CPAC: DARBY - Post-Tropical

[Alyono]

12Z ECMWF is within 30 miles of the Big Island

[Kingarabian]

Looks like landfall somewhere in between the 24 hr frames.

[Alyono]

It does not make landfall. It goes around the Big Island

[gatorcane]

Image showing 12Z ECMWF track which is very close to Hawaii. EPAC tracks from other storms also shown

[Alyono]

I have a feeling this may end up SOUTH of the Big Island. Starting to see a shift just like we saw with Ana, Kilo, and Neki

[Kingarabian]

I have a feeling this may end up SOUTH of the Big Island. Starting to see a shift just like we saw with Ana, Kilo, and Neki

I'm not sure. Kilo and Neki missed Hawaii becsuse they took too long to develop. Even Ana to some extent.

[Alyono]

I have a feeling this may end up SOUTH of the Big Island. Starting to see a shift just like we saw with Ana, Kilo, and Neki

I'm not sure. Kilo and Neki missed Hawaii becsuse they took too long to develop. Even Ana to some extent.

One difference though between this one and those

If this ends up south of the Big Island, it probably means Oahu landfall

[gatorcane]

saved vis loop:

[cycloneye]

TROPICAL STORM DARBY DISCUSSION NUMBER 37
NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI EP052016
1100 AM HST WED JUL 20 2016

Darby's satellite signature has degraded slightly since the previous
advisory but convective cloud tops as cold as -65C persist near the
center. However, microwave and conventional satellite imagery
indicate that the low level center is located on the southwest side
of the deep convection, with a 1615Z SSMI/S pass helpful in locating
the center. Since then, visible imagery shows that the center has
become partially exposed. Subjective Dvorak intensity estimates
from PHFO/SAB both indicated a current intensity of 3.0/45 kt, and
the initial intensity has been lowered to 45 kt for this advisory.

The initial motion vector is 270/11 kt, with Darby located well
south of a mid-level ridge that is supported by anticyclones
centered far north of the Hawaiian islands and over the central
mainland U.S. The forecast philosophy with respect to track has
changed little from the previous advisory. Through the first 48 to
72 hours of the forecast, Darby's motion is expected to be
increasingly influenced by the ridge building southeastward from
north of the Hawaiian Islands. This is expected to impart a motion
slightly south of due west until early Saturday. After that time, a
break in the ridge is still expected in response to a southward-
digging low aloft. This is expected to cause Darby to turn sharply
toward the northwest and north on days 4 and 5 as it becomes
embedded within deep southerly flow between the low aloft and a
ridge extending westward from the eastern Pacific. The updated
forecast track is close to the previous, especially through 48
hours, and lies between the previous and TVCN thereafter. This
forecast slightly delays the turn toward the northwest, bringing the
center a little closer to the Big Island of Hawaii, influenced by
the latest ECMWF guidance.

The intensity forecast continues to be complicated by competing
environmental factors. Darby will be moving over slightly warmer
waters during the next couple of days, but this is still expected to
be counteracted by moderate southwesterly shear and a generally dry
environment. The SHIPS and LGEM models show steady weakening that
briefly levels off in the 60 to 72 hour time frame, while the
dynamical models are showing some restrengthening in the 36 to 60
hour time frame, primarily due to a brief reduction in vertical wind
shear. Given this, the official forecast shows little change in
intensity during the first 36 hours, with a slight strengthening on
days 2 and 3 before increasing shear at the end of the period leads
to steady weakening. The official forecast lies close to the IVCN
consensus, which lies between the much stronger HWRF/GHM and the
weaker SHIPS/LGEM.


FORECAST POSITIONS AND MAX WINDS

INIT 20/2100Z 19.9N 141.3W 45 KT 50 MPH
12H 21/0600Z 19.7N 143.0W 45 KT 50 MPH
24H 21/1800Z 19.3N 145.5W 45 KT 50 MPH
36H 22/0600Z 19.0N 147.9W 45 KT 50 MPH
48H 22/1800Z 18.8N 149.9W 50 KT 60 MPH
72H 23/1800Z 19.4N 152.8W 50 KT 60 MPH
96H 24/1800Z 21.3N 154.3W 45 KT 50 MPH
120H 25/1800Z 24.7N 156.0W 40 KT 45 MPH

$$
Forecaster Birchard

[Alyono]

18Z MU maybe 20-25 miles closer to the Big Island

[Hammy]

What impact is the terrain/resulting wind flow around the big island going to play in turning this north?

