ATL: GASTON - Post-Tropical - Discussion

[cycloneye]

TROPICAL STORM GASTON DISCUSSION NUMBER 7
NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016
500 AM AST WED AUG 24 2016

A 0414 UTC AMSR2 microwave image revealed that Gaston's structure
has improved with the development of a well-defined low-level cloud
ring and a mid-level microwave eye. However, the mid-level center
is displaced about 25 n mi to the northeast of the low-level
center. Dvorak intensity estimates from TAFB and SAB are both
T4.0/65 kt, but objective numbers are still between 55-60 kt. Given
the tilted structure of the cyclone, the initial intensity is only
raised to 60 kt on this advisory.

University of Wisconsin CIMSS shear analyses indicate that 10 kt
of southwesterly shear is affecting Gaston, which could explain
the cyclone's tilted structure. The shear is expected to remain low
enough during the next 24 hours to allow Gaston to strengthen to a
hurricane later today. However, the intensification trend should be
interrupted after 24 hours, continuing through day 3, when Gaston
moves into a higher shear zone to the east of a mid-/upper-level
low. Some strengthening is then probable on days 4 and 5 when
Gaston moves north of the upper low into a lower-shear environment.
The intensity models are in very good agreement on this general
scenario, and the official NHC forecast was only lowered a bit at
36 and 48 hours to be more in line with the IVCN consensus.

Tracking the low-level center observed in microwave data yields a
motion of 290/15 kt. Gaston is approaching a break in the
subtropical ridge caused by the aforementioned mid-/upper-level
low, and the steering currents should cause the motion to become
northwestward later today and continue along that heading for the
next four days. By day 5, Gaston is expected to slow down and turn
northward to the west of a mid-level high. There is lower-than-
normal spread among the track guidance for the entire five-day
forecast period, and the NHC forecast continues to closely follow
the TVCN multi-model consensus.

FORECAST POSITIONS AND MAX WINDS

INIT 24/0900Z 14.9N 38.6W 60 KT 70 MPH
12H 24/1800Z 16.2N 40.6W 65 KT 75 MPH
24H 25/0600Z 18.2N 42.9W 70 KT 80 MPH
36H 25/1800Z 20.4N 45.1W 70 KT 80 MPH
48H 26/0600Z 22.9N 47.2W 70 KT 80 MPH
72H 27/0600Z 26.9N 52.3W 70 KT 80 MPH
96H 28/0600Z 29.5N 55.5W 80 KT 90 MPH
120H 29/0600Z 31.5N 55.5W 85 KT 100 MPH

$$
Forecaster Berg

[Kazmit]

I wonder if this will pull a Fiona and not take a path directly NW as predicted, and instead go more WNW. But then again, there's a weakness in the ridge.

[cycloneye]

TROPICAL STORM GASTON DISCUSSION NUMBER 8
NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016
1100 AM AST WED AUG 24 2016

Gaston's overall cloud pattern has not changed much in organization
since the last advisory. The low-level center is located underneath
a small central dense overcast (CDO), with hints of a prototype eye
within the convective mass. A solid band, attached to the CDO,
wraps around the eastern half of the circulation. A 1234 UTC ASCAT
pass supports keeping the initial wind estimate at 60 kt.

Gaston could still reach hurricane strength today. However, the
shear associated with a potent mid- to upper-level trough along 52W
is forecast to greatly increase over Gaston and peak in 36 to 48
hours. Even though the cyclone will gradually be moving over warmer
waters at that time, the shear should be enough to arrest its
development or perhaps even result in weakening. By 72 hours,
Gaston's involvement with the trough should lessen and the shear
should decrease. Much lower westerly shear is forecast toward the
end of the forecast period, which should give Gaston an opportunity
to re-strengthen, and perhaps significantly, as it moves over waters
of around 29 deg C. The new NHC intensity forecast is reduced some
compared to the previous one through 48 hours, but shows slightly
greater intensification by 120 hours. The official forecast is in
generally in good agreement with the multi-model consensus.

The latest fixes indicate that Gaston's heading has a much greater
northerly component, and the long-term initial motion estimate is
305/14. Gaston is expected to move northwestward between a
subtropical ridge over the eastern Atlantic and the aforementioned
trough to the west during the next 3 days. After 72 hours, Gaston
should gradually turn north-northwestward and northward with a
considerable decrease in forward speed, when it encounters a more
substantial break in the Atlantic subtropical ridge along 60W. The
ECMWF shows less ridging and makes Gaston vulnerable to the mid-
latitude westerlies earlier, with the model showing recurvature
before 120 hours. The bulk of the guidance, however, lies farther
west, with these models showing more ridging north of Gaston through
day 5. The new track forecast is a little right of the previous
one through 72 hours in response to the cyclone's abrupt change in
heading since the last advisory and is close to a blend of the
ECMWF and GFS solutions. The track beyond 72 hours was adjusted
westward and remains close to a ECMWF/GFS model blend.

