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Hurricane Larry Discussion Number  13
NWS National Hurricane Center Miami FL       AL122021
500 PM AST Fri Sep 03 2021
 
Over the course of the day, Larry's appearance has become more 
impressive. The eye has been gradually becoming better defined on 
visible satellite imagery as the center cloud top temperature has 
been warming on the infrared channel. Several microwave passes have 
been received since the last advisory. The most recent pass at 1900 
UTC suggested the eye and surrounding eyewall convection is becoming 
better defined, though still weaker on the east side. 1800 
UTC subjective Dvorak classifications were still both CI 4.5/77 kt 
from TAFB and SAB. However, the objective satellite estimates have 
been rising quickly, with the most recent UW-CIMSS SATCON estimate 
up to 84 kt. Thus, the NHC advisory intensity has been raised to 85 
kt for this advisory, making Larry a category 2 hurricane.
 
Larry's motion has remained fairly steady to the west-northwest 
throughout the day, at 285/14 kt. The latest track reasoning remains 
unchanged, with Larry being steered to the west-northwest around the 
southern periphery of an expansive mid-level ridge centered  north 
of the hurricane. This ridge axis will gradually become oriented to 
the northeast of Larry with time, allowing the tropical cyclone to 
gradually gain more latitude. Because the ridge is quite large, even 
as its axis shifts eastward, a portion of the ridge will still 
remain to the north of the storm. The end result is that Larry 
should only slowly gain a more poleward component of motion as the 
system also gradually slows down. While the track guidance spread 
remains small for the first 48-60 hours of the forecast, a bit more 
longitudinal spread becomes apparent thereafter, which appears 
related to how much mid-level ridging remains poleward of the 
cyclone. The latest 12z ECMWF run has a distinctly stronger ridge, 
oriented more poleward in comparison to the 12z GFS run. Thus, it is 
not surprising to see that the ECMWF track is a bit further west and 
slower than the GFS. The latest NHC track forecast this cycle was 
also adjusted a bit slower, following the TVCN and HCCA consensus 
aids. However, it is interesting to note the latest 12z ECMWF 
ensembles are even slower than this track forecast, and future 
slower adjustments may be needed if these track solutions verify. It 
remains 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.

With the recent improvement in Larry's structure on satellite 
imagery, the hurricane may be starting another rapid intensification 
(RI) cycle, as suggested by the high RI probabilities given in the 
DTOPS guidance. However, the earlier microwave data also suggested 
that there were a lot of concentric bands outside of the primary 
eyewall, and it is not out of the question that another eyewall 
replacement cycle could begin in the next 24-36 hours. Instead 
of attempting to forecast these often stochastic processes, the 
latest NHC intensity forecast will instead show a healthy rate of 
deepening over the next several days, now taking Larry to 120 kt in 
48 hours. Afterwards, there still remain questions as to how much 
the vertical wind shear will increase over Larry, with the 
ECMWF-SHIPS indicating higher shear that could affect Larry's 
inner-core structure, while the GFS-SHIPS showing much lower shear. 
Warmer sea-surface temperatures also await the hurricane in the 
60-96 hour period. Due to these competing factors, the intensity 
forecast during this period will only show very gradual weakening. 
The latest NHC intensity forecast remains on the high end of the 
guidance, but still remains lower than the higher HAFS-B and 
COAMPS-TC models. It should be noted that the majority of the 
guidance also shows Larry becoming a very large hurricane, with a 
significant expansion of the 34-, 50-, and 64-kt wind radii also 
forecast.
 
Significant ocean swells generated by the increasingly 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  03/2100Z 15.0N  42.0W   85 KT 100 MPH
 12H  04/0600Z 15.8N  44.1W   95 KT 110 MPH
 24H  04/1800Z 16.9N  46.5W  105 KT 120 MPH
 36H  05/0600Z 18.1N  48.7W  110 KT 125 MPH
 48H  05/1800Z 19.4N  50.8W  120 KT 140 MPH
 60H  06/0600Z 20.7N  52.8W  120 KT 140 MPH
 72H  06/1800Z 21.9N  54.5W  115 KT 130 MPH
 96H  07/1800Z 24.7N  57.6W  115 KT 130 MPH
120H  08/1800Z 28.7N  60.5W  105 KT 120 MPH
 
$$
Forecaster Papin
 
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