EPAC: GILMA - Post-Tropical

[Kingarabian]

We have Gilma.

[cycloneye]

Tropical Storm Gilma Discussion Number 3
NWS National Hurricane Center Miami FL EP082018
200 AM PDT Fri Jul 27 2018

The cloud pattern associated with the cyclone has lost organization
during the past several hours, with the low-level center now located
at the northern edge of a smaller area of convection. Despite the
ragged appearance in satellite imagery, recent ASCAT-A data showed
an area of 35 kt winds to the northeast of the center. Based on
this, the cyclone has been upgraded to Tropical Storm Gilma.

The initial motion is 285/14. Gilma should be steered westward to
west-northwestward along the southwestern flank of a large mid-level
ridge extending westward from the southwestern United States. As
the cyclone decays to a shallow remnant low late in the period, a
more westward motion is expected. The new track forecast is
similar to, but a little faster than the previous forecast, and it
is a blend of the previous forecast and the multi-model consensus
aids.

Gilma is located just to the east of an upper-level trough, which
the large-scale models forecast to move or re-form westward just
ahead of the storm for the next couple of days. This evolution is
expected to keep the cyclone in an area of moderate vertical wind
shear, but with some upper-level divergence to maintain convection.
Based on this, the intensity forecast shows a little more
strengthening in agreement with the overall trend of the intensity
guidance. After 36-48 h, Gilma is likely to move into strong and
dry northwesterly upper-level flow west of the upper-level trough
axis, which should lead to weakening and eventual dissipation. The
new intensity forecast now shows dissipation by 120 h in agreement
with the dynamical models.


FORECAST POSITIONS AND MAX WINDS

INIT 27/0900Z 14.4N 126.7W 35 KT 40 MPH
12H 27/1800Z 14.6N 128.7W 40 KT 45 MPH
24H 28/0600Z 15.0N 130.9W 40 KT 45 MPH
36H 28/1800Z 15.5N 132.6W 40 KT 45 MPH
48H 29/0600Z 16.1N 134.5W 35 KT 40 MPH
72H 30/0600Z 17.5N 138.5W 30 KT 35 MPH
96H 31/0600Z 17.5N 143.0W 20 KT 25 MPH...POST-TROP/REMNT LOW
120H 01/0600Z...DISSIPATED

$$
Forecaster Beven

[NDG]

Last night's ASCAT looks worst than yesterday's, IMO, with an elongated circulation but those 36 knot winds to the NE of coc is what made the NHC upgrade it. The satellite presentation is not great at all this morning with most of the convection to the SE of the coc.

[wxman57]

Gilma's center is outrunning its diminishing convection this morning. Definitely weakening.

[NDG]

The NHC has been more than generous this morning in keeping it as a TS, first visible satellite pics show an exposed broad circulation with multiple vorticities rotating around it.

[wxman57]

Yeah, NDG, doesn't look like it qualifies for TS status any longer. Maybe not for TD status.

[cycloneye]

And 18z Best Track agrees.

EP, 08, 2018072718, , BEST, 0, 147N, 1294W, 30, 1007, TD

[wxman57]

Bye, Gilma. Nice knowing you. Circulation may be about to open up. It's not long for this world...

[cycloneye]

Tropical Depression Gilma Discussion Number 5
NWS National Hurricane Center Miami FL EP082018
200 PM PDT Fri Jul 27 2018

Gilma is feeling the effects of northwesterly shear. During the
past several hours, the center of the system has been exposed at
times with much of the convective activity being displaced to the
southeast of the center. The initial intensity is lowered to 30 kt
based on a very recent ASCAT pass and the latest Dvorak estimates.
This makes Gilma a tropical depression. Although Gilma is expected
to be over relatively warm SSTs during the next couple of days, the
northwesterly shear is forecast to become even more hostile.
Therefore, continued weakening is anticipated, and Gilma is now
forecast to become a remnant low by 48 hours when the shear is
expected to be in excess of 30 kt. The remnant low is predicted to
open into a trough by day 4 as shown by many of the global models.

