Atlantic Basin Tropical Cyclones

Hurricane Sam Forecast Discussion


000
WTNT43 KNHC 261445
TCDAT3

Hurricane Sam Discussion Number  16
NWS National Hurricane Center Miami FL       AL182021
1100 AM AST Sun Sep 26 2021

The satellite presentation of Hurricane Sam has been quite steady
over the past 6 hours.  Sam continues to have a well-defined, 12 n 
mi wide eye on visible and infrared satellite imagery this morning, 
with cloud tops colder than -70C completely surrounding the center. 
The latest subjective satellite intensity estimates from TAFB and 
SAB were both T-6.5/127 kt, while the objective estimate from ADT is 
near 122 kt. Based on these data, there is no reason to change the 
125-kt initial intensity at this time, especially given that a NOAA 
Hurricane Hunter aircraft is scheduled for a late afternoon/early 
evening mission into Sam today.

Sam is moving toward the west-northwest, or 290/7 kt.  A deep-layer
subtropical ridge to the north and northeast of Sam should generally
steer the hurricane west-northwestward to northwestward for the next
several days.  By midweek, an amplifying upper-level trough will 
move off the east coast of the U.S. and dig southward over the 
western Atlantic, which will erode the western extent of the 
steering ridge.  The deep-layer southerly flow ahead of the trough 
should cause Sam to begin recurving, with a somewhat faster 
north-northwestward motion forecast by day 5.  The latest NHC track 
forecast was adjusted a little to the left, or southwest of the 
previous official forecast, mainly at days 4 and 5, but not quite as 
far southwest as the model consensus aids.

The hurricane will likely experience some fluctuations in intensity 
during the next few days.  Guidance, however, indicates that the 
chance of an eyewall replacement cycle during the next 36 hours is 
below climatology.  Sam will remain over warm sea-surface 
temperatures and in a low wind shear environment for the next 3 
days, with less than 10 kt of shear diagnosed by the SHIPS guidance. 
It is possible that Sam's slow forward motion during the next 2 to 3 
days could cause some upwelling of cooler water, but this will 
probably not result in significant weakening.  The official NHC 
intensity forecast lies on the high end of the guidance for the 
first 36-48 h of the forecast period.  Thereafter, the NHC forecast 
is closer to the middle of the guidance envelope.  Some gradual 
weakening is forecast later in the period as southwesterly vertical 
wind shear begins to increase, but Sam is still forecast to remain a 
major hurricane through the 5-day period.

The radii were expanded slightly by 5-10 n mi in the western 
semicircle from the previous advisory based on a 1155 UTC ASCAT-A 
pass.


FORECAST POSITIONS AND MAX WINDS

INIT  26/1500Z 13.9N  50.2W  125 KT 145 MPH
 12H  27/0000Z 14.4N  51.2W  125 KT 145 MPH
 24H  27/1200Z 15.2N  52.4W  120 KT 140 MPH
 36H  28/0000Z 16.0N  53.5W  120 KT 140 MPH
 48H  28/1200Z 16.8N  54.6W  115 KT 130 MPH
 60H  29/0000Z 17.7N  55.7W  115 KT 130 MPH
 72H  29/1200Z 18.6N  57.1W  115 KT 130 MPH
 96H  30/1200Z 21.4N  60.3W  110 KT 125 MPH
120H  01/1200Z 25.6N  63.1W  110 KT 125 MPH

$$
Forecaster Hagen/Pasch



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