[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 262035
TCDAT3

Hurricane Sam Discussion Number  17
NWS National Hurricane Center Miami FL       AL182021
500 PM AST Sun Sep 26 2021

Sam has found a way to strengthen some more today. The ring of
intense convection surrounding the 15-n-mi-diameter eye has expanded
in size over the past several hours, while cloud tops colder than
-70C still completely surround the center. An earlier GMI microwave
overpass revealed very strong eyewall convection tightly wrapped
around the small eye, a feature oftentimes seen with intense
hurricanes. The latest Dvorak intensity estimates from TAFB and SAB
remain T6.5/127 kt. However, the UW-CIMSS ADT estimate has recently
increased to T6.6/130 kt. Based on the expansion of the ring of
convection and a blend of these Dvorak estimates, the initial
intensity is raised to 130 kt for this advisory. Sam remains
compact, with winds of tropical storm force extending only 70 n mi
from the center. A NOAA Hurricane Hunter aircraft is en route to
investigate Sam and will provide more detailed intensity data.

It would not take much further expansion of the convection and
cooling of the cloud tops over the inner-most core of Sam for it to
become a rare Category 5 hurricane on the Saffir-Simpson hurricane
wind scale. Although plausible given the ideal environmental
conditions over the next couple of days, this strengthening is not
explicitly forecast to occur. The only factors within the next 2-3
days that could cause Sam to weaken would be an eyewall replacement
cycle or the upwelling of cooler waters due to the hurricane's
relatively slow forward motion. By 72 h, vertical wind shear is
forecast to increase, which should cause gradual weakening. However,
given the current strength of Sam, the new forecast calls for it to
remain a major hurricane throughout the 5-day forecast period. The
latest NHC intensity forecast was increased by 5 kt through 60 h due
to the change in the initial intensity and remains on the high end
of the guidance through that time. At 72 h and beyond, the intensity
forecast is near the HFIP corrected consensus approach (HCCA) model.

Sam has been wobbling a little north-northwestward and has slowed
down over the past few hours, but the 12-h-average motion is about
305/06 kt. A subtropical ridge centered to the northeast of the
hurricane should steer it northwestward for the next 3-4 days. By
late Thursday, Sam is expected to reach the western periphery of the
ridge and turn north-northwestward and begin to increase its forward
speed as the cyclone starts to get caught up in the southerly flow
between the ridge and an approaching mid-latitude trough. The
various track model guidance is in better agreement today compared
to yesterday, especially in the day 3-5 time frame. The latest NHC
track forecast is little changed from the previous one, and lies
near the various consensus track model solutions.


FORECAST POSITIONS AND MAX WINDS

INIT  26/2100Z 14.2N  50.5W  130 KT 150 MPH
 12H  27/0600Z 14.7N  51.3W  130 KT 150 MPH
 24H  27/1800Z 15.5N  52.5W  125 KT 145 MPH
 36H  28/0600Z 16.3N  53.6W  125 KT 145 MPH
 48H  28/1800Z 17.2N  54.7W  120 KT 140 MPH
 60H  29/0600Z 18.1N  55.8W  120 KT 140 MPH
 72H  29/1800Z 19.2N  57.4W  115 KT 130 MPH
 96H  30/1800Z 22.1N  60.5W  110 KT 125 MPH
120H  01/1800Z 26.6N  62.9W  110 KT 125 MPH

$$
Forecaster Latto
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