EPAC: GILMA - Post-Tropical - Discussion

[DorkyMcDorkface]

Donut watch in effect

[aspen]

The eye was kinda cloudy and ragged for a bit this morning, but now it’s quickly improved again in the last few hours, so it’s probably a Cat 4.

[Yellow Evan]

B ring was broken at 18z. Likely no Category 4 support from Dvorak.

[Yellow Evan]

Classic EPAC hurricane under some easterly shear, with the CDO not expanding outward and the eye less defined than what you'd expect in a stronger storm. Sustaining well so far and will likely continue until its upwells from its cold wake due to low OHC.

[Yellow Evan]

EP, 07, 2024082218, , BEST, 0, 169N, 1247W, 110, 955, HU, 34, NEQ, 100, 90, 80, 100, 1013, 260, 10, 0, 0, E, 0, , 0, 0, GILMA, D, 0, , 0, 0, 0, 0, genesis-num, 018,

[DorkyMcDorkface]

## ANNULAR HURRICANE INDEX (AHI) EP072024 GILMA 08/22/24 18 UTC ##
## PASSED SCREENING STEP, MIGHT BE ANNULAR, CALCULATE AHI FROM DISCRIMINANT ANALYSIS ##
## AHI= 1 (AHI OF 100 IS BEST FIT TO ANN. STRUC., 1 IS MARGINAL, 0 IS NOT ANNULAR) ##

https://ftp.nhc.noaa.gov/atcf/stext/240 ... _ships.txt

[Kingarabian]

Close to being annular that's for sure. Will wrinkle the forecast.

[aspen]

Rip, stuck at 110 kt for 18z despite the warmer, clearer eye relative to earlier today. It looks like a marginal Cat 4 IMO, but without a deeper CDO and a more consistent B ring, Dvorak will never support it.

[Yellow Evan]

Surrounded B embedded LG with WMG yields T6.0 which at this split second we have.

[Kingarabian]

:uarrow: Surrounded B embedded LG with WMG yields T6.0 which at this split second we have.

Recent frames are > 6.0 since the W ring is embedded and halfway wrapped.

[cycloneye]

Hurricane Gilma Discussion Number 18
NWS National Hurricane Center Miami FL EP072024
1100 AM HST Thu Aug 22 2024

Gilma has maintained an impressive satellite appearance, with a
clear warm eye surrounded by a ring of -60 to -70 C cold cloud
tops. An SSMIS microwave pass received after the prior advisory
showed the eyewall remains well organized and mostly closed, with
only a limited number of rainbands observed outside of the inner
core. Subjective Dvorak satellite estimates were CI 5.5/102 kt from
SAB and 6.0/115 kt from TAFB, while objective intensity estimates
largely fell in between these values. Thus, the initial intensity
is being held at 110 kt for this advisory. A helpful scatterometer
pass was used to help adjust some of the 34-kt and 50-kt radii this
advisory.

The hurricane continues to move slowly west-northwestward across the
central portion of the East Pacific basin, with its estimated motion
at 290/6 kt. This general motion is expected to continue over the
next several days as Gilma is primarily influenced by a narrow
mid-level ridge to its north, though a gradual turn more westward
with some increase in forward motion is anticipated by early next
week as mid-level ridging builds in a bit more towards the end of
the forecast period. The track guidance this cycle is a bit more
poleward and a little faster than the prior cycle, and the NHC track
forecast was nudged in that direction, blending the previous
forecast track with the track consensus aids.

While I can not rule out a bit of additional intensification in the
short-term, the slow motion of Gilma could also make it susceptible
to upwelling cooler ocean waters underneath it. However, at the same
time, vertical wind shear as diagnosed by the GFS-SHIPS is expected
to remain quite low, under 10 kt for at least the next 48-60 h. Some
of the regional-hurricane models also suggest that Gilma may take
on some annular characteristics, with limited outer bands as the
eyewall remains intact, and current satellite imagery suggests that
appearance may already be taking shape. Since these storms are known
to weaken slower than expected by the cooler sea-surface
temperatures underneath, the NHC intensity forecast only shows very
gradual weakening over the next couple of days, which is higher than
the majority of the intensity guidance, aside from the HAFS-A/B
runs. Thereafter, westerly shear is forecast to increase as SSTs
remain in the 25-26 C range, and a faster rate of weakening is
anticipated by the end of the forecast period, more in line with the
intensity consensus.