The depression is moving westward at 15 kt steered by the flow on
the south side of a low- to mid-level ridge. A west to
west-northwest motion at a slower forward speed is expected during
the next few days as the weak and increasingly shallow system is
steered by the low-level flow. The NHC track forecast is slightly
south of the previous one to come into better agreement with the
latest consensus models.

FORECAST POSITIONS AND MAX WINDS

INIT 27/2100Z 14.7N 129.9W 30 KT 35 MPH
12H 28/0600Z 14.8N 131.5W 30 KT 35 MPH
24H 28/1800Z 15.3N 133.3W 25 KT 30 MPH
36H 29/0600Z 15.8N 135.0W 25 KT 30 MPH
48H 29/1800Z 16.5N 137.0W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 30/1800Z 17.0N 140.0W 20 KT 25 MPH...POST-TROP/REMNT LOW
96H 31/1800Z...DISSIPATED

$$
Forecaster Cangialosi/Avila

[cycloneye]

Tropical Depression Gilma Discussion Number 6
NWS National Hurricane Center Miami FL EP082018
800 PM PDT Fri Jul 27 2018

The satellite presentation of Gilma has not changed much over the
past several hours. Strong west-northwesterly shear continues to
affect tropical cyclone, and deep convection is limited to the
southeast quadrant of the otherwise exposed low-level circulation.
The initial intensity has been held at 30 kt based on earlier ASCAT
data, but recent Dvorak intensity estimates from TAFB and SAB
suggest this could be generous.

Virtually no change has been made to the track or intensity
forecasts. All of the dynamical models indicate that the shear
affecting Gilma will increase through the next several days, and
the cyclone will be moving into a drier environment. Continued
weakening is therefore still forecast, in line with all of the
intensity guidance. While the NHC forecast currently shows that
Gilma will be a closed low through 72 hours, it is entirely possible
that Gilma will dissipate sooner than currently indicated due to
the hostile environment.

As expected, Gilma has slowed slightly and the initial
motion estimate is now 280/13 kt. A continued west to
west-northwest heading with a decrease in forward speed is still
forecast for the next few days while the shallow depression is
steered primarily by the low-level flow associated with the
subtropical high to the north-northeast. The NHC forecast remains
in close agreement with the various track consensus models.

FORECAST POSITIONS AND MAX WINDS

INIT 28/0300Z 15.0N 131.3W 30 KT 35 MPH
12H 28/1200Z 15.2N 132.7W 30 KT 35 MPH
24H 29/0000Z 15.7N 134.5W 25 KT 30 MPH
36H 29/1200Z 16.2N 136.2W 25 KT 30 MPH
48H 30/0000Z 16.5N 138.0W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 31/0000Z 16.5N 142.0W 20 KT 25 MPH...POST-TROP/REMNT LOW
96H 01/0000Z...DISSIPATED

$$
Forecaster Zelinsky

[cycloneye]

Tropical Depression Gilma Discussion Number 7
NWS National Hurricane Center Miami FL EP082018
200 AM PDT Sat Jul 28 2018

Gilma continues to be affected by strong westerly vertical shear,
with the low level center remaining exposed to the northwest of an
area of weakly banded convection. Satellite intensity estimates
are in the 25-35 kt range, so the initial intensity remains a
possibly generous 30 kt.

The initial motion is 280/11. The low- to mid-level subtropical
ridge to the north of the cyclone should continue steering the
cyclone generally westward to west-northwestward for the next three
days with some decrease in forward speed. The new track forecast
is similar to the previous forecast and lies a little to the north
of the center of the guidance envelope and the various consensus
models.

The ongoing shear is due to an upper-level trough just to the west
and northwest of the tropical cyclone. The dynamical models suggest
that this trough will move westward in close proximity to Gilma,
which will allow the shear to continue. In addition, the cyclone
should be moving into a drier airmass during the next several days.
These factors should cause Gilma to weaken and eventually dissipate
during the forecast period even though it will stay over warm
water. The new intensity forecast is mostly an update of the
previous forecast, and it now calls for Gilma to decay to a remnant
low by 36 h and dissipate completely after 72 h.


