[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT42 KNHC 022049
TCDAT2

Hurricane Larry Discussion Number   9
NWS National Hurricane Center Miami FL       AL122021
500 PM AST Thu Sep 02 2021

This afternoon, Larry's appearance has evolved from a small central
dense overcast to a tightly coiled spiral banding pattern, with
these bands wrapping into the estimated center. I finally received a
useful SSMIS microwave overpass at 1857 UTC which provided evidence
that Larry could be undergoing an eyewall replacement cycle (ERC).
The imagery suggests a large outer eyewall is taking shape on the
low-level 37-GHz channel, while an inner eyewall fragment is still
seen on the mid-level 91-GHz channel. Despite these structural
changes, the 1800 UTC subjective Dvorak estimates from TAFB and SAB
were unchanged from this morning at CI 4.0/65 kt. In addition, the
latest objective intensity estimates from SATCON and ADT have only
varied slightly at 73 kt and 74 kt respectively. Taking a blend of
the subjective and objective estimates still yields 70 kt for this
advisory.

Larry continues to move just north of due west, with the latest
motion estimated at 280/16 kt. There is not much new to report with
the track reasoning, with a large mid-level ridge currently north of
Larry expected to steer the hurricane to the west or west-northwest
over the next 2-3 days. Over time, the ridge axis is forecast to
slowly shift further northeast of Larry's position, allowing the
hurricane to gradually gain a more rightward component of motion by
the end of the forecast period. In general, the track guidance
remains tightly clustered for the first 2-3 days. There was a
notable shift west in the latest 12z GFS deterministic track, while
the 12z ECMWF shifted ever so slightly right. A quick look at the
12z ECMWF ensemble guidance reveals that the deterministic run is a
bit to the northeast of the ensemble mean. The latest NHC track has
been shifted slightly to the south and west of the previous track,
following the reliable HFIP corrected consensus aid (HCCA) which
also shifted a bit south and west this cycle.

Once Larry completes the ongoing ERC, significant or even rapid
intensification is anticipated. The short-term intensity forecast
has been raised a bit more, since it appears more likely the present
ERC will finish in the next 6-12 h. The latest NHC intensity
forecast now makes Larry a major hurricane by tomorrow night, with
further intensification up to Category 4 intensity anticipated by
Sunday. Once again, there remains some discrepancy in the vertical
wind shear forecast between the GFS- and ECMWF-based SHIPS guidance,
with the ECMWF forecasting significantly more southwesterly shear
beyond 72 hours. Both guidance tools also indicate a decrease in
mid-level relative humidity which, if the shear materializes, could
disrupt Larry's inner core structure. Thus, the NHC intensity
forecast still shows gradual weakening from 72 to 120 hours. The
latest intensity forecast is just a hair above HCCA in the short
term, with the intensity forecast most closely following the latest
COAMPS-TC (CTCI) guidance in the first three days.

FORECAST POSITIONS AND MAX WINDS

INIT  02/2100Z 13.7N  35.8W   70 KT  80 MPH
 12H  03/0600Z 14.0N  38.4W   80 KT  90 MPH
 24H  03/1800Z 14.7N  41.3W   95 KT 110 MPH
 36H  04/0600Z 15.7N  43.9W  105 KT 120 MPH
 48H  04/1800Z 16.9N  46.3W  110 KT 125 MPH
 60H  05/0600Z 18.3N  48.6W  120 KT 140 MPH
 72H  05/1800Z 19.7N  50.6W  120 KT 140 MPH
 96H  06/1800Z 22.2N  54.0W  115 KT 130 MPH
120H  07/1800Z 25.1N  57.0W  110 KT 125 MPH

$$
Forecaster Papin
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