WTPA33 PHFO 220237
TCPCP3

BULLETIN
TROPICAL STORM DARBY ADVISORY NUMBER 42
NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI EP052016
500 PM HST THU JUL 21 2016

…DARBY STILL MOVING TOWARD HAWAII…

SUMMARY OF 500 PM HST…0300 UTC…INFORMATION
———————————————-
LOCATION…18.9N 147.5W
ABOUT 500 MI…800 KM E OF HILO HAWAII
ABOUT 695 MI…1115 KM ESE OF HONOLULU HAWAII
MAXIMUM SUSTAINED WINDS…65 MPH…100 KM/H
PRESENT MOVEMENT…W OR 270 DEGREES AT 13 MPH…20 KM/H
MINIMUM CENTRAL PRESSURE…997 MB…29.44 INCHES

WATCHES AND WARNINGS
——————–
CHANGES WITH THIS ADVISORY:

None.

SUMMARY OF WATCHES AND WARNINGS IN EFFECT:

A Tropical Storm watch is in effect for…
* Hawaii County
* Maui County, including the islands of Maui, Molokai, Lanai and
Kahoolawe

A Tropical Storm watch means that tropical storm conditions are
possible within the watch area within 48 hours.

Interests elsewhere in the Hawaiian Islands should monitor the
progress of Darby. Watches may be required for additional islands
later tonight or Friday.

For storm information specific to your area, please monitor
products issued by the National Weather Service in Honolulu.

DISCUSSION AND 48-HOUR OUTLOOK
——————————
At 500 PM HST (0300 UTC), the center of Tropical Storm Darby was
located near latitude 18.9 North, longitude 147.5 West. Darby is
moving toward the west near 13 mph (20 km/h). This general motion
is expected to continue over the next 48 hours, with a slight
decrease in forward speed expected late Friday into Saturday.
On the forecast track, the center of Darby will pass very close to,
or over, the Big Island on Saturday.

Maximum sustained winds are near 65 mph (100 km/h) with higher
gusts. Little change in strength is forecast during the next 48
hours.

Tropical-storm-force winds extend outward up to 105 miles (165 km)
from the center.

The estimated minimum central pressure is 997 mb (29.44 inches).

HAZARDS AFFECTING LAND
———————-
WIND: Tropical-storm-force winds are possible over the Big Island
and portions of Maui county on Saturday.

SURF: Swells generated by Darby are expected to impact the
Hawaiian Islands over the next couple of days, possibly becoming
damaging along some coastlines Friday and Saturday.

RAINFALL: Heavy rains associated with Darby are expected to reach
the Big Island and portions of Maui county late Friday, potentially
impacting the remainder of the state Saturday. These rains could
cause life-threatening flash floods as well as rock and mud slides.

NEXT ADVISORY
————-
Next intermediate advisory at 800 PM HST.
Next complete advisory at 1100 PM HST.

$$
Forecaster Birchard
WTPA43 PHFO 220303
TCDCP3

TROPICAL STORM DARBY DISCUSSION NUMBER 42
NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI EP052016
500 PM HST THU JUL 21 2016

Darby’s satellite signature has improved somewhat since the
previous advisory, with a curved convective band wrapping into the
center from the north, and outflow increasing in the north and
northwest semicircles. Dvorak intensity estimates continue to be
3.0/45 kt from HFO/SAB/GTW, but the initial intensity has been
maintained at 55 kt for this advisory, as earlier intensity
estimates may have been a little generous. Initial reconnaissance
into Darby by the 53rd Weather Reconnaissance Squadron is scheduled
for tomorrow morning, and their data will give us a much better
idea as to Darby’s intensity and size.

The initial motion for this advisory is estimated to be 270/11 kt,
with Darby currently being steered by a retrograding mid-level ridge
centered well north of Hawaii. This ridge will be the primary
steering mechanism over the next day or so, after which time a deep
layer low digging southward will weaken the ridge, resulting in a
reduction in Darby’s forward speed. As the ridge weakens, Darby is
expected to turn toward the northwest, but the turn is not expected
to be a sharp as earlier forecast. As Darby begins to gain latitude,
it will move north of a mid-level ridge axis extending toward Hawaii
from the eastern Pacific, and become embedded within deep-layer
south to southeast flow that takes Darby toward the northwest.
Latest track guidance shifted toward the previous forecast, and the
new track forecast is essentially an update of the previous one.
Although still on the right hand side of a tighter guidance suite,
the official forecast is now very close to the multi-model consensus
TVCN.

The intensity forecast offers little change, with light to moderate
shear inhibiting strengthening while Darby traverses slightly warmer
water over the next 48 hours. Gradually cooling water temperatures
and increasing shear are expected to lead to a weakening trend on
days 4 and 5, and the updated forecast is close to FSSE guidance,
and a little higher than the intensity consensus IVCN. Of note,
while the official track forecast indicates Darby tracking over
portions of the Hawaiian Islands, the intensity forecast is thus far
not indicating significant weakening due to interaction with land
due to uncertainties associated with the track forecast.

Interests outside of the watch area in the Hawaiian islands should
monitor the progress of Darby, as it could eventually have impacts
on all islands through early next week. Remember, 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 22/0300Z 18.9N 147.5W 55 KT 65 MPH
12H 22/1200Z 18.8N 149.1W 55 KT 65 MPH
24H 23/0000Z 18.8N 151.1W 55 KT 65 MPH
36H 23/1200Z 19.2N 152.9W 55 KT 65 MPH
48H 24/0000Z 19.7N 154.5W 55 KT 65 MPH
72H 25/0000Z 21.3N 156.9W 50 KT 60 MPH
96H 26/0000Z 23.6N 159.1W 40 KT 45 MPH
120H 27/0000Z 27.0N 162.0W 35 KT 40 MPH

$$
Forecaster Birchard
H