WTPA33 PHFO 230548
TROPICAL STORM DARBY INTERMEDIATE ADVISORY NUMBER 46A
NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI EP052016
800 PM HST FRI JUL 22 2016
…DARBY EXPECTED TO REACH THE BIG ISLAND SATURDAY…
SUMMARY OF 800 PM HST…0600 UTC…INFORMATION
ABOUT 180 MI…290 KM ESE OF HILO HAWAII
ABOUT 390 MI…630 KM ESE OF HONOLULU HAWAII
MAXIMUM SUSTAINED WINDS…60 MPH…95 KM/H
PRESENT MOVEMENT…W OR 280 DEGREES AT 12 MPH…19 KM/H
MINIMUM CENTRAL PRESSURE…1001 MB…29.56 INCHES
WATCHES AND WARNINGS
CHANGES WITH THIS ADVISORY:
SUMMARY OF WATCHES AND WARNINGS IN EFFECT:
A Tropical Storm Warning is in effect for…
* Hawaii County
* Maui County, including the islands of Maui, Molokai, Lanai and
A Tropical Storm Watch is in effect for…
A Tropical Storm Warning means that tropical storm conditions are
expected within the warning area within 36 hours.
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 and warnings may be required for
additional islands later tonight or Saturday.
For storm information specific to your area, please monitor
products issued by the National Weather Service office in
DISCUSSION AND 48-HOUR OUTLOOK
At 800 PM HST (0600 UTC), the center of Tropical Storm Darby was
located near latitude 18.7 North, longitude 152.6 West. Darby is
moving toward the west near 12 mph (19 km/h). Darby’s forward
motion is expected to slow slightly Saturday followed by a turn
toward the northwest Saturday night. On the forecast track, the
center of Darby is forecast to pass over or near the Big Island on
Saturday, and Maui on Saturday night.
Maximum sustained winds are near 60 mph (95 km/h) with higher gusts.
Some weakening is forecast during the next 48 hours.
Tropical-storm-force winds extend outward up to 125 miles (205 km)
from the center.
The estimated minimum central pressure is 1001 mb (29.56 inches).
HAZARDS AFFECTING LAND
WIND: Tropical storm force winds are expected over the Big Island
starting late tonight, and over portions of Maui County on
Saturday. Tropical storm force winds are possible over Oahu
Saturday night or Sunday.
SURF: Swells generated by Darby are expected to impact the
Hawaiian Islands over the next couple of days.
RAINFALL: Storm total rainfall of 10 to 15 inches with locally
higher amounts…could cause life-threatening flash floods as well
Next complete advisory at 1100 PM HST.
WTPA43 PHFO 230250
TROPICAL STORM DARBY DISCUSSION NUMBER 46
NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI EP052016
500 PM HST FRI JUL 22 2016
Tropical Storm Darby is very asymmetric in the satellite images this
afternoon with deep convection mainly within a band wrapping around
its western semicircle. This was also seen in the 2231 UTC GPM
pass, along with a separate rainband southeast of the center. High
clouds have been streaming toward the northeast which indicates the
cyclone is beginning to feel an approaching upper level trough. One
of the later passes through Darby by the 53rd Weather Reconnaissance
Squadron’s WC-130J helped confirm the center position of the system
toward the end of the morning mission though the aircraft had to fly
lower to find it. Based on the morning recon mission and the
maintenance of deep convection, the initial intensity has been held
at