[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 021451
TCDAT2

Hurricane Larry Discussion Number   8
NWS National Hurricane Center Miami FL       AL122021
1100 AM AST Thu Sep 02 2021

Larry continues to gradually become better organized this morning,
with a small but cold CDO near the estimated center, and well
defined curved bands rotating completely around.
Unfortunately, there have not been any recent microwave
passes over the center in the last 6-9 hours. However, I
did receive a helpful ASCAT-B pass at 1128 UTC which indicated that
while the inner core remains quite small, the 34-kt wind radii have
expanded dramatically in all quadrants. The most recent Dvorak
subjective intensity estimates were both CI 4.0/65 kt from SAB and
TAFB. However, the objective intensity estimates from SATCON and
ADT are higher, at 70 kt and 77 kt respectively. Thus, the current
intensity of Larry was nudged upward to 70 kt for this advisory.

The hurricane's heading has changed little this morning, estimated
at 280/15 kt. There also remains little change to the forecast track
reasoning. A dominant mid-level ridge is located north of Larry
and should maintain the hurricane on a west to west-northwest
heading over the next 72 hours. The track guidance is in excellent
agreement over this time period. After 72 hours, a bit more track
guidance spread begins to take shape, which appears related to the
evolution of the the steering ridge orientation. For example, the
most recent GFS run begins a more pronounced rightward bend, as the
ridge becomes positioned northeast of the tropical cyclone. In
contrast, the ECMWF and a number of its left-leaning ensemble
members maintain more ridging directly north of Larry, keeping the
hurricane on a more leftward track with only a gradual bend to the
west-northwest and northwest. These differences notwithstanding, the
consensus aids has changed little from the previous forecast cycle,
and the NHC forecast track is nearly identical to the previous
track. This track lies very close to the HCCA consensus aid, which
favors a track solution a bit closer to the ECMWF versus the GFS.

Larry's environmental conditions appear very favorable for
additional intensification over the next 60-72 hours. In fact, the
primarily forecast challenge relates to how Larry's core structure
evolves over the next several days. Right now, the inner-core and
hurricane-force wind radii are very small relative to the expanding
tropical-storm-force wind field around the storm. Thus, it appears
likely that Larry will undergo some form of an eyewall replacement
cycle (ERC) in the next 12 to 24 hours, which may slow down the
short term intensification rate. However, once this cycle is
complete, very light easterly vertical wind shear between 2-8 kt,
abundant mid-level moisture, and sufficently warm sea-surface
temperatures between 27-28 C should favor steady to rapid
intensification. The latest NHC intensity forecast now calls for a
peak intensity of 120 kt in roughly 72 hours, which is a faster and
slightly higher peak than the prior forecast. Afterwards, the
guidance is a bit conflicted. The GFS-based SHIPS continues to
suggests low vertical wind shear through the end of the forecast
period. However, the ECMWF-based SHIPS shows much higher
southwesterly vertical wind shear beginning in 84 hours as Larry
also moves into a drier environment. It is also possible additional
ERCs may occur in the latter portion of this forecast, resulting in
additional intensity fluctuations. For these reasons, the intensity
forecast at the end of the period shows some modest weakening. This
intensity forecast is a bit higher than the HCCA corrected consensus
aid, but remains lower than some of the more aggressive guidance
(COAMPS-TC, the experimental HAFS-B).

FORECAST POSITIONS AND MAX WINDS

INIT  02/1500Z 13.5N  34.2W   70 KT  80 MPH
 12H  03/0000Z 13.8N  36.7W   75 KT  85 MPH
 24H  03/1200Z 14.4N  39.7W   85 KT 100 MPH
 36H  04/0000Z 15.3N  42.6W   95 KT 110 MPH
 48H  04/1200Z 16.5N  45.2W  105 KT 120 MPH
 60H  05/0000Z 17.9N  47.6W  115 KT 130 MPH
 72H  05/1200Z 19.2N  49.7W  120 KT 140 MPH
 96H  06/1200Z 21.8N  53.1W  115 KT 130 MPH
120H  07/1200Z 24.6N  56.0W  110 KT 125 MPH

$$
Forecaster Papin
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