[stormwise]

[gatorcane]

18Z MU

[Alyono]

HWRF has a HURRICANE passing JUST SOUTH of the Big Island

[gatorcane]

HWRF looks just south of Hawaii right?

[Alyono]

:uarrow: HWRF looks just south of Hawaii right?

I see that on the maps. I got my latitudes a bit crossed up

[cycloneye]

TROPICAL STORM DARBY DISCUSSION NUMBER 38
NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI EP052016
500 PM HST WED JUL 20 2016

For most of the day, Darby's satellite signature was indicative of a
system experiencing southwesterly vertical wind shear, with deep
convection located northeast of the partially-exposed low-level
center. In the last couple of hours, however, a banding feature has
been wrapping around the west side of the center, with latest shear
analyses from UW-CIMSS and SHIPS indicating that the shear has
shifted to the south, and weakened to near 10 kt. Latest subjective
Dvorak intensity estimates from PHFO and SAB indicated a current
intensity of 3.0/45 kt, which is also supported by an ASCAT pass
from around 1900Z that detected winds near 45 kt in the northwest
semicircle. The initial intensity for this advisory has been
maintained at 45 kt, but this may be low if current trends
continue.

Overall, the ongoing track forecast philosophy remains unchanged,
and the updated track forecast through 48 hours is similar to the
previous. After 48 hours, the forecast has been shifted slightly
west once again, due to a westward shift in the typically-reliable
GFEX and TVCN multi-model consensus. Through the first 48 to 72
hours of the forecast, Darby's motion is expected to be increasingly
influenced by a mid-level ridge building southeastward from far
north of the Hawaiian Islands, imparting a motion slightly south of
due west through Friday. After that time, a break in the ridge is
still expected in response to a southward-digging low aloft. This is
expected to cause Darby's forward motion to slow before turning
toward the northwest and north on days 4 and 5, as it becomes
embedded within a deep southerly flow between the low aloft and a
ridge extending westward from the eastern Pacific. It's the timing
of the anticipated turn that has implications for weather in Hawaii,
and the details surrounding that introduce a greater than normal
amount of uncertainty to the forecast, especially with Darby slowing
just east of the Big Island. Noteworthy that the HWRF indicates
that the turn will not occur until after passing the longitude of
the Big Island.

The intensity forecast continues to be complicated by what appears
to be competing environmental factors. Darby will be moving over
slightly warmer water the next couple of days, but this is still
expected to be counteracted by moderate southwesterly shear, with
SHIPS guidance indicating an increase in shear over the next 24
hours before relaxing again in 48 to 72 hours. The official
forecast responds by indicating little change in strength through 24
hours, slight strengthening in 36 hours, then remaining steady state
until weakening later in the forecast period. The weakening on days
4 and 5 is expected to occur as shear increases and Darby begins to
move over cooler waters. The official forecast lies close to the
IVCN consensus, which lies between the much stronger HWRF/GHM which
make Darby a hurricane again, and the weaker SHIPS/LGEM that
continue to indicate a slow but steady weakening.

An ASCAT pass around 1900Z was helpful in estimating the size of
Darby's wind field, and a CryoSat pass around 1500Z was utilized
to increase the radii of 12 foot seas in the northern semicircle.

Interests in the Hawaiian islands should monitor the progress of
Darby. However, it is important not to focus too closely on the
exact track and intensity forecasts because the average track error
72 hours out is near 100 miles, while the average intensity error is
about 15 kt. In addition, the hazards of a tropical cyclone can
extend over a broad area well away from the center.


FORECAST POSITIONS AND MAX WINDS

INIT 21/0300Z 19.8N 142.6W 45 KT 50 MPH
12H 21/1200Z 19.6N 144.5W 45 KT 50 MPH
24H 22/0000Z 19.2N 146.9W 45 KT 50 MPH
36H 22/1200Z 18.9N 149.1W 50 KT 60 MPH
48H 23/0000Z 18.9N 150.9W 50 KT 60 MPH
72H 24/0000Z 19.7N 153.4W 50 KT 60 MPH
96H 25/0000Z 21.7N 155.2W 45 KT 50 MPH
120H 26/0000Z 25.0N 157.0W 40 KT 45 MPH

$$
Forecaster Birchard

[Kingarabian]

Very good discussion by the CPHC.

[Alyono]

quite a bit more intense this run, though it looks to be a bit to the north and has shifted about 20 miles east

[Kingarabian]

Going to be a very interesting Euro run.