CPAC: DARBY - Post-Tropical

[Yellow Evan]

Since there's a system established, I don't think dry air will be the fatal issue. We've seen Isellle from 2014 maintain its strength in a similar environment. The SHIPS output does suggest there could be moderate wind shear however not as strong as the NHC makes it sound. I'd be surprised if this was a hurricane by the time it approaches the Big Island, but a moderate tropical storm is not out of the question.

[Alyono]

We really need recon scheduled for late this week for Darby

[Kingarabian]

15kts if vertical shear and 15kts of mid level shear near Hawaii may be enough to take down Darby.

[Alyono]

I'm quite surprised

Recon is scheduled for Friday

[Alyono]

12Z EC has shifted a little closer to the islands and is slightly more intense

The models are now indicating that intensification will likely occur as this makes the north turn

[cycloneye]

TROPICAL STORM DARBY DISCUSSION NUMBER 33
NWS NATIONAL HURRICANE CENTER MIAMI FL EP052016
200 PM PDT TUE JUL 19 2016

Although Darby continues to exhibit a well-defined circulation, deep
convection, in the form of ragged bands, continues to gradually
diminish in coverage. The initial intensity is subsequently lowered
to 55 kt and reflects the Dvorak intensity estimates from both TAFB
and SAB. The cyclone is forecast to move over marginal SSTs and
into a drier, more stable air mass with increasing modest
southwesterly shear during the next several days. The official
forecast indicates slight weakening during the next day or so, with
very little change in strength through 96 hours as Darby traverses
a slightly warmer ocean. Thereafter, Darby is expected to again
weaken as it gains latitude and moves into a region of strong
southwesterly shear. The official forecast is weighed heavily on
the IVCN consensus and lies above the LGEM and Decay SHIPS. It
should be noted that longer-range, i.e. day 4 and 5, intensity
predictions have little skill.

A series of microwave images reveal an apparent northward
tilt with height of the cyclone. Consequently, the conventional
satellite position estimates have been consistently a little north
of the microwave fixes. Using a blend of these fixes, the initial
motion is estimated at 280/11 kt. The synoptic reasoning for the
track forecast has not changed, as Darby will be steered generally
westward during the next 4 days. Through the remaining portion of
the forecast, the cyclone is expected to decrease its forward speed
and gradually begin to recurve northwestward into a growing weakness
caused by an amplifying mid- to upper-level trough north of the
Hawaiian Islands. The ECMWF global model has shifted farther south
than the previous runs. This shift has also nudged the multi-model
consensus south as well, and the official forecast follows suit and
is close the GFS and ECMWF ensemble mean guidance.

FORECAST POSITIONS AND MAX WINDS

INIT 19/2100Z 19.9N 136.6W 55 KT 65 MPH
12H 20/0600Z 20.0N 138.3W 50 KT 60 MPH
24H 20/1800Z 20.0N 140.6W 45 KT 50 MPH
36H 21/0600Z 19.7N 142.8W 45 KT 50 MPH
48H 21/1800Z 19.4N 145.1W 45 KT 50 MPH
72H 22/1800Z 19.2N 149.3W 45 KT 50 MPH
96H 23/1800Z 19.9N 152.0W 45 KT 50 MPH
120H 24/1800Z 21.6N 153.7W 40 KT 45 MPH

$$
Forecaster Roberts

[gatorcane]

The 12Z UKMET is similar to the ECMWF which shows a turn east of Hawaii:

[Alyono]

what is worrying me is there is potential for rapid intensification right when this starts making the turn.

A hit on Hawaii likely would be as an intensifying hurricane. I wish the EC has trended east, but it shifted slightly west at 12Z. Hope the early recurvature bias that we saw in 2009 and again in 2014 with Ana is not going to happen this time as if it does, we are looking at a landfall

[gatorcane]

18z MU slightly stronger but still east of Hawaii

[cycloneye]

TROPICAL STORM DARBY DISCUSSION NUMBER 34
NWS NATIONAL HURRICANE CENTER MIAMI FL EP052016
800 PM PDT TUE JUL 19 2016

Darby continues to maintain deep convection near its center, and in
fact the convective cloud tops have cooled a bit over the past
several hours. The current intensity estimate, 55 kt, is unchanged
from the past advisory and is consistent with the latest Dvorak
estimate from TAFB. The storm should encounter modestly increasing
vertical shear and drier air over the next several days, but will be
moving over slightly warmer waters. These factors should result in
only a slow rate of weakening over the forecast period, and this is
reflected in the official intensity forecast, which is near the
model consensus and above the latest SHIPS and LGEM guidance. It
is worth noting however, that longer-range intensity prediction has
little skill.

