[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT42 KNHC 041459
TCDAT2

Hurricane Larry Discussion Number  16
NWS National Hurricane Center Miami FL       AL122021
1100 AM AST Sat Sep 04 2021

Overnight and this morning, Larry appears to have gone through a
rather quick eyewall replacement cycle (ERC). The most recent 1000
UTC GMI microwave pass suggests that Larry now has a much larger eye
and surrounding eyewall, with less evidence of concentric bands as
noted yesterday. On geostationary satellite the larger eye is also
becoming apparent on visible and infrared channels, though some left
over inner eyewall debris remains. Consistent with the larger eye,
an ASCAT-B pass received at 1226 UTC indicated that the 34-, 50-,
and 64-kt wind radii have expanded with Larry this morning. While
the most recent SAB and TAFB subjective Dvorak intensity estimates
have remain at 90 kt and 100 kt respectively, the
objective estimates are higher, with an earlier UW-CIMSS SATCON
estimate up to 112 kt, though it has recently been adjusted lower.
With the eye beginning to clear out on the most recent satellite
images, I am favoring the higher objective estimates, bringing Larry
up to 110 kt for this advisory.

Larry's track has remained fairly steady toward the west-northwest,
but just a touch slower and more poleward with the latest estimated
motion at 300/13 kt. This west-northwest motion is expected to
continue for the next several days with only a very gradual poleward
bend as Larry remains positioned to the south of an expansive
mid-level ridge extending westward ahead of Larry's expected path.
As has been the case the last several days, the track guidance is in
excellent agreement over the next 36-48 hours with more spread
becoming apparent thereafter. As discussed yesterday, the increase
in spread in the latter portion of the forecast appears to be
related to how much mid-level ridging stays north of Larry. The
ECMWF and its ensembles maintain more ridging that results in a
slower and more westward track, while the GFS and its ensembles
place more ridging east of Larry that results in a faster more
eastward track. The UKMET favors a solution closer to the ECMWF
while the Canadian favors a solution closer to the GFS. For now, the
latest NHC track has elected to remain close to the HFIP corrected
consensus approach (HCCA) which slightly favors the ECMWF solution
over the GFS. This latest track forecast is quite similar to the
previous one, but just a touch slower. Based on this forecast, Larry
will continue moving across the central Atlantic in the coming days,
and be approaching the latitude of Bermuda by the end of the
forecast period.

Conditions continue to remain favorable for additional
intensification over the next few days, with vertical wind shear
remaining under 10 kts, sea-surface temperatures gradually
increasing, and mid-level relatively humidity staying fairly moist.
However, by 48 hours, Larry will be impinging on the eastern edge of
a large upper-level mid-oceanic trough, which could result in an
increase in westerly vertical wind shear over the hurricane.
However, the models handle Larry's interaction with this synoptic
feature differently. The GFS suggests this upper-level trough will
fracture to the west fairly quickly, keeping a more favorable
upper-level pattern over Larry, while the ECMWF has a stronger
trough that takes longer to give way to Larry's upper-level outflow,
providing more westerly shear over the hurricane. Additional eyewall
replacement cycles are also possible in the coming days, which could
result in additional intensity fluctuations that are challenging to
predict ahead of time. For now, the latest NHC intensity forecast
keeps Larry intensifying over the next 24-36 hours, with only a very
gradual decay thereafter due to the somewhat less favorable
dynamical environment. The latest intensity forecast remains on the
high end of the guidance, noting that there still remain higher
outliers making Larry stronger than the 125 kt peak intensity
(HAFS-B, COAMPS-TC). Regardless of the details, Larry is expected to
remain a large major hurricane through the forecast period.

Significant ocean swells generated by Larry's growing wind field are
expected to reach the Lesser Antilles tomorrow, and then spread
westward to portions of the Greater Antilles, the Bahamas, and
Bermuda on Monday and Tuesday.  These swells could cause
life-threatening rip currents and high surf conditions.  Large
swells are also likely to spread to the east coast of the United
States by midweek.


FORECAST POSITIONS AND MAX WINDS

INIT  04/1500Z 16.8N  45.8W  110 KT 125 MPH
 12H  05/0000Z 17.7N  47.5W  115 KT 130 MPH
 24H  05/1200Z 19.1N  49.6W  120 KT 140 MPH
 36H  06/0000Z 20.3N  51.5W  125 KT 145 MPH
 48H  06/1200Z 21.5N  53.2W  120 KT 140 MPH
 60H  07/0000Z 22.6N  54.8W  120 KT 140 MPH
 72H  07/1200Z 23.8N  56.4W  115 KT 130 MPH
 96H  08/1200Z 27.0N  59.4W  110 KT 125 MPH
120H  09/1200Z 31.6N  62.0W  100 KT 115 MPH

$$
Forecaster Papin
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