Atlantic Basin Tropical Cyclones

Hurricane Sam Forecast Discussion


000
WTNT43 KNHC 260251
TCDAT3

Hurricane Sam Discussion Number  14
NWS National Hurricane Center Miami FL       AL182021
1100 PM AST Sat Sep 25 2021

It's been an interesting evening with regards to analyzing the 
various data from a NOAA Hurricane Hunter research flight into 
Hurricane Sam. Dropsonde data in both the southeast and northwest 
quadrants indicate that small eyewall mesovortices and possibly even 
tornado-scale vortices were present based on the wind profiles 
showing sharply opposite-direction winds from what would normally be 
expected in those regions of the hurricane. Some dropsonde surface 
winds have been as high as 162 kt, which is more representative of a 
gust, while SFMR surface winds have been as high as 133 kt. However, 
the strongest 700-mb flight-level winds have been 138-139 kt in the 
northeastern quadrant, which equates to about 125-kt tangential 
surface winds. Three dropsondes released in the eye indicate that 
the pressure had remained steady at 943-944 mb during the duration 
of the aircraft reconnoiter. That pressure equates to about 125 kt 
based on various pressure-wind relationships. Based on that estimate 
and the 700-mb flight-level to surface-wind conversion, the advisory 
intensity is 125 kt, which is representative of the mean tangential 
winds and no localized wind perturbations.

The initial motion is toward the west-northwest, or 295/07 kt.  
Not to sound like a broken record, but no significant changes were 
made to the previous track forecast and reasoning. Sam is expected 
to move slowly west-northwestward and northwestward over the next 
few days around the southwestern periphery of a deep-layer 
subtropical ridge that is situated to the north and northeast of the 
small hurricane. On days 4 and 5, an upper-level trough/low is 
forecast to dig southward and amplify off the U.S. east coast and 
extend all the way to the Bahamas. This feature should act to lift 
Sam northward at a faster forward speed. The latest NHC model 
guidance based on 12Z and 18Z model runs has shifted noticeably to 
the east of the previous runs, and the new NHC track forecast has 
been nudged in that direction as well. However, since the NOAA G-IV 
jet aircraft has been out there sampling the environment around 
Sam, it's best to remain conservative and not shift the track any 
farther to the east until the new 00Z model runs with that new 
aircraft data come in for the next advisory package at 0600Z. The 
new NHC track forecast lies about halfway between the previous 
advisory track on the left and the tightly packed consensus track 
models on the right.

The radar images from the reconnaissance aircraft indicated that the 
eyewall was thin in many locations due to dry-air intrusions, and 
the latest SHIPS intensity output indicates that Sam will remain 
within a fairly dry mid-level environment. Also, the depth of the 
warm water beneath the hurricane isn't overly deep, which could 
result in cold upwelling owing to Sam's slow forward motion of only 
5-7 kt during the next couple of days. Eyewall replacement cycles 
are also likely now due to the hurricane's small size and strong 
intensity. Thus, fluctuations in intensity seem likely for the next 
couple of days even though the vertical wind shear is expected to 
remain quite low at only 5-10 kt. On days 3-5, however, the shear is 
forecast to increase to 15-20 kt from the southwest, which is 
expected to induce a slow weakening trend. However, it is likely 
that Sam will remain a major hurricane through 120 h, even on days 
3-5 due to the cyclone moving over warmer and deeper water during 
that 3-day period. The new official intensity forecast is 
essentially the same as the previous advisory, and remains above the 
consensus model and is near the higher end of the intensity 
guidance.

FORECAST POSITIONS AND MAX WINDS

INIT  26/0300Z 13.5N  49.0W  125 KT 145 MPH
 12H  26/1200Z 14.0N  49.9W  130 KT 150 MPH
 24H  27/0000Z 14.7N  51.0W  125 KT 145 MPH
 36H  27/1200Z 15.4N  52.1W  120 KT 140 MPH
 48H  28/0000Z 16.3N  53.2W  120 KT 140 MPH
 60H  28/1200Z 17.1N  54.3W  115 KT 130 MPH
 72H  29/0000Z 18.0N  55.6W  115 KT 130 MPH
 96H  30/0000Z 20.1N  58.5W  110 KT 125 MPH
120H  01/0000Z 23.7N  61.4W  110 KT 125 MPH

$$
Forecaster Stewart



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