Based on geostationary and microwave fixes, the westward motion,
280/11 kt, continues. The track forecast reasoning has not
changed. A mid-level subtropical ridge extending westward from the
southern Great Plains should continue to steer Darby on a westward
or slightly south-of-westward heading for the next few days. In the
latter part of the forecast period, a mid-level cyclone to the north
of the Hawaiian Islands should cause Darby to turn toward the
northwest. The official forecast is very close to the multi-model
consensus. However, the GFS and ECMWF ensemble tracks show
considerable spread on days 3-5, indicating a significant amount of
uncertainty in the forecast track late in the period.

FORECAST POSITIONS AND MAX WINDS

INIT 20/0300Z 20.0N 137.8W 55 KT 65 MPH
12H 20/1200Z 20.0N 139.5W 50 KT 60 MPH
24H 21/0000Z 19.8N 141.8W 50 KT 60 MPH
36H 21/1200Z 19.5N 144.1W 50 KT 60 MPH
48H 22/0000Z 19.2N 146.3W 45 KT 50 MPH
72H 23/0000Z 19.1N 150.2W 45 KT 50 MPH
96H 24/0000Z 20.2N 152.2W 45 KT 50 MPH
120H 25/0000Z 22.0N 154.0W 40 KT 45 MPH

$$
Forecaster Pasch

[Kingarabian]

However, the GFS and ECMWF ensemble tracks show
considerable spread on days 3-5, indicating a significant amount of
uncertainty in the forecast track late in the period.

[Kingarabian]

Euro continues trending west. Very close call.

[cycloneye]

TROPICAL STORM DARBY DISCUSSION NUMBER 35
NWS NATIONAL HURRICANE CENTER MIAMI FL EP052016
200 AM PDT WED JUL 20 2016

Darby has changed little in structure during the past few hours.
Deep convection with cloud tops as cold as -60C continue near the
center, and recent microwave images showed a nearly closed
mid-level ring. Dvorak Current Intensity numbers remain 3.5 from
TAFB and SAB, and the initial intensity is therefore held at 55 kt.
Darby is over SSTs around 25C and is heading toward warmer waters.
However, vertical shear is expected to increase in the next 24-36
hours. Interestingly, the intensity models aren't in agreement on
what will happen with Darby's intensity during this period. While
the SHIPS and LGEM models indicate steady weakening over the next
few days, the HWRF and Florida State Superensemble actually show
some re-intensification in 24-36 hours. Due to these competing
signals, the NHC official forecast shows little change in intensity
during the next 48 hours, followed by gradual weakening on days
3-5. This updated forecast is not too different from the previous
one, except that it delays steady weakening until after 48 hours.

Darby is moving westward with an initial motion of 270/11 kt. The
cyclone is located to the south of the subtropical ridge, which
should maintain a westward motion for the next three days. After
that time, a retrograding mid- to upper-level low north of the
Hawaiian Islands will create a break in the ridge, causing Darby to
turn northwestward and north-northwestward on days 4 and 5. With
the exception of the GFDL, there is very little cross-track spread
among the model guidance. However, there are some noticeable speed
differences, with the ECMWF and UKMET showing more acceleration
after the turn compared to the GFS and HWRF models. For now, the
NHC official forecast, which is very similar to the previous
forecast, splits the difference and is near a consensus of the GFS
and ECMWF models.

