EPAC: GREG - Post-Tropical

[Sciencerocks]

000
WTPZ42 KNHC 220253
TCDEP2

Tropical Storm Greg Discussion Number 19
NWS National Hurricane Center Miami FL EP072017
800 PM PDT Fri Jul 21 2017

Greg is continuing to experience significant vertical shear, as the
deep convection is only observed in the northeastern semicircle and
its center is on the edge of the convective overcast. A blend of
subjective Dvorak estimates from SAB/TAFB, objective Dvorak from
CIMSS, and an ATMS analysis all suggest about 45 kt maximum
sustained winds, unchanged from the previous advisory.

Vertical shear analysis from CIMSS suggests substantially stronger
tropospheric vertical shear than analyzed by either the GFS or
ECMWF global models. The southwesterly shear appears to due to a
vigorous upper-level trough north-northwest of Greg. While the
shear should subside some as the upper trough and Greg move away
from each other, the atmosphere that Greg will move through should
become progressively drier and more stable. Around day 5, the
hostile thermodynamics are expected to cause Greg to lose deep
convection completely and thus become a remnant low. The official
intensity forecast - showing little to no intensification before a
gradual decay starting around day 3 - is based upon the HCCA
corrected consensus technique and is slightly lower than that of the
previous advisory.

Greg is moving toward the west at about 12 kt, as it is moving
along the southern periphery of a subtropical ridge. A gradual
bend toward the west-northwest is anticipated around day 4 and 5,
as a weakening Greg should be advected along by the low-level
trades. The model guidance is in good agreement for this scenario -
except for the GFS which has a sharp turn toward the northwest.
This appears to be due to a stronger, deeper Greg being more
affected by an approaching upper-level trough. This GFS solution is
not, however, mirrored by most of the GEFS ensemble members which
show more of a west to west-northwest track. The official track
prediction is based upon the TVCN multi-model consensus with
the GFS replaced by the GEFS ensemble mean and is nudged slightly
southwest from the previous advisory.

The initial wind radii was contracted inward some based upon the
ATMS analysis from 2129Z. The official wind radii forecast is
closest to the RVCN consensus approach.

FORECAST POSITIONS AND MAX WINDS

INIT 22/0300Z 15.1N 121.8W 45 KT 50 MPH
12H 22/1200Z 15.3N 123.5W 45 KT 50 MPH
24H 23/0000Z 15.5N 125.7W 45 KT 50 MPH
36H 23/1200Z 15.5N 128.0W 50 KT 60 MPH
48H 24/0000Z 15.5N 130.4W 50 KT 60 MPH
72H 25/0000Z 15.8N 133.6W 45 KT 50 MPH
96H 26/0000Z 17.1N 136.5W 35 KT 40 MPH
120H 27/0000Z 18.0N 139.0W 25 KT 30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Landsea/Ramos

[Sciencerocks]

[Sciencerocks]

000
WTPZ42 KNHC 220833
TCDEP2

Tropical Storm Greg Discussion Number 20
NWS National Hurricane Center Miami FL EP072017
200 AM PDT Sat Jul 22 2017

Microwave imagery indicates that the center of the tropical storm
is located to the south of the main area of deep convection. The
current intensity estimate is kept at 45 kt, based on a Dvorak
estimate from TAFB. Greg is being affected by southerly
shear associated with an upper-level low/trough located to the
north-northwest, which is clearly seen in water vapor imagery. The
global dynamical guidance indicates that the trough will lift
northward over the next day or two, resulting in a decrease of
shear. However, Greg is expected to reach slightly cooler waters
and begin to ingest drier mid-level air within a couple of days.
Therefore, only slight strengthening is anticipated, followed by
weakening later in the forecast period. By the end of the period,
the hostile environment should result in the system degenerating
into a remnant low. The official forecast is near or a little above
the intensity model consensus.

Greg is moving westward or 275/11 kt. A mid-level ridge to the
north of the tropical cyclone is likely to maintain the westward
motion for the next several days. In 4-5 days the ridge is
predicted to weaken somewhat, and this should result in a slowing
of the forward speed along with a turn toward the right. The
official track forecast is close to the simple and corrected
dynamical consensus tracks and is not too different from its
predecessor.