FORECAST POSITIONS AND MAX WINDS

INIT 28/0900Z 15.1N 132.4W 30 KT 35 MPH
12H 28/1800Z 15.4N 133.9W 25 KT 30 MPH
24H 29/0600Z 16.0N 135.7W 25 KT 30 MPH
36H 29/1800Z 16.3N 137.4W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 30/0600Z 16.4N 139.3W 20 KT 25 MPH...POST-TROP/REMNT LOW
72H 31/0600Z 16.5N 143.5W 20 KT 25 MPH...POST-TROP/REMNT LOW
96H 01/0600Z...DISSIPATED

$$
Forecaster Beven

[cycloneye]

Tropical Depression Gilma Discussion Number 8
NWS National Hurricane Center Miami FL EP082018
800 AM PDT Sat Jul 28 2018

Despite being in an environment of strong westerly shear, Gilma is
maintaining its status as a tropical cyclone. Deep convection has
been persisting on the eastern side of the system, and this
activity has even moved a little closer to the center, aided by
upper-level diffluence. The initial wind speed is held at 30 kt,
in agreement with the Dvorak classifications of 2.0/30 kt from TAFB
and SAB. This intensity estimate is a little lower than the 2.5/35
kt ADT values from CIMSS at the University of Wisconsin.

Westerly shear of about 30 kt is likely to continue during the
next few days, and the global models indicate that the upper-level
flow should become less diffluent as Gilma approaches the Central
Pacific basin. These hostile winds aloft combined with cooler
SSTs and a more stable air mass should cause Gilma to gradually
weaken during the next few days. The NHC forecast continues to show
Gilma becoming a remnant low in about 36 hours and degenerating into
a trough by day 4, following the trend in the model guidance.

The depression is moving westward at 10 kt steered by the flow on
the south side of a mid-level ridge. A west to west-northwest
motion is expected during the next few days as the increasingly
shallow system moves within the low- to mid-level flow. The NHC
track forecast is a little south of the previous one to come
into better agreement with the latest consensus aids.

FORECAST POSITIONS AND MAX WINDS

INIT 28/1500Z 15.2N 133.0W 30 KT 35 MPH
12H 29/0000Z 15.4N 134.3W 30 KT 35 MPH
24H 29/1200Z 15.7N 136.0W 25 KT 30 MPH
36H 30/0000Z 15.8N 137.9W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 30/1200Z 15.7N 140.0W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 31/1200Z 15.4N 144.4W 20 KT 25 MPH...POST-TROP/REMNT LOW
96H 01/1200Z...DISSIPATED

$$
Forecaster Cangialosi

[cycloneye]

Tropical Depression Gilma Discussion Number 9
NWS National Hurricane Center Miami FL EP082018
200 PM PDT Sat Jul 28 2018

Increasing westerly shear is taking its toll on Gilma. Deep
convection that has been persisting on the eastern side of the
system has become farther separated from the center, with an exposed
low-level swirl centered more than 75 n mi from the edge of
the convection. Although the system appears to be a little weaker
than earlier today, there is no scatterometer data to confirm that.
Therefore, the initial wind speed has been held at 30 kt, in
agreement with the Dvorak classifications of 2.0/30 kt from both
TAFB and SAB.

Westerly shear of about 30 to 35 kt is expected to continue during
the next few days, and the global models indicate that the
upper-level flow should become less diffluent as Gilma passes west
of the axis of an upper-level trough tonight. The combination of
these hostile winds aloft, cooler SSTs, and a more stable air mass
should cause Gilma to gradually weaken, and the NHC forecast
now shows Gilma becoming a remnant low in about 24 hours and
degenerating into a trough by day 4. Gilma could become a remnant
low even sooner than currently forecast, as the unfavorable
upper-level flow to the west of the upper trough axis could
completely shear off the deep convection from the center.

