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Hurricane Sam Forecast Discussion

WTNT43 KNHC 271459

Hurricane Sam Discussion Number  20
NWS National Hurricane Center Miami FL       AL182021
1100 AM AST Mon Sep 27 2021

Sam appeared weaker on satellite images earlier this morning, and
the eye was not apparent in visible or infrared satellite imagery.
However, a 27/0741 UTC SSMIS pass still showed a well-defined eye.
Recent visible and infrared images from the past couple of
hours however, show that the structure is becoming better organized
again, with the eye again becoming apparent on visible satellite.  
A NOAA Hurricane Hunter aircraft has made one pass through the 
hurricane so far and found that the central pressure has risen to
about 966 mb.  The current intensity estimate is reduced slightly 
to 110 kt.  The aircraft has not yet sampled the northeast 
quadrant, where the strongest winds are likely occurring.

The initial motion is estimated to be northwestward, or 310/7.  A
subtropical ridge to the north and northeast of Sam will remain the
primary steering mechanism over the next several days, and Sam is
expected to continue on a similar heading through the next 72 hours
or so.  A mid- to upper-level trough is forecast to emerge off the
U.S. Northeast and Mid-Atlantic coast and dig southward over the
western Atlantic late this week.  The deep-layer southerly flow
ahead of this feature should cause Sam to accelerate to the
north-northwest by late Thursday and then turn northward on Friday.
The NHC forecast track is largely unchanged from the previous,
except it is shifted ever so slightly westward at the hour 72 and 96
points, about midway between the previous NHC forecast and the
latest HCCA consensus.  The confidence in the track forecast is
medium to high through 3 to 4 days. Beyond that time, confidence is
about average, as there is some increase in model spread noted.

Regarding the intensity forecast, Sam is forecast to travel over
warm ocean temperatures while it remains in an environment of low
vertical wind shear through the next 4 days.  Some mid-level dry
air noted on water vapor channels to the west of Sam could play a
role in limiting Sam's intensity through hour 48.  However, the
environment could become more moist again after that time.  Due to
these competing factors, I opted to hold the intensity steady for
the next several days, although fluctuations in intensity can be
expected.  The NHC intensity forecast is above all model
guidance through hour 60, and is in agreement with the model
consensus at hour 72-96.  After that time, increasing wind shear
induced by the approaching mid- to upper-level trough to the
northwest of the cyclone should cause weakening.


INIT  27/1500Z 15.7N  52.1W  110 KT 125 MPH
 12H  28/0000Z 16.4N  52.8W  110 KT 125 MPH
 24H  28/1200Z 17.3N  53.9W  110 KT 125 MPH
 36H  29/0000Z 18.1N  55.1W  110 KT 125 MPH
 48H  29/1200Z 19.1N  56.5W  110 KT 125 MPH
 60H  30/0000Z 20.2N  58.1W  110 KT 125 MPH
 72H  30/1200Z 21.7N  59.8W  110 KT 125 MPH
 96H  01/1200Z 25.9N  62.2W  110 KT 125 MPH
120H  02/1200Z 32.0N  61.3W  100 KT 115 MPH

Forecaster Hagen/Pasch

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