[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 080243
TCDAT2

Hurricane Larry Discussion Number  30
NWS National Hurricane Center Miami FL       AL122021
1100 PM AST Tue Sep 07 2021

Larry's eye has become a little better defined since the previous
advisory, and the eyewall convective cloud tops have cooled, with a
nearly solid ring of -65C to -70C now surrounding the eye. Data from
an Air Force Reserve Hurricane Hunter aircraft indicate that the
central pressure has risen slightly to 966-968 mb and that peak
700-mb flight-level winds measured were 106 kt in the northeast
quadrant. These data would suggest that the intensity has decreased
a little to 95-96 kt. However, the peak flight-level wind of 106 kt
was measured in the wake of a strong eyewall convective burst,
suggesting that stronger winds could have been present in that
convection. Also, if the surrounding eyewall convective ring closes
off during the nocturnal convective maximum period in another six
hours, then Dvorak intensity estimates would increase from the
current T5.0/90 kt to T5.5/102 kt. Thus, Larry's intensity has been
maintained at 100 kt for this advisory in order to avoid possible
intensity vacillations.

The initial motion estimate is 325/10 kt, based on reconnaissance
aircraft fixes over the past 12 hours. There remains no significant
change to the track forecast or reasoning over the past 48 hours.
The latest NHC model guidance remains in strong agreement on Larry
continuing to move northwestward and then northward around the
western periphery of a deep-layer subtropical ridge during the next
48 hours or so.  After passing east of Bermuda by early day 3, the
hurricane is forecast to accelerate northeastward and move into the
mid-latitude westerlies ahead of a large, eastward-moving trough
that is currently approaching the northeastern United States and
Atlantic Canada. By day 5, Larry is expected to move across the far
north Atlantic.  The new NHC track forecast is similar to the
previous advisory track, and lies along the eastern edge of the
tightly packed simple- and corrected-consensus track models.

Larry's upper-level outflow pattern remains impressive, and has
even expanded and accelerated on the east side owing to a
southward-digging upper-level trough, which has enhanced the
outflow in that part of the hurricane. The vertical wind shear
affecting Larry is expected to remain low for the next 24 hours or
so while the hurricane passes over even warmer sea-surface
temperatures (SST) approaching 29.7 deg C, even though the oceanic
heat content will be decreasing. However, entrainment of dry
mid-level air along with eye fluctuations are expected to offset the
warmer SSTs. Thus, a slow but steady decrease in intensity through
48-60 hours is expected. Late in the forecast period, colder waters
and increasing southwesterly vertical wind shear exceeding 30 kt
should induce a faster rate of weakening.  By 96-120 hours, the
global models also indicate Larry will be merging with a frontal
zone.  Therefore, the new intensity forecast continues to show
extratropical transition during that time period.  The official
intensity forecast is similar to the previous advisory, and closely
follows the simple- and corrected-consensus intensity models IVCN
and NOAA-HCCA.

Key Messages:

1. Large swells generated by Larry will continue to affect the
Lesser Antilles, portions of the Greater Antilles, and the Bahamas
through midweek.  Significant swells should reach the east coast of
the United States and Atlantic Canada by midweek and continue
affecting these shores through the end of the week.  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 guidance of lifeguards and local officials this week.

2. The center of Larry is forecast to pass east of Bermuda on
Thursday, but given Larry's large size, tropical storm conditions
are possible there Thursday, along with a risk of heavy rainfall
and coastal flooding.  A Tropical Storm Watch is in effect for
Bermuda and interests there should closely monitor the latest
forecast updates.

3. Larry is forecast to move near or over portions of southeastern
Newfoundland late Friday and Friday night as it transitions to a
hurricane-force post-tropical cyclone. There is a risk of impacts
from high winds, rainfall, and storm surge in portions of
Newfoundland, and interests there should monitor the progress of
Larry and updates to the forecast.


FORECAST POSITIONS AND MAX WINDS

INIT  08/0300Z 25.8N  56.8W  100 KT 115 MPH
 12H  08/1200Z 27.2N  58.0W   95 KT 110 MPH
 24H  09/0000Z 29.2N  59.6W   95 KT 110 MPH
 36H  09/1200Z 31.5N  61.3W   90 KT 105 MPH
 48H  10/0000Z 34.6N  61.8W   90 KT 105 MPH
 60H  10/1200Z 39.1N  60.1W   85 KT 100 MPH
 72H  11/0000Z 44.5N  55.7W   80 KT  90 MPH
 96H  12/0000Z 56.2N  43.6W   65 KT  75 MPH...POST-TROP/EXTRATROP
120H  13/0000Z 62.8N  33.6W   50 KT  60 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart
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