[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT43 KNHC 242057
TCDAT3

Hurricane Sam Discussion Number   9
NWS National Hurricane Center Miami FL       AL182021
500 PM AST Fri Sep 24 2021

Sam's structure on satellite has improved this afternoon, with
overshooting cloud tops rotating quickly around the small central
dense overcast, providing the appearance that the hurricane is
mixing out the dry air that affected its core structure this
morning. In fact, the last few visible satellite images suggest that
a small eye is clearing out within the cirrus canopy. Interestingly,
a recent 1930 UTC SSMIS pass suggests concentric bands are also
forming in Sam's core structure and it remains to be seen how that
structure will affect the wind field evolution of the hurricane. The
latest subjective Dvorak intensities estimates form TAFB and SAB
were a consensus T4.5/75 kt, while the latest SATCON and ADT
estimates from UW-CIMSS have also increased this afternoon. The
current advisory intensity has been set to 75-kt. Given the
recent trends on satellite, this may be conservative.

The motion of Sam has remained on the same heading, though the
storm is beginning to slow down with the estimate now at 280/10
kt. An additional slowdown in forward motion is anticipated in the
short-term as the mid-level ridging shuffles to the northwest of
the tropical cyclone, impeding its forward motion. However, after
72 hours, this same ridge is expected to shift back to the
northeast of Sam as a deep-layer trough off the Eastern United
States coastline becomes established. Track guidance spread
increases towards the end of the forecast, with the ECMWF and GFS
remaining at odds, with the former on the equatorward side, and
the latter on the poleward side of the track envelope. The
consensus aids, however, have changed little from the previous
advisory and the latest NHC track forecast is quite similar to the
previous one.

Now that Sam appears to have mixed out the dry air seen this
morning, intensification, likely rapid, is resuming. The only fly in
the ointment is the current concentric banding structure on
microwave imagery, which hints at the possibility of an eyewall
replacement cycle. Assuming the smaller eye will not collapse in the
short term, rapid intensification appears likely over the next 12-24
hours and the intensity has been raised in the short term, taking
Sam to major hurricane intensity by tomorrow. The latest NHC
intensity forecast now shows a slightly higher peak at 120-kt in 48
hours followed by very gradual decay, due to a subtle increase in
vertical wind shear, potentially some upwelling effects due to the
slow forward notion, and eyewall replacement cycles that could lead
to a broadening of the wind field. The latest NHC intensity forecast
starts out on the high side of the intensity guidance but ends up
very close to the HCCA consensus aid by the end of the forecast
period.  Regardless of the details, Sam is expected to be a
significant hurricane through the forecast period.


FORECAST POSITIONS AND MAX WINDS

INIT  24/2100Z 12.1N  44.8W   75 KT  85 MPH
 12H  25/0600Z 12.3N  46.5W   90 KT 105 MPH
 24H  25/1800Z 12.7N  48.1W  105 KT 120 MPH
 36H  26/0600Z 13.1N  49.3W  115 KT 130 MPH
 48H  26/1800Z 13.5N  50.5W  120 KT 140 MPH
 60H  27/0600Z 14.3N  51.7W  115 KT 130 MPH
 72H  27/1800Z 15.1N  52.9W  115 KT 130 MPH
 96H  28/1800Z 16.9N  55.1W  110 KT 125 MPH
120H  29/1800Z 18.9N  58.1W  110 KT 125 MPH

$$
Forecaster Papin
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