The depression made a jog to the northwest during the day today,
but it is expected to resume a west to west-northwestward motion
at around 10 kt through tonight, steered by mid-level ridging
to the north. Thereafter, as the system becomes increasingly
shallow, it should turn to the west, steered by low-level ridging to
its northwest. The NHC forecast track is very close to the previous
forecast, and it is close to the clustering of the consensus aids.

FORECAST POSITIONS AND MAX WINDS

INIT 28/2100Z 15.8N 134.3W 30 KT 35 MPH
12H 29/0600Z 16.0N 135.6W 30 KT 35 MPH
24H 29/1800Z 16.3N 137.4W 25 KT 30 MPH...POST-TROP/REMNT LOW
36H 30/0600Z 16.3N 139.6W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 30/1800Z 16.1N 141.8W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 31/1800Z 15.7N 145.8W 20 KT 25 MPH...POST-TROP/REMNT LOW
96H 01/1800Z...DISSIPATED

$$
Forecaster Cangialosi/Latto

[cycloneye]

Tropical Depression Gilma Discussion Number 10
NWS National Hurricane Center Miami FL EP082018
800 PM PDT Sat Jul 28 2018

Gilma's structure has not changed much over the past six hours.
Deep convection remains limited to small, but frequent, bursts just
east of the depression's exposed low-level center. The initial
intensity has been held at 30 kt, based primarily on continuity, but
this is also supported by the most recent Dvorak classification from
TAFB. No change has been made to the official intensity forecast.
The wind shear, as measured in SHIPS diagnostic data from the GFS
and ECMWF, is already over 30 kt and is forecast to continue
increasing over the next day or two. In addition, the tropical
cyclone will be moving through a progressively drier environment
during the next couple of days, which should also contribute to the
suppression of deep convection. Gilma is therefore still forecast
to become a remnant low tomorrow, and then dissipate entirely by
early next week.

The depression is moving a little quicker than previously
anticipated, and the initial motion estimate is 285/12. Aside from
the faster initial speed, there is no change to the reasoning
behind the track forecast. Gilma is still expected to turn back
toward the west and slow its forward speed for the next
couple of days while being steered primarily by the low-level flow
associated with the subtropical ridge to the north. The NHC track
forecast is very close to the various consensus models, and minimal
changes were made from the previous advisory.


FORECAST POSITIONS AND MAX WINDS

INIT 29/0300Z 16.1N 135.7W 30 KT 35 MPH
12H 29/1200Z 16.3N 137.2W 30 KT 35 MPH
24H 30/0000Z 16.4N 139.2W 25 KT 30 MPH...POST-TROP/REMNT LOW
36H 30/1200Z 16.3N 141.4W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 31/0000Z 16.1N 143.8W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 01/0000Z...DISSIPATED

$$
Forecaster Zelinsky

[cycloneye]

Tropical Depression Gilma Discussion Number 11
NWS National Hurricane Center Miami FL EP082018
200 AM PDT Sun Jul 29 2018

Gilma continues to generate convection to the east of the exposed
low-level center despite the ongoing 30 kt or more of westerly
vertical wind shear. Satellite intensity estimates are in the 30-40
kt range, and the initial intensity will be held at 30 kt based on
these data and the unchanged organization since the last advisory.

The initial motion is now 280/13. Gilma is expected to move
westward and eventually south of westward during the next 2-3 days
in the low-level flow associated with the subtropical ridge to the
north. There has been a slight southward shift of the guidance
since the last advisory, and the forecast track is thus nudged a
little southward. The new track continues to lie close to the
various consensus models.

An upper-level trough located over the cyclone has been providing
support for the central convection to persist despite the shear.
The large-scale models forecast that Gilma will move west of the
trough axis during the next 12-24 h, which will expose the tropical
cyclone to 40-50 kt northwesterly upper-level winds and an
environment of upper-level convergence. These developments should
bring an end to the central convection and cause Gilma to decay to
a remnant low. The remnants of the cyclone should weaken to a
trough between 48-72 h as indicated by the large-scale models.


