[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 031456
TCDAT2

Hurricane Larry Discussion Number  12
NWS National Hurricane Center Miami FL       AL122021
1100 AM AST Fri Sep 03 2021

Larry continues to have the presentation of a hurricane that is
gradually becoming better organized. Its structure this morning
consists of well defined spiral banding around a somewhat asymmetric
central dense overcast. More recently, a warm spot appears to be
forming on both infrared and visible satellite channels which could
foreshadow the development of a better-defined eye, as hinted at on
an earlier 0934 UTC SSMIS microwave pass. The most recent subjective
Dvorak classifications at 1200 UTC were still CI 4.5/77 kt from TAFB
and SAB. Interestingly, the objective satellite estimates from ADT
and SATCON remain on the lower side, though these seem conservative
given the structural improvements seen on recent satellite images.
Favoring the subjective estimates, the initial intensity is being
maintained at 80 kt for this advisory.

Larry continues to move to the west-northwest at 285/14 kt. A
prominent mid-level ridge located to the north of the hurricane is
expected to continue this heading over the next 24 to 36 hours,
though with a gradual decrease in forward speed. As Larry continues
to move west-northwest, the ridge axis will gradually re-position to
the northeast of the tropical cyclone, providing an avenue for the
hurricane to begin gaining more latitude. Starting around 72 hours,
a bit more cross-track spread begins to emerge in the track
guidance, related to both how much ridging remains directly poleward
of Larry, and also the outer-core size of the tropical cyclone
itself. For example, the most recent GFS run shifts most of the
ridging to the east of Larry and also expands the outer radius of
cyclone dramatically, helping to widen the poleward weakness
allowing a more northward track by 120 hours. By contrast, the ECMWF
maintains a more compact hurricane in the mid-levels, and maintains
more mid-level ridging to the north of the cyclone. These
differences in the ECMWF allow a bit more of a westward track closer
to the Island of Bermuda. For now, the latest NHC track forecast is
in between these solutions, though with a slight preference towards
the ECMWF, which is also close to the latest HCCA consensus aid
forecast. This track forecast is quite similar to the previous
forecast. Given the track uncertainty by day 5, it is too soon to
determine what impacts Larry may pose to the Island of Bermuda, but
interests there should monitor updates in the forecast in the
subsequent days.

Larry remains embedded in a favorable environment for
intensification, with low vertical wind shear under 10 kt and a
sufficently moist mid-level environment. However, the hurricane has
had difficulty closing off its inner-core structure, which might be
preventing more rapid development from taking place. Assuming its
inner core becomes better established, Larry is expected to
intensity at a decent clip over the next 36-48 hours, and is
forecast to become a major hurricane this weekend with a peak
intensity of 120 kt by Monday. Afterwards, both the ECMWF-SHIPS
guidance and latest HWRF run suggest that westerly vertical wind
shear, from an upper-level mid-oceanic trough positioned northwest
of Larry, could begin to undercut the favorable upper-level
environment of the large tropical cyclone. Thus, some gradual
weakening is shown from days 3-5 in the NHC intensity forecast. This
latest forecast is quite similar to the previous one and is on the
upper-end of the guidance envelope, though still not as high as the
most recent COAMPS-TC and experimental HAFS-B runs. This intensity
forecast also does not account for possible eyewall replacement
cycles, which could cause additional intensity fluctuations that are
difficult to predict several days in advance.

Significant ocean swells generated by the large wind field of Larry
are expected to reach the Lesser Antilles on Sunday, increasing the
risk of life-threatening rip currents and high surf conditions on
those islands early next week. Large swells are likely to spread to
areas surrounding the western Atlantic later in the week as well.


FORECAST POSITIONS AND MAX WINDS

INIT  03/1500Z 14.8N  40.7W   80 KT  90 MPH
 12H  04/0000Z 15.4N  42.8W   90 KT 105 MPH
 24H  04/1200Z 16.5N  45.3W  100 KT 115 MPH
 36H  05/0000Z 17.6N  47.6W  110 KT 125 MPH
 48H  05/1200Z 19.0N  49.8W  115 KT 130 MPH
 60H  06/0000Z 20.2N  51.8W  120 KT 140 MPH
 72H  06/1200Z 21.4N  53.6W  120 KT 140 MPH
 96H  07/1200Z 24.2N  56.9W  115 KT 130 MPH
120H  08/1200Z 27.8N  60.0W  105 KT 120 MPH

$$
Forecaster Papin

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