FORECAST POSITIONS AND MAX WINDS

INIT 22/2100Z 17.0N 125.0W 110 KT 125 MPH
12H 23/0600Z 17.4N 126.0W 110 KT 125 MPH
24H 23/1800Z 17.7N 127.4W 105 KT 120 MPH
36H 24/0600Z 18.1N 129.0W 100 KT 115 MPH
48H 24/1800Z 18.5N 130.6W 95 KT 110 MPH
60H 25/0600Z 18.7N 132.4W 90 KT 105 MPH
72H 25/1800Z 18.9N 134.4W 80 KT 90 MPH
96H 26/1800Z 19.5N 138.5W 60 KT 70 MPH
120H 27/1800Z 20.0N 143.5W 45 KT 50 MPH

$$
Forecaster Papin

[aspen]

Starting to see a smoothed-out CDO and a stadium effect appear, just like Douglas, Felicia, Darby, and Dora. Definitely a Cat 4 now and yet another system like the ones I mentioned that ADT is going to under-estimate.

[WaveBreaking]

[ElectricStorm]

07E GILMA 240823 0000 17.2N 125.2W EPAC 105 959

[aspen]

07E GILMA 240823 0000 17.2N 125.2W EPAC 105 959

Rip, there go its chances of getting operationally assessed as a low-end 4.

I doubt it’ll get upgraded in post because Calvin wasn’t.

[Kingarabian]

Under what basis are they dropping the intensity though? Dvorak at the high end can still support a 6.0. I would've kept it at 110kts. Currently the eye is less defined as earlier may be going through a ERC.

[cycloneye]

Hurricane Gilma Discussion Number 19
NWS National Hurricane Center Miami FL EP072024
500 PM HST Thu Aug 22 2024

Gilma's satellite presentation has degraded this evening, with the
eye becoming cloud filled and more irregularly shaped. Dvorak
Current Intensity estimates remain unchanged from TAFB (6.0/115 kt)
and SAB (5.5/102 kt) from earlier today, but most objective numbers
from UW-CIMSS now range between 95 and 105 kt. 105 kt is set as the
initial intensity as a blend of these numbers and the general
degradation of the eye definition.

Gilma's forward speed is now down to 5 kt toward the west-northwest.
A deep-layer trough over the U.S. West Coast is expected to
gradually move eastward, allowing mid-level high pressure to build
offshore. This should cause Gilma to very gradually accelerate
toward the west-northwest and then west through the entire 5-day
forecast period. There is high confidence in Gilma's forecast
track given a tightly packed guidance suite, and the updated NHC
track prediction has only been pushed northward a shade, following
the latest consensus trends. On the current forecast track, Gilma
would move into the central Pacific basin late Monday.

The ocean heat content near Gilma is only about 8 units (for
reference, ocean heat content in the northwestern Caribbean Sea and
Gulf of Mexico exceeds 100 units). Therefore, it is likely that
the hurricane's very slow motion is upwelling cold water from
below, which may be contributing to the recent degradation in the
satellite appearance. Cold water upwelling will probably continue
for the next couple of days while Gilma's forward speed remains 8
kt or less, and despite a relatively low-shear environment,
gradual weakening is forecast to continue through the entire
forecast period. The new NHC forecast has been adjusted downward,
but it still lies near the northern bound of the guidance suite.
Global model guidance suggests that the cool waters, a drier
environment, and increased shear may cause Gilma to struggle to
maintain deep convection by early next week, and the new forecast
now shows the cyclone becoming post tropical by day 5.


FORECAST POSITIONS AND MAX WINDS

INIT 23/0300Z 17.3N 125.5W 105 KT 120 MPH
12H 23/1200Z 17.6N 126.6W 100 KT 115 MPH
24H 24/0000Z 18.0N 128.1W 95 KT 110 MPH
36H 24/1200Z 18.4N 129.8W 90 KT 105 MPH
48H 25/0000Z 18.8N 131.5W 85 KT 100 MPH
60H 25/1200Z 19.0N 133.3W 80 KT 90 MPH
72H 26/0000Z 19.2N 135.3W 70 KT 80 MPH
96H 27/0000Z 19.8N 139.8W 55 KT 65 MPH
120H 28/0000Z 20.5N 145.3W 35 KT 40 MPH...POST-TROPICAL

$$
Forecaster Berg

[cycloneye]

Hurricane Gilma Discussion Number 20
NWS National Hurricane Center Miami FL EP072024
1100 PM HST Thu Aug 22 2024

The satellite presentation of Gilma has not changed much since the
previous advisory. The ragged eye of the hurricane is still
surrounded by deep convection, with the coldest cloud tops located
over the western and southern portions of the inner core. Based on
consensus T-5.5/102 kt Dvorak current intensity estimates from TAFB
and SAB, and similar estimates from the various UW-CIMSS objective
techniques, the initial intensity is held at 105 kt.