FORECAST POSITIONS AND MAX WINDS

INIT 24/1500Z 16.1N 39.4W 55 KT 65 MPH
12H 25/0000Z 17.5N 41.2W 65 KT 75 MPH
24H 25/1200Z 19.6N 43.5W 60 KT 70 MPH
36H 26/0000Z 21.9N 45.6W 55 KT 65 MPH
48H 26/1200Z 24.3N 47.9W 55 KT 65 MPH
72H 27/1200Z 27.8N 53.1W 65 KT 75 MPH
96H 28/1200Z 30.5N 55.9W 80 KT 90 MPH
120H 29/1200Z 32.2N 56.5W 90 KT 105 MPH

$$
Forecaster Kimberlain

[cycloneye]

IMO,Gaston has underperformed as it was supposed to be a Hurricane at this point and here we are looking still at a Tropical Storm.

...GASTON HAS CHANGED LITTLE IN STRENGTH...
...EXPECTED TO ENCOUNTER STRONG SHEAR IN A DAY OR TWO...


SUMMARY OF 1100 AM AST...1500 UTC...INFORMATION
-----------------------------------------------
LOCATION...16.1N 39.4W
ABOUT 1020 MI...1645 KM W OF THE CABO VERDE ISLANDS
MAXIMUM SUSTAINED WINDS...65 MPH...100 KM/H
PRESENT MOVEMENT...NW OR 305 DEGREES AT 16 MPH...26 KM/H
MINIMUM CENTRAL PRESSURE...999 MB...29.50 INCHES

[HurricaneBelle]

This has been the year of "almost but not quite a hurricane", as several storms in the EPac reached 65-70 MPH and stayed there for several advisories. Gaston should still break through, though.

[TheStormExpert]

I'd be somewhat suprised if Gaston becomes a hurricane before 72-96hrs. since shear is now forecasted to increase and weaken it some. Though once beyond 72-96hrs. it should have no problem obtaining hurricane status and could still make a run at major status before it heads off into the North Atlantic hurricane graveyard.

[Extratropical94]

The NHC corrected the intensity to 60 knots / 70 mph.

CORRECTED INITIAL INTENSITY

...GASTON HAS CHANGED LITTLE IN STRENGTH...
...EXPECTED TO ENCOUNTER STRONG SHEAR IN A DAY OR TWO...


SUMMARY OF 1100 AM AST...1500 UTC...INFORMATION
-----------------------------------------------
LOCATION...16.1N 39.4W
ABOUT 1020 MI...1645 KM W OF THE CABO VERDE ISLANDS
MAXIMUM SUSTAINED WINDS...70 MPH...110 KM/H
PRESENT MOVEMENT...NW OR 305 DEGREES AT 16 MPH...26 KM/H
MINIMUM CENTRAL PRESSURE...999 MB...29.50 INCHES

[Kazmit]

Now showing a peak intensity of 105 mph while east of Bermuda on Monday.

FORECAST POSITIONS AND MAX WINDS

INIT 24/1500Z 16.1N 39.4W 60 KT 70 MPH
12H 25/0000Z 17.5N 41.2W 65 KT 75 MPH
24H 25/1200Z 19.6N 43.5W 60 KT 70 MPH
36H 26/0000Z 21.9N 45.6W 55 KT 65 MPH
48H 26/1200Z 24.3N 47.9W 55 KT 65 MPH
72H 27/1200Z 27.8N 53.1W 65 KT 75 MPH
96H 28/1200Z 30.5N 55.9W 80 KT 90 MPH
120H 29/1200Z 32.2N 56.5W 90 KT 105 MPH

[RL3AO]

Big westward shift from the 12Z GFS. Seems that the N Atlantic trough may be a little quicker in the past couple runs meaning that Gaston misses it. Not sure how much I buy it.

Here's the trend.

[Hammy]

This will be the second storm I have money riding on--there's no way this becomes a hurricane during the forecast period with those 50kt of shear to the north and I'll be surprised if it makes it past a Cat 1 at it's peak. The models seem to be severely overdoing everything north of 20N just as they did last year.

[Nimbus]

Lots of dry air.

[cycloneye]

12z UKMET is with GFS more west.Bermuda has to watch this very closely.

[Kazmit]

Looking good... it's easy to make out the eye in this loop.

[weathernerdguy]

Looking good... it's easy to make out the eye in this loop.

That's not an eye, that is it starting to be sheared.

