Hurricane Sam Forecast Discussion Number 14

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September 25, 2021

Issued at 1100 PM AST Sat Sep 25 2021

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

Click here to read more at The National Hurricane Center

News date : 09/25/2021    Category : Hurricane, Weather

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