[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 010858
TCDAT2

Tropical Storm Larry Discussion Number   3
NWS National Hurricane Center Miami FL       AL122021
800 AM CVT Wed Sep 01 2021

Deep convection with cloud tops colder than -80C have increased over
and to the west of the low-level center since the previous advisory.
Subjective satellite intensity estimates at 0600 UTC from TAFB and
SAB were T2.5/35 kt and T3.0/45 kt, while the most recent objective
intensity estimates from UW-CIMSS are T2.8/41 kt from ADT and 37 kt
from SATCON. An average of these intensity estimates support
increasing the advisory intensity to 40 kt, making the cyclone
Tropical Storm Larry. In addition, 0300 UTC and 0700 UTC
observations from ship VRNF3, which recently passed through the
center of Larry, reported a pressure of 1006.8 mb and winds near 25
kt. These data were the basis for the estimated central pressure of
1003 mb, a pressure value that also supports an intensity of 40 kt.

Larry has turned more westward over the past several hours, and the
new motion estimate is 280/17 kt. Larry is expected to move around
the southern and southwestern periphery of the sprawling
Bermuda-Azores ridge for the next 5 days, resulting in a general
west motion for the next 36 hours or so, followed by a turn toward
the west-northwest on Friday, and a northwestward motion over the
weekend and continuing into early next week. There has been a
pronounced westward shift in the track guidance for this cycle, with
the greatest shift coming from the GFS model. Over the past 36
hours, the GFS has shifted its track westward by more than 500 nmi,
and even the latest shift still keeps the GFS model the easternmost
track forecast in the guidance suite. In contrast, the ECMWF and
UKMET models, which lie along the westernmost portion of the
guidance envelope, have been fairly stable. Owing to the westward
shift in the overall guidance envelope, and considering the GFS
solution as an outlier model, the new NHC forecast track has also
been shifted westward, and lies between the NOAA-HCCA
corrected-consensus model to the south, and the tightly packed TVCA
simple-consensus model and FSSE corrected-consensus model to the
north. Given the poor handling of the ridge to the north of Larry by
the GFS, subsequent NHC forecast tracks may have to be shifted
farther west.

Given the improved inner-core wind field based on earlier ASCAT wind
data and reports from ship VRNF3, along with warm sea-surface
temperatures of 28 deg C and light easterly to southeasterly
vertical shear of around 5 kt, steady strengthening is expected for
the next 24 hours or so. By 36 hours when Larry is expected to be a
hurricane and have a well-established and tighter inner-core wind
field and possibly an eye, rapid intensification is forecast, with
Larry becoming a major hurricane by 72 hour. This in large part due
to the massive equatorward upper-level outflow pattern that all
of the global and regional models are forecasting, which is the
same type of outflow pattern that recently occurred with Hurricane
Ida. The new official intensity forecast is above the previous
advisory forecast by about 10 kt at all forecast times, and
conservatively follows an average of the Decay-SHIPS, COAMPS-TC,
FSSE, and ECMWF models. This intensity forecast is near the upper
end of the guidance envelope and is above the other consensus
intensity models.


FORECAST POSITIONS AND MAX WINDS

INIT  01/0900Z 12.3N  24.8W   40 KT  45 MPH
 12H  01/1800Z 12.5N  27.5W   50 KT  60 MPH
 24H  02/0600Z 12.6N  30.6W   60 KT  70 MPH
 36H  02/1800Z 13.1N  33.7W   70 KT  80 MPH
 48H  03/0600Z 13.6N  36.8W   85 KT 100 MPH
 60H  03/1800Z 14.4N  39.7W   95 KT 110 MPH
 72H  04/0600Z 15.5N  42.0W  100 KT 115 MPH
 96H  05/0600Z 18.2N  46.9W  105 KT 120 MPH
120H  06/0600Z 21.3N  50.6W  105 KT 120 MPH

$$
Forecaster Stewart
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