[cycloneye]

TROPICAL STORM GASTON DISCUSSION NUMBER 9
NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016
500 PM AST WED AUG 24 2016

Gaston is being affected by southwesterly vertical shear associated
with a strong mid- to upper-level trough and cut-off low seen in
water vapor imagery near 26n 51w. The shear has caused the
low-level center to become partially exposed while much of the
deep convection has been shunted to eastern half of the circulation.
In spite of the degraded satellite presentation, dropsonde data
from the unmanned NASA Global Hawk aircraft investigating Gaston
support keeping the intensity at 60 kt. In fact, additional
observations from the ongoing mission might reveal that the system
is even a little stronger than this estimate.

Gaston is in for a prolonged period of strong southwesterly shear
for the next 36 to 48 hours, with the shear possibly peaking around
30 kt during that time. The considerable shear is expected to
induce weakening, and it is possible that more weakening could occur
than indicated in the forecast despite the cyclone's moving over
warmer waters. Once Gaston's interaction with the trough lessens in
about 2 days, the cyclone should reach 29 deg C water when the
shear diminishes. This should give Gaston an opportunity to
re-intensify for at least a couple of days before the models
indicate an increase in shear at the end of the forecast period.
The intensity forecast is reduced relative to the previous one and
is a little below the multi-model consensus through 48 hours, but
then reverses and is above the consensus aids from 72-120 hours.

Gaston's heading is now definitively northwestward, and the initial
motion estimate is 315/14. A continued northwestward motion is
likely for the next few days as Gaston moves between one cell of the
subtropical ridge centered over the eastern Atlantic and the cut-off
low to the northwest. After 72 hours, Gaston should approach a more
significant weakness in the Atlantic subtropical ridge around 60w
and slow while turning north-northwestward to northward. There are
substantial model differences again this cycle, with the ECMWF
indicating a weaker subtropical ridge. The weaker ridge makes
Gaston more vulnerable to the mid-latitude westerly flow over
the North Atlantic, which results in an earlier recurvature.
However, the bulk of the guidance has a stronger ridge, and thus
have solutions that go much farther west. No major changes have
been made to the previous forecast in the short term but the track
has been adjusted much farther to the left after 72 hours, in the
direction of but not as far left as the multi-model consensus.

FORECAST POSITIONS AND MAX WINDS

INIT 24/2100Z 17.4N 40.6W 60 KT 70 MPH
12H 25/0600Z 18.8N 42.3W 55 KT 65 MPH
24H 25/1800Z 21.0N 44.5W 45 KT 50 MPH
36H 26/0600Z 23.5N 46.6W 45 KT 50 MPH
48H 26/1800Z 25.7N 49.2W 50 KT 60 MPH
72H 27/1800Z 28.3N 54.6W 65 KT 75 MPH
96H 28/1800Z 30.1N 57.4W 80 KT 90 MPH
120H 29/1800Z 31.6N 58.2W 85 KT 100 MPH

$$
Forecaster Kimberlain/Franklin

[Hammy]

I feel like I'm watching a replay of 2013, with everything struggling and severely under-performing the models. And this was supposed to be the ACE pump and in all likelihood it's already peaked (I'm not buying the re-intensification forecast--Humberto in 2013 was forecast the same with a wall of shear to the north and we saw how that turned out.)

[TheStormExpert]

I feel like I'm watching a replay of 2013, with everything struggling and severely under-performing the models. And this was supposed to be the ACE pump and in all likelihood it's already peaked (I'm not buying the re-intensification forecast--Humberto in 2013 was forecast the same with a wall of shear to the north and we saw how that turned out.)

This season is really starting to get on my nerves and it's August 24th, what is happening?

[weathernerdguy]

Somewhat of a phase 6 MJO

[JaxGator]

I feel like I'm watching a replay of 2013, with everything struggling and severely under-performing the models. And this was supposed to be the ACE pump and in all likelihood it's already peaked (I'm not buying the re-intensification forecast--Humberto in 2013 was forecast the same with a wall of shear to the north and we saw how that turned out.)

This season is really starting to get on my nerves and it's August 24th, what is happening?

Really? We have 8 storms already and Earl strengthening into a dangerous hurricane that hit Belize and Mexico. It is weird maybe that's August 24th though we still have time but remember, this season has been a bad one for a lot of folks so far.. Sorry, but imo, it's TOO soon for any season cancel posts or ones that imply it.

[TheStormExpert]

I feel like I'm watching a replay of 2013, with everything struggling and severely under-performing the models. And this was supposed to be the ACE pump and in all likelihood it's already peaked (I'm not buying the re-intensification forecast--Humberto in 2013 was forecast the same with a wall of shear to the north and we saw how that turned out.)

This season is really starting to get on my nerves and it's August 24th, what is happening?

Really? We have 8 storms already and Earl strengthening into a dangerous hurricane that hit Belize and Mexico. It is weird maybe that's August 24th though we still have time but remember, this season has been a bad one for a lot of folks so far.. Sorry, but imo, it's TOO soon for any season cancel posts or ones that imply it.

Not when yet another storm is underperforming in the MDR.