[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 060252
TCDAT2

Hurricane Larry Discussion Number  22
NWS National Hurricane Center Miami FL       AL122021
1100 PM AST Sun Sep 05 2021

A series of afternoon and evening passive microwave imagery indicate
that Larry has evolved from an earlier stable eyewall structure to
one with concentric eyewalls. The most recent microwave pass, a 2201
UTC SSMIS overpass, showed that Larry is likely in the latter stages
of this most recent eyewall replacement cycle (ERC), with a new
outer eyewall radius of nearly 50 nautical miles. However,
geostationary satellite imagery still shows quite a bit of inner
eyewall convection and it may take a while longer before this ERC is
complete. A recent ASCAT-A pass at 2339 UTC also indicated that
Larry's inner-core wind field had expanded further, likely related
to this recent ERC. The latest Dvorak intensity estimates from
TAFB/SAB remain unchanged from this afternoon, and the initial
intensity remains 110 kt this advisory. A NOAA P-3 aircraft will be
conducting a research mission into the Larry tomorrow and should
provide some helpful in-situ measurements to verify the hurricane's
intensity.

Larry's motion to the northwest continues to gradually slow down,
with the latest estimated motion at 310/10 kt. The forecast track
reasoning remains the same. Larry should maintain its northwestward
heading for the next 2-3 days as it remains steered around the
southern periphery of a mid-level ridge to the hurricane's
northeast. After 72 hours, Larry will be reaching the western extent
of this ridge while an amplifying mid-latitude trough will be
approaching from the northeastern United States. The combination of
these features should help Larry recurve, first to the north, and
then northeast with accelerating forward motion by the end of the
forecast period. The track guidance continues to be in excellent
agreement, and very few changes were made to the track forecast this
cycle, staying close to the consensus aids HCCA and TVCA.

The intensity forecast with Larry is somewhat tricky. Larry's wind
field continues to grow, following the multiple ERCs that have
occurred over the past few days. At the same time, Larry's forward
motion has gradually been slowing down, with the latest estimate
down to 10 kts. The combination of Larry's widening wind field with
its slower forward motion opens the storm up to possible upwelling
of cooler ocean waters near its inner-core. In fact, a drifting buoy
that Larry moved over in the past 24 hours showed a significant drop
in sea-surface temperatures from 27.5 C to less than 26 C. These
in-situ measurements are helpful, because they are lower than the
SHIPS-derived sea-surface temperature values and more consistent
with the upwelling recently forecast by the HWRF model. Adding to
the forecast complexity is the ongoing ERC which could be ending
over the next 6-12 hours. The latest NHC intensity forecast is
similar to the previous one for the first 24 hours, showing little
change in intensity after ERC completion. Thereafter, while
dynamical conditions near Larry are expected to become more
favorable (lower vertical wind shear), the anticipated upwelling
from Larry's large wind field as the storm moves slowly
northwestward is forecast to result in gradual decay. The latest NHC
intensity forecast is a bit lower than the previous one after
48 hours, following the latest HCCA consensus aid closely. However,
Larry's wind field is also forecast to expand over this time period,
making the cyclone a prolific swell and surf producer.

Key Messages:

1. Large swells generated by Larry are already affecting the Lesser
Antilles and will spread to portions of the Greater Antilles, the
Bahamas, and Bermuda on Monday and Tuesday.  Significant swells
should reach the east coast of the United States and Atlantic Canada
by 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 this week.

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


FORECAST POSITIONS AND MAX WINDS

INIT  06/0300Z 21.0N  51.4W  110 KT 125 MPH
 12H  06/1200Z 22.0N  52.6W  110 KT 125 MPH
 24H  07/0000Z 23.2N  54.1W  110 KT 125 MPH
 36H  07/1200Z 24.6N  55.4W  105 KT 120 MPH
 48H  08/0000Z 26.0N  56.7W  100 KT 115 MPH
 60H  08/1200Z 27.6N  58.2W   95 KT 110 MPH
 72H  09/0000Z 29.4N  59.9W   90 KT 105 MPH
 96H  10/0000Z 34.4N  61.6W   85 KT 100 MPH
120H  11/0000Z 42.9N  55.8W   80 KT  90 MPH

$$
Forecaster Papin
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