[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT43 KNHC 260847
TCDAT3

Hurricane Sam Discussion Number  15
NWS National Hurricane Center Miami FL       AL182021
500 AM AST Sun Sep 26 2021

Sam remains a small but intense hurricane this morning, with a
well-defined 10 n mi-wide eye. A 0500 UTC AMSR-2 microwave pass
reveals Sam has a compact inner core of deep convection surrounding
its eye, with curved convective bands primarily extending around the
northern and eastern portions of the circulation. Infrared cloud top
temperatures briefly warmed overnight in portions of Sam's central
dense overcast, perhaps due to some intrusions of drier mid-level
air. The 06z objective and subjective satellite estimates ranged
from 113-127 kt, but Sam's satellite presentation has improved in
recent infrared imagery as cloud top temperatures are cooling around
the eye. Thus, the initial intensity is held at 125 kt for this
advisory. NOAA aircraft are scheduled to investigate Sam later this
afternoon, which should provide helpful information about the
structure and intensity of the hurricane.

Sam is moving toward the west-northwest, or 295/7 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 is very
similar to the previous one and generally lies between the HFIP
corrected consensus approach (HCCA) and TVCA aids. At days 4-5,
there is more spread in the track guidance, with the ECMWF on the
far left side of the guidance envelope and the GFS on the far right.
The NHC forecast track has been nudged just slightly to the left at
96 and 120 h, but it still lies to the right of HCCA.

The hurricane will likely experience some fluctuations in intensity
during the next day or two. On the one hand, Sam remains over warm
sea-surface temperatures of around 28.5 deg C, with vertical wind
shear less than 10 kt as diagnosed from the SHIPS guidance. However,
GOES-16 water vapor imagery shows some drier mid-level air in the
surrounding environment that may periodically entrain into the inner
core of Sam. Additionally, the onset of an eyewall replacement cycle
remains a distinct possibility at this stage of Sam's life cycle,
although the latest microwave data does not suggest one is imminent.
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 trends closer to the HCCA and IVCN consensus aids.
Some gradual weakening is forecast later in the period as the
southwesterly vertical wind shear begins to increase, but Sam is
still forecast to remain a major hurricane through the 5-day period.


FORECAST POSITIONS AND MAX WINDS

INIT  26/0900Z 13.8N  49.7W  125 KT 145 MPH
 12H  26/1800Z 14.3N  50.6W  125 KT 145 MPH
 24H  27/0600Z 15.0N  51.8W  120 KT 140 MPH
 36H  27/1800Z 15.8N  52.9W  120 KT 140 MPH
 48H  28/0600Z 16.6N  53.9W  115 KT 130 MPH
 60H  28/1800Z 17.5N  55.0W  115 KT 130 MPH
 72H  29/0600Z 18.4N  56.3W  115 KT 130 MPH
 96H  30/0600Z 20.8N  59.3W  110 KT 125 MPH
120H  01/0600Z 24.5N  62.0W  110 KT 125 MPH

$$
Forecaster Reinhart
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