FORECAST POSITIONS AND MAX WINDS

INIT 22/0900Z 15.2N 123.1W 45 KT 50 MPH
12H 22/1800Z 15.3N 124.8W 45 KT 50 MPH
24H 23/0600Z 15.4N 127.2W 45 KT 50 MPH
36H 23/1800Z 15.3N 129.6W 50 KT 60 MPH
48H 24/0600Z 15.2N 131.6W 50 KT 60 MPH
72H 25/0600Z 15.7N 134.6W 45 KT 50 MPH
96H 26/0600Z 17.2N 137.0W 35 KT 40 MPH
120H 27/0600Z 18.0N 139.0W 25 KT 30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Pasch

[cycloneye]

Tropical Storm Greg Discussion Number 21
NWS National Hurricane Center Miami FL EP072017
800 AM PDT Sat Jul 22 2017

A large and strong burst of convection has formed on the northern
side of the center of Greg during the past several hours. However,
microwave data suggest the center remains mostly exposed on the
southern side of the thunderstorm activity. Satellite intensity
estimates range between 35-55 kt, so the initial wind speed will be
kept at 45 kt.

While shear is forecast to decrease near Greg over the next 36
hours as a trough lifts out, SSTs are also likely to decrease
along with more dry air in the environment of the cyclone. The net
result of these competing factors is that little significant
intensity change is expected over the next couple of days. After
that, most of the models increase the shear again while the storm
moves over colder waters. Thus Greg should weaken at long range,
and will likely be a remnant low by day 5. Little change was made
to the previous NHC forecast, which is near or slightly higher than
the model consensus.

Greg is moving westward or 270/11 kt. A mid-level ridge to the
north of the tropical cyclone is likely to maintain the westward
motion for the next couple of days. In 3-5 days the ridge is
predicted to weaken somewhat, and this should result Greg slowing
down and turning west-northwestward. There has been little change
to the model tracks, with the ECMWF/UKMET on the left side of the
guidance, and the GFS-based guidance on the right side. The
GFS-based guidance has been trending westward, so it makes some
sense to stay on the left side of the consensus. The latest NHC
forecast is very close to the previous forecast, leaning a little
heavier on the ECMWF, UKMET, their ensembles, and the HFIP-corrected
consensus model.


FORECAST POSITIONS AND MAX WINDS

INIT 22/1500Z 15.1N 124.2W 45 KT 50 MPH
12H 23/0000Z 15.1N 126.0W 45 KT 50 MPH
24H 23/1200Z 15.0N 128.2W 45 KT 50 MPH
36H 24/0000Z 14.9N 130.3W 50 KT 60 MPH
48H 24/1200Z 15.1N 132.0W 50 KT 60 MPH
72H 25/1200Z 16.0N 134.8W 45 KT 50 MPH
96H 26/1200Z 17.3N 137.2W 35 KT 40 MPH
120H 27/1200Z 18.0N 140.0W 25 KT 30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Blake

[Sciencerocks]

[Kingarabian]

[mg]http://imageshack.com/a/img923/1049/tnLK87.gif[/img]

Convection warming but at least it has some sort of banding features now.

[Sciencerocks]

000
WTPZ42 KNHC 222037
TCDEP2

Tropical Storm Greg Discussion Number 22
NWS National Hurricane Center Miami FL EP072017
200 PM PDT Sat Jul 22 2017

Similar to the past few days, deep convection from the morning
hours has weakened, with the center of Greg peeking out of the
cirrus canopy on the southern side of the central dense overcast.
Although the latest satellite estimates have risen a bit, the
initial wind speed will remain 45 kt after considering the recent
degradation of the cloud pattern.

It seems like dry air has been getting into the central core of
Greg, with some outflow boundaries present in the southern
semicircle. This dry air presents a problem for future
intensification, since it would take some time to mix out of
the storm's circulation and additional environmental dry and
stable air is in the forecast path. Thus, the window for
strengthening may have ended, and it is becoming more likely that
Greg will just slowly weaken as environmental conditions gradually
deteriorate. None of the historically reliable guidance strengthen
the cyclone now, and the NHC forecast is decreased from the previous
one, consistent with the latest model consensus guidance.

