[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 042056
TCDAT2

Hurricane Larry Discussion Number  17
NWS National Hurricane Center Miami FL       AL122021
500 PM AST Sat Sep 04 2021

Larry remains a formidable hurricane this afternoon. Both visible
and infrared satellite bands show the hurricane has a well-defined
and warm eye (greater than 10 C) surrounded by a cold ring of
eyewall convection (-60 to -70 C). An AMSR2 microwave pass received
at 1630 UTC showed the well-defined eye of Larry, though the 89 GHz
channel hinted that the eyewall was weaker on its eastern side.
Taking a look at the high-density atmospheric motion vectors (AMVs),
available thanks to a GOES-16 1-minute updating domain over Larry,
there is some restriction of Larry's outflow to the southwest.
Flight-level wind data from the NASA-DC8 aircraft conducting a
research mission around Larry also showed some light southwesterly
flow between 10-20 kt just a few degrees to the south and west of
the storm center. These data suggest that the upper-level wind
environment is not as pristine as earlier suggested by SHIPS
guidance, with the southwesterly flow likely restricting Larry's
outflow in that quadrant. The SAB/TAFB subjective Dvorak intensity
estimates are unchanged from this morning, and the latest UW-CIMSS
ADT and SATCON estimates have also plateaued in the 105-110 kt
range. Therefore, Larry was maintained as a 110 kt hurricane this
advisory.

Larry remains on a west-northwest track this afternoon, but a bit
slower at 300/12 kt. There has been little change to the track
philosophy over the next several days, as the guidance is in good
agreement that Larry will maintain a continued west-northwest
heading while gradually slowing down as it rounds the southern
periphery of a large mid-level ridge. However, there has been a
notable eastward shift in the track guidance in the short-term. The
latest ECMWF run, which had previously been on the southwest side of
the track guidance envelope, is now very similar or even a tad east
of the latest GFS run. This shift has also resulted in an eastward
adjustment in the consensus aids this afternoon. The latest NHC
track forecast was adjusted a bit to the right early on, but not as
far right as the HCCA and TVCA aids. After 72 hours, the track
guidance actually converges very close to the previous track
forecast, and few changes were needed after this time period. Based
on this forecast, Larry will continue moving across the central
Atlantic in the coming days, and be approaching Bermuda from the
southeast in the day 4 to 5 forecast period.

The existence of some light upper-level southwesterly flow ahead of
Larry today was bit of a surprise, since the SHIPS guidance from the
last few days suggested the shear-vector would be out of the east
and weak. Indeed, the latest ECMWF-SHIPS guidance now shows moderate
southwesterly vertical wind shear beginning earlier, and peaking
between 20-25 knots in 36 to 48 hours. While the GFS-SHIPS shear
remains much lower, given what I'm seeing from the latest
upper-level flow in front of Larry, the ECMWF seems closer to
correct. For this reason, the latest NHC intensity guidance now
shows a bit of weakening after 24 hours, when the shear magnitude is
expected to peak as the hurricane interacts with a large tropical
upper-tropospheric trough (TUTT) located to its northwest. However,
Larry has a large and vertically-deep circulation, and ultimately it
will win the battle against the more vertically shallow TUTT, which
is forecast to cut off and move away from the hurricane's expansive
upper-level outflow. By 60 hours, this change in the synoptic
pattern should once again reduce the vertical wind shear over Larry,
and it will have a chance to achieve a secondary peak between the
60-84 hour time-frame. However, eyewall replacement cycles could
also occur at any time over the next 2-5 days, providing additional
intensity fluctuations that make this a challenging intensity
forecast. The latest NHC intensity forecast is lower than the
previous one, but still remains higher than the latest HCCA
consensus aid. Regardless of the details, Larry is expected to
remain a large major hurricane over the next 3-4 days.

Key Messages:

1. Large swells generated by Larry are expected to first reach the
Lesser Antilles on Sunday and will spread to portions of the Greater
Antilles, the Bahamas, and Bermuda Monday and Tuesday.  Significant
swells will likely reach the east coast of the United States and
Atlantic Canada around midweek.  These swells will likely cause
life-threatening surf and rip current conditions, and beachgoers
and other interests along these coasts are urged to follow the
advice of lifeguards and local officials through the upcoming week.

2. Larry is forecast to approach Bermuda during the next several
days, possibly as a major hurricane, bringing a risk of strong
winds, heavy rainfall, and coastal flooding to the island by the
middle of next week.  While it is too soon to determine the
magnitude of these hazards and potential impacts on Bermuda,
interests there should monitor changes to the forecast during the
next several days.


FORECAST POSITIONS AND MAX WINDS

INIT  04/2100Z 17.4N  47.1W  110 KT 125 MPH
 12H  05/0600Z 18.4N  48.6W  115 KT 130 MPH
 24H  05/1800Z 19.8N  50.6W  120 KT 140 MPH
 36H  06/0600Z 21.0N  52.3W  115 KT 130 MPH
 48H  06/1800Z 22.2N  54.0W  110 KT 125 MPH
 60H  07/0600Z 23.4N  55.6W  110 KT 125 MPH
 72H  07/1800Z 24.7N  57.0W  115 KT 130 MPH
 96H  08/1800Z 28.3N  59.9W  110 KT 125 MPH
120H  09/1800Z 33.4N  61.9W   95 KT 110 MPH

$$
Forecaster Papin
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