FORECAST POSITIONS AND MAX WINDS

INIT 20/0900Z 20.0N 138.9W 55 KT 65 MPH
12H 20/1800Z 19.8N 140.6W 50 KT 60 MPH
24H 21/0600Z 19.5N 142.9W 50 KT 60 MPH
36H 21/1800Z 19.2N 145.2W 50 KT 60 MPH
48H 22/0600Z 18.9N 147.5W 50 KT 60 MPH
72H 23/0600Z 19.0N 151.0W 45 KT 50 MPH
96H 24/0600Z 20.0N 152.5W 45 KT 50 MPH
120H 25/0600Z 23.0N 154.0W 40 KT 45 MPH

$$
Forecaster Berg

[cycloneye]

TROPICAL STORM DARBY DISCUSSION NUMBER 36
NWS NATIONAL HURRICANE CENTER MIAMI FL EP052016
800 AM PDT WED JUL 20 2016

Darby continues to be relatively steady state, with convective cloud
tops of -50C to -60C near the center. An AMSR2 pass at 1017Z showed
that the low-level center is located southwest of the mid-level
center seen in geostationary imagery. The initial intensity is set
to 50 kt based on a blend of the latest Dvorak estimates of T3.5/55
kt from TAFB and T3.0/45 kt from SAB. Darby will move over slightly
warmer waters in the next couple of days, but this should be
counteracted by moderate southwesterly shear and a generally dry
atmospheric environment. The SHIPS and LGEM models show steady
weakening, while the dynamical models are showing some
restrengthening through 48 hours. Given this, the official forecast
continues to show little change in intensity during the first 48
hours. The shear increases late in the period while SSTs will cool
below 26C along the track, which should result in some slow
weakening. The new NHC forecast is close to the latest intensity
consensus aid.

The initial motion estimate is 270/10. Darby is expected to move a
little south of due west for the next 48 hours under the influence
of a mid-level ridge building southeastward from north of the
Hawaiian Islands. After that time, a break in the ridge develops as
an upper-level low retrogrades westward well north of Hawaii, which
should cause Darby to turn sharply northwestward by day 4 and then
north-northwestward on day 5. Most of model guidance remains in
good agreement on this track scenario, however, there are still some
forward speed differences late in the period. The ECMWF and UKMET
are faster by days 4 and 5, while the GFS and GEFS ensemble mean are
slower. Also, the latest HWRF is well south and west of the rest of
the guidance envelope. The new NHC track forecast is near the
previous one through 48 hours and has been shifted a little to the
left after that time, and lies between the GFS and ECMWF solutions
and their respective ensemble means.

The next advisory on Darby will be issued by the Central Pacific
Hurricane Center in Honolulu, Hawaii.

FORECAST POSITIONS AND MAX WINDS

INIT 20/1500Z 19.9N 139.8W 50 KT 60 MPH
12H 21/0000Z 19.7N 141.6W 50 KT 60 MPH
24H 21/1200Z 19.4N 144.0W 50 KT 60 MPH
36H 22/0000Z 19.0N 146.4W 50 KT 60 MPH
48H 22/1200Z 18.8N 148.6W 50 KT 60 MPH
72H 23/1200Z 19.0N 152.0W 45 KT 50 MPH
96H 24/1200Z 20.5N 153.5W 45 KT 50 MPH
120H 25/1200Z 24.0N 155.0W 40 KT 45 MPH

$$
Forecaster Brennan

[bg1]

TROPICAL STORM DARBY DISCUSSION NUMBER 36
NWS NATIONAL HURRICANE CENTER MIAMI FL EP052016
800 AM PDT WED JUL 20 2016

...

Darby is expected to move a
little south of due west for the next 48 hours under the influence
of a mid-level ridge building southeastward from north of the
Hawaiian Islands. After that time, a break in the ridge develops as
an upper-level low retrogrades westward well north of Hawaii, which
should cause Darby to turn sharply northwestward by day 4 and then
north-northwestward on day 5. Most of model guidance remains in
good agreement on this track scenario, ...

$$
Forecaster Brennan

Hawaii has really been pulling out all the stops since 2014; it even slowed a hurricane down so it would weaken to a TS.

[Alyono]

12Z MU slightly stronger and a half degree to the west. Still a miss though

Looks like it is close enough so that the mountains are starting to affect the track

[gatorcane]

Turns NNW at last second on 12Z GFS

[cycloneye]

 https://twitter.com/EricBlake12/status/755797508668792832

[Kingarabian]

00z Euro ensembles were more left as well. Super close call.

[Alyono]

As small as this is, if it were to hit the Big Island, it likely would dissipate over the island