Greg continues moving westward or 270/11 kt. A mid-level ridge to
the north of the tropical cyclone is likely to maintain the westward
motion for the next couple of days. In 3-5 days, the ridge is
predicted to weaken somewhat, and this should result in Greg slowing
down and turning west-northwestward. The GFS-based guidance has
made a large westward shift toward the ECMWF/UKMET on this cycle,
similar to its overnight trend. Thus, the NHC forecast is shifted
westward as well and remains near or west of the model consensus at
long range.



FORECAST POSITIONS AND MAX WINDS

INIT 22/2100Z 15.1N 125.2W 45 KT 50 MPH
12H 23/0600Z 15.0N 126.9W 45 KT 50 MPH
24H 23/1800Z 14.9N 129.2W 45 KT 50 MPH
36H 24/0600Z 15.0N 131.3W 40 KT 45 MPH
48H 24/1800Z 15.2N 133.1W 40 KT 45 MPH
72H 25/1800Z 16.3N 135.9W 35 KT 40 MPH
96H 26/1800Z 17.3N 138.6W 30 KT 35 MPH
120H 27/1800Z 18.0N 141.5W 25 KT 30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Blake

[Sciencerocks]

[Sciencerocks]

000
WTPZ42 KNHC 230233
TCDEP2

Tropical Storm Greg Discussion Number 23
NWS National Hurricane Center Miami FL EP072017
800 PM PDT Sat Jul 22 2017

Greg's cloud pattern remains unimpressive, with a rather
disorganized area of convection found to the west and northwest of
the low-level center. The latest Dvorak estimates are about the same
as before, so the initial intensity estimate remains 45 kt.

Little change in strength is expected during the first 24 hours, as
the near-storm environment is not expected to change much during
that time. Afterward, however, the usually lethal combination of
cooling SSTs and a drying low- to mid-level atmosphere, along with
increasing shear at days 4-5, should bring about the gradual demise
of Greg. Remnant low status is now forecast by 96 hours, and the new
NHC intensity forecast is in line with the bulk of the latest
guidance.

A timely SSMIS pass from 23/0005Z was helpful in establishing the
initial position of Greg, which is a little south and west of
previous estimates. Based on this, the initial motion estimate is
265/11, as Greg continues moving to the south of an expansive low-
to mid-level ridge over the eastern Pacific. This ridge will begin
to weaken in 48-72 hours, which should result in Greg turning a
little more poleward during that time with a slight decrease in
forward speed. By the end of the period, the shallow remnants of
Greg should turn westward in the low-level trade wind flow over the
central Pacific. Based on the initial position and the trend in the
latest guidance, the new NHC track forecast is faster and has been
adjusted to the left of the previous forecast, especially at days 3
through 5. The NHC prediction is close to the latest consensus aids
through day 3, but lies a bit to the right of those aids at days 4
and 5 out of respect to continuity.

FORECAST POSITIONS AND MAX WINDS

INIT 23/0300Z 14.9N 126.5W 45 KT 50 MPH
12H 23/1200Z 14.9N 128.3W 45 KT 50 MPH
24H 24/0000Z 14.8N 130.8W 45 KT 50 MPH
36H 24/1200Z 14.9N 132.9W 40 KT 45 MPH
48H 25/0000Z 15.2N 134.6W 35 KT 40 MPH
72H 26/0000Z 16.3N 137.2W 30 KT 35 MPH
96H 27/0000Z 17.5N 140.0W 25 KT 30 MPH...POST-TROP/REMNT LOW
120H 28/0000Z 17.5N 143.5W 25 KT 30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Brennan

[Sciencerocks]

[Sciencerocks]

000
WTPZ42 KNHC 230851
TCDEP2

Tropical Storm Greg Discussion Number 24
NWS National Hurricane Center Miami FL EP072017
200 AM PDT Sun Jul 23 2017

The center of Greg has been difficult to locate tonight, but earlier
microwave data and an ASCAT pass that just barely caught the center
indicate that the tropical storm remains sheared. At the time of
those passes, the center was located just southeast of the main
convective mass and there is no reason to believe that has changed.
Although the ASCAT pass missed the western portion of the
circulation, it still showed winds of 38 kt in the northeast
quadrant. Assuming the winds are a little higher within the
convection, the initial intensity has been held at 45 kt.

