.  In the bigger picture, the intensity
guidance calls for a slow weakening during the 24-120 h forecast
period, and the new intensity forecast does the same.  However, it
is likely that Sam will remain a major hurricane through 120 h.


INIT  25/2100Z 13.3N  48.5W  120 KT 140 MPH
 12H  26/0600Z 13.7N  49.4W  130 KT 150 MPH
 24H  26/1800Z 14.3N  50.5W  125 KT 145 MPH
 36H  27/0600Z 14.9N  51.6W  120 KT 140 MPH
 48H  27/1800Z 15.7N  52.7W  120 KT 140 MPH
 60H  28/0600Z 16.6N  53.9W  115 KT 130 MPH
 72H  28/1800Z 17.4N  55.0W  115 KT 130 MPH
 96H  29/1800Z 19.5N  58.0W  110 KT 125 MPH
120H  30/1800Z 22.5N  61.0W  110 KT 125 MPH

Forecaster Beven

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