FORECAST POSITIONS AND MAX WINDS

INIT 29/0900Z 16.1N 137.0W 30 KT 35 MPH
12H 29/1800Z 16.2N 138.6W 30 KT 35 MPH
24H 30/0600Z 16.1N 140.8W 25 KT 30 MPH...POST-TROP/REMNT LOW
36H 30/1800Z 16.0N 143.0W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 31/0600Z 15.8N 145.3W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 01/0600Z...DISSIPATED

$$
Forecaster Beven

[cycloneye]

Tropical Depression Gilma Discussion Number 12
NWS National Hurricane Center Miami FL EP082018
800 AM PDT Sun Jul 29 2018

Gilma is on its way in becoming a remnant low. Very strong
west-northwesterly shear has pushed all of the associated convection
well away from the center, leaving a completely exposed low-level
swirl. The initial intensity is held at 30 kt, in agreement with a
blend of the Dvorak Final T- and CI-numbers from TAFB and SAB. Now
that Gilma is located to the west of an upper-level trough axis,
strong west-northwesterly shear is expected to continue. These
hostile winds aloft, plus marginal SSTs and dry air, should cause
Gilma to become a remnant low later today and ultimately degenerate
into a trough in 2 to 3 days.

The depression is moving westward at 12 kt, steered by the flow on
the south side of a low- to mid-level ridge. This general motion
with a slight decrease in forward speed is expected until the
system dissipates. Little change was made to the previous NHC
track forecast, and this one lies near the middle of the guidance
envelope.

FORECAST POSITIONS AND MAX WINDS

INIT 29/1500Z 15.9N 138.5W 30 KT 35 MPH
12H 30/0000Z 16.0N 140.3W 25 KT 30 MPH...POST-TROP/REMNT LOW
24H 30/1200Z 15.9N 142.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
36H 31/0000Z 15.7N 144.8W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 31/1200Z 15.5N 147.2W 20 KT 25 MPH...POST-TROP/REMNT LOW
72H 01/1200Z...DISSIPATED

$$
Forecaster Cangialosi

[cycloneye]

Post-Tropical Cyclone Gilma Discussion Number 13
NWS National Hurricane Center Miami FL EP082018
200 PM PDT Sun Jul 29 2018

Gilma has lacked organized deep convection near the center for at
least 12 hours now. Although there has been a cluster of
thunderstorms lingering about 90 n mi southeast of the exposed
center, this convection has been pulling away from the system and
lacks sufficient organization necessary for a tropical cyclone.
Therefore, this is the last advisory issued by the National
Hurricane Center on Gilma. Very strong west-northwesterly shear is
expected to continue during the next few days. These hostile winds
combined with marginal SSTs and dry air should cause the remnant low
to degenerate into a trough in 36 to 48 hours.

The remnant low is moving westward at 13 kt steered by the low-level
flow on the south side of a ridge, and it should cross into the
Central Pacific basin shortly. This general motion is expected to
continue until the system dissipates.

For additional information on the remnant low please see High Seas
Forecasts issued by the National Weather Service, under AWIPS
header NFDHSFEPI, WMO header FZPN02 KWBC, and on the web at
https://ocean.weather.gov/shtml/NFDHSFEPI.shtml.

FORECAST POSITIONS AND MAX WINDS

INIT 29/2100Z 16.2N 139.7W 30 KT 35 MPH...POST-TROP/REMNT LOW
12H 30/0600Z 16.2N 141.4W 25 KT 30 MPH...POST-TROP/REMNT LOW
24H 30/1800Z 16.1N 143.8W 25 KT 30 MPH...POST-TROP/REMNT LOW
36H 31/0600Z 15.9N 146.3W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 31/1800Z...DISSIPATED

$$
Forecaster Cangialosi