The center of the hurricane has jogged westward during the past few
hours, but the longer-term motion is still west-northwestward at
about 5 kt. A deep-layer trough near the west coast of the United
States is expected to lift out over the next couple of days, which
will allow a high pressure ridge to build westward over the eastern
North Pacific. As a result, Gilma should gradually move faster
toward the west-northwest and west during the next several days. The
track guidance remains tightly clustered, and the NHC forecast is
essentially an update of the previous one with no notable changes.

As previously discussed, the slow forward speed of Gilma is likely
resulting in some upwelling of cooler waters underneath the
hurricane. Although the vertical wind shear is forecast to remain
relatively weak during the next couple of days, the track of Gilma
will bring the hurricane over even cooler waters and into a drier
mid-level environment by this weekend. These factors suggest at
least gradual weakening should continue in the near term, followed
by more significant weakening beyond 72 h as the cyclone encounters
stronger shear and the upper air pattern becomes more convergent.
Some slight downward adjustments were made to the NHC prediction
later in the period. Gilma is forecast to become a post-tropical
cyclone by 120 h, but the lack of organized convection in GFS and
ECMWF model-simulated satellite imagery indicates this transition
could occur as early as day 4.


FORECAST POSITIONS AND MAX WINDS

INIT 23/0900Z 17.4N 126.1W 105 KT 120 MPH
12H 23/1800Z 17.6N 127.2W 100 KT 115 MPH
24H 24/0600Z 18.0N 128.8W 95 KT 110 MPH
36H 24/1800Z 18.4N 130.5W 90 KT 105 MPH
48H 25/0600Z 18.7N 132.2W 80 KT 90 MPH
60H 25/1800Z 19.0N 134.1W 70 KT 80 MPH
72H 26/0600Z 19.3N 136.1W 65 KT 75 MPH
96H 27/0600Z 19.9N 140.8W 50 KT 60 MPH
120H 28/0600Z 20.5N 146.0W 35 KT 40 MPH...POST-TROPICAL

$$
Forecaster Reinhart

[cycloneye]

Hurricane Gilma Discussion Number 21
NWS National Hurricane Center Miami FL EP072024
500 AM HST Fri Aug 23 2024

The cloud pattern associated with Gilma has lost some organization
since the last advisory, with the eye become less distinct and the
central dense overcast becoming more ragged. The various objective
and subjective satellite intensity estimates have trended downward
during the past 6 h, and based on these data the initial intensity
is decreased to 100 kt.

The westward jog mentioned in the previous advisory continues, with
the initial motion now 275/7 kt. A deep-layer trough north of Gilma
near the west coast of the United States is expected to lift out
over the next couple of days, which will allow a high pressure ridge
to gradually build westward over the eastern North Pacific. As a
result, Gilma should move generally west-northwestward to westward
at a faster forward speed during the next several days. Due to the
current westward jog, the guidance envelope has shifted a little to
the south and west since the previous advisory. Based on this
guidance, the new track forecast, which lies close to the consensus
models, will also shift a bit to the south.

Gilma is gradually moving toward cooler water, with the center
expected to cross the 26C isotherm in about 36 h. The hurricane is
also moving into a drier and more stable air mass. This combination
should lead to gradual weakening for the next 60 h or so. Beyond
that time, increasing shear and sea surface temperatures near 25C
should cause a faster weakening rate, and Gilma is now expected to
degenerate to a remnant low by 120 h. The new intensity forecast
follows the trend of the intensity guidance, but is nudged downward
a little from the previous forecast.


FORECAST POSITIONS AND MAX WINDS

INIT 23/1500Z 17.4N 126.8W 100 KT 115 MPH
12H 24/0000Z 17.6N 128.0W 95 KT 110 MPH
24H 24/1200Z 18.0N 129.6W 90 KT 105 MPH
36H 25/0000Z 18.3N 131.3W 85 KT 100 MPH
48H 25/1200Z 18.6N 133.1W 75 KT 85 MPH
60H 26/0000Z 18.9N 134.9W 70 KT 80 MPH
72H 26/1200Z 19.2N 137.0W 60 KT 70 MPH
96H 27/1200Z 19.9N 141.7W 45 KT 50 MPH
120H 28/1200Z 20.6N 147.0W 30 KT 35 MPH...POST-TROP/REMNT LOW

$$
Forecaster Beven

[Kingarabian]

Models have also shifted south with this in the medium range. Euro now keeps it intact entering the CPAC.