Although Greg is moving away from an upper-level low and the worst
of the vertical shear, the near-storm environment is still fairly
dry, as diagnosed in the GFS and ECMWF-based SHIPS. This should
be enough to prevent the cyclone from strengthening, and very
gradual weakening is supported by most of the guidance. After
about 48 hours, cooler SSTs and increasing shear should result in
additional weakening, and Greg is forecast to become a remnant low
by day 4. The new intensity forecast is very similar to the
previous advisory.

The aforementioned ASCAT pass suggested that the center of Greg is a
little farther south than previously estimated, and the track
forecast has been adjusted accordingly. Aside from that, little
change has been made and the official forecast is near the middle
of the tightly clustered guidance. All of the global models still
show Greg moving along the southern periphery of a low- to mid-level
ridge for the next couple of days, followed by a turn toward the
northwest as the ridge weakens slightly. By the end of the
forecast period a turn back to the west as a remnant low is
expected, when Greg becomes steered primarily by low-level trade
winds.


FORECAST POSITIONS AND MAX WINDS

INIT 23/0900Z 14.7N 127.7W 45 KT 50 MPH
12H 23/1800Z 14.6N 129.5W 45 KT 50 MPH
24H 24/0600Z 14.6N 131.9W 40 KT 45 MPH
36H 24/1800Z 14.8N 133.9W 40 KT 45 MPH
48H 25/0600Z 15.2N 135.5W 35 KT 40 MPH
72H 26/0600Z 16.1N 138.0W 30 KT 35 MPH
96H 27/0600Z 16.9N 140.4W 25 KT 30 MPH...POST-TROP/REMNT LOW
120H 28/0600Z 17.0N 143.5W 25 KT 30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Zelinsky

[cycloneye]

Tropical Storm Greg Discussion Number 25
NWS National Hurricane Center Miami FL EP072017
800 AM PDT Sun Jul 23 2017

Greg's convective pattern is looking a little better this morning,
with two distinct bursts of thunderstorm activity located near and
to the west of the center. The latest Dvorak intensity estimates
range from T2.0 to T3.0, and the initial intensity will remain 45
kt since the cyclone's structure has improved somewhat during the
past several hours.

Vertical shear over Greg is less than 10 kt at the moment, but the
cyclone's biggest challenge is dry air in the surrounding
environment. GOES-16 lower-level water vapor imagery shows that
Greg is running into a very dry low- to mid-level air mass to its
west, and this environment will likely curtail strengthening. Greg
is forecast to maintain its intensity for another 24 hours,
followed by gradual weakening due to the dry air, increasing shear,
and cooler sea surface temperatures after 48 hours. This scenario
is captured by all of the intensity guidance, and Greg is likely to
degenerate into a remnant low by 96 hours.

A little more southward adjustment of the center location was needed
based on additional ASCAT and other microwave data received since
the last advisory. Still, low- to mid-level ridging to the north
of Greg is expected to maintain a westward motion through 36 hours.
The ridge is forecast to weaken after 36 hours, which should allow
the cyclone to gain a little more latitude on days 2 through 4.
The low-level trade winds are then expected to push the remnant low
west-southwestward by day 5. Except for a southward adjustment in
the track forecast to account for the initial position, the model
guidance is in good agreement, and no other significant changes
were required from the previous advisory.

FORECAST POSITIONS AND MAX WINDS

INIT 23/1500Z 14.3N 129.0W 45 KT 50 MPH
12H 24/0000Z 14.3N 130.7W 45 KT 50 MPH
24H 24/1200Z 14.5N 133.0W 45 KT 50 MPH
36H 25/0000Z 14.7N 134.8W 40 KT 45 MPH
48H 25/1200Z 15.0N 136.2W 35 KT 40 MPH
72H 26/1200Z 16.1N 138.6W 30 KT 35 MPH
96H 27/1200Z 16.5N 141.0W 25 KT 30 MPH...POST-TROP/REMNT LOW
120H 28/1200Z 15.9N 143.6W 25 KT 30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Berg

[Ntxw]

Greg's had a tough life battling shear. But he's been resilient enough as a named storm that he may end up as much ACE as Dora which was a formidable hurricane but shorter span.

[Kingarabian]

UW - CIMSS
ADVANCED DVORAK TECHNIQUE
ADT-Version 8.2.1
Tropical Cyclone Intensity Algorithm

----- Current Analysis -----
Date : 23 JUL 2017 Time : 180000 UTC
Lat : 14:17:36 N Lon : 129:34:23 W


CI# /Pressure/ Vmax
3.1 / 997.2mb/ 47.0kt


Final T# Adj T# Raw T#
3.1 3.0 4.1

Center Temp : -73.6C Cloud Region Temp : -72.4C

Scene Type : UNIFORM CDO CLOUD REGION

Positioning Method : FORECAST INTERPOLATION

Making a run at hurricane status?

[Kingarabian]

Center is embedded at the center of the convection mass. Interesting to see if it can develop a core.

[cycloneye]

Tropical Storm Greg Discussion Number 26
NWS National Hurricane Center Miami FL EP072017
200 PM PDT Sun Jul 23 2017

Greg has had a large burst of deep convection during the past
several hours, although cloud tops have begun to warm just
recently. A 1748 UTC ASCAT-B pass showed maximum winds of 40 kt,
but subjective and objective Dvorak estimates have all increased to
T3.0/45 kt. Therefore, the initial intensity will remain 45 kt,
with the assumption that the resolution of the scatterometer missed
higher winds.

Greg is moving westward, or 270/12 kt, to the southeast of a low-
to mid-level anticyclone centered northeast of Hawaii. Although
the ridging is not expected to weaken, it is forecast to expand and
shift westward, which could allow Greg to turn west-northwestward
in about 36 hours. A turn back toward the west is expected by day 4
when it becomes a remnant low and is steered by the low-level trade
winds. The track models have moved northward on this cycle,
leaving the previous official forecast along the southern edge of
the guidance envelope. This necessitated that the new NHC forecast
be nudged northward as well, but it is still south of the TVCN
multi-model consensus for at least the first 48 hours.

The thermodynamic environment appears to be the limiting factor on
Greg's intensity, since the vertical shear is expected to remain
low for the next 48 hours. Visible satellite imagery shows a large,
stable stratocumulus field out ahead of the cyclone, and
lower-level water vapor imagery from GOES-16 shows a lot of
mid-level dry air. Therefore, Greg is likely to only maintain its
intensity for the next 24 hours and then gradually weaken due to
the dry air, increasing shear, and cooler sea surface temperatures.
No significant changes were required to the NHC intensity forecast,
and Greg is still expected to degenerate into a remnant low on day
4.

FORECAST POSITIONS AND MAX WINDS

INIT 23/2100Z 14.4N 130.1W 45 KT 50 MPH
12H 24/0600Z 14.5N 131.8W 45 KT 50 MPH
24H 24/1800Z 14.8N 133.8W 45 KT 50 MPH
36H 25/0600Z 15.2N 135.4W 40 KT 45 MPH
48H 25/1800Z 15.9N 136.8W 40 KT 45 MPH
72H 26/1800Z 17.1N 139.6W 30 KT 35 MPH
96H 27/1800Z 17.1N 142.7W 25 KT 30 MPH...POST-TROP/REMNT LOW
120H 28/1800Z 16.3N 145.6W 20 KT 25 MPH...POST-TROP/REMNT LOW

$$
Forecaster Berg

[Sciencerocks]

[cycloneye]

Tropical Storm Greg Discussion Number 27
NWS National Hurricane Center Miami FL EP072017
800 PM PDT Sun Jul 23 2017

Greg has changed little since the previous advisory, with the
cyclone still producing intermittent bursts of deep convection near
the center. Subjective and objective satellite intensity estimates
remain at T3.0/45 kt, so the initial intensity will remain
unchanged at 45 kt for this advisory.

Greg's initial motion estimate is 275/12 kt. No significant changes
to the previous forecast track were required through 72 hours. Greg
is expected to continue moving west-northwestward to westward for
the next 3 days while it skirts along the southern periphery of a
deep-layer subtropical ridge centered northeast of the Hawaiian
Islands. The small cyclone is forecast to weaken to a remnant low
around 72 hours, with the shallow system being forced
west-southwestward by brisk northeasterly tradewind flow. The new
NHC forecast track closely follows the TVCN consensus model.

Little change in strength is expected for the next 24 hours or so.
Gradual weakening is expected thereafter due to drier and more
stable air getting entrained into the cyclone, followed by
increasing vertical wind shear. By 72 hours and beyond, the shear is
expected to exceed 25-30 kt, which should result in Greg
degenerating into a remnant low. The new intensity forecast is
similar to the previous advisory, and closely follows the intensity
consensus model ICON and IVCN.

FORECAST POSITIONS AND MAX WINDS

INIT 24/0300Z 14.4N 131.4W 45 KT 50 MPH
12H 24/1200Z 14.6N 132.9W 45 KT 50 MPH
24H 25/0000Z 15.0N 134.7W 45 KT 50 MPH
36H 25/1200Z 15.6N 136.2W 40 KT 45 MPH
48H 26/0000Z 16.4N 137.6W 35 KT 40 MPH
72H 27/0000Z 17.0N 140.5W 30 KT 35 MPH...POST-TROP/REMNT LOW
96H 28/0000Z 16.9N 143.7W 25 KT 30 MPH...POST-TROP/REMNT LOW
120H 29/0000Z 16.1N 146.5W 20 KT 25 MPH...POST-TROP/REMNT LOW

$$
Forecaster Stewart

[Sciencerocks]

[Sciencerocks]

CZC MIATCDEP2 ALL
TTAA00 KNHC DDHHMM

Tropical Storm Greg Discussion Number 28
NWS National Hurricane Center Miami FL EP072017
200 AM PDT Mon Jul 24 2017

ASCAT passes at 0542 and 0636 UTC suggest that Greg has weakened a
little. The highest wind vector in either pass was 36 kt in the
northeast quadrant, and no winds exceeding 32 kt were observed in
any other quadrants. Based on the ASCAT data, and sampling
considerations, the intensity has been decreased to 40 kt.

The ASCAT passes were very helpful in locating the center of Greg
and the initial motion estimate is 285/8 kt. No significant change
has been made to the NHC track forecast. There remains fair
agreement between the models that Greg will continue west-northwest
or northwestward, steered by an extension of a mid-layer ridge
centered to the northwest. Once the tropical storm weakens to a
remnant low, the low-level flow should turn the remnant circulation
back toward the west or west-southwest until it dissipates.

Greg is not expected to recover from the dry air that is
contributing to its current weakening trend. The vertical shear is
expected to increase as well in about 24-36 hours, which should
cause further weakening. The new forecast is a little lower than
the previous one, mainly to account for the lower initial
intensity. A few models, especially the GFS, suggest that Greg
could open into a trough within the 5-day forecast period, but this
isn't supported by the ECMWF, UKMET, or HWRF. Out of respect for
those models, the forecast currently calls for Greg to remain a
closed circulation throughout the forecast period, but it would not
be surprising if it dissipated by day 5.

FORECAST POSITIONS AND MAX WINDS

INIT 24/0900Z 14.4N 132.1W 40 KT 45 MPH
12H 24/1800Z 14.8N 133.5W 40 KT 45 MPH
24H 25/0600Z 15.4N 135.0W 40 KT 45 MPH
36H 25/1800Z 16.2N 136.4W 35 KT 40 MPH
48H 26/0600Z 16.9N 137.8W 30 KT 35 MPH
72H 27/0600Z 17.4N 140.8W 25 KT 30 MPH...POST-TROPICAL
96H 28/0600Z 17.0N 144.1W 25 KT 30 MPH...POST-TROP/REMNT LOW
120H 29/0600Z 16.0N 147.0W 20 KT 25 MPH...POST-TROP/REMNT LOW

$$
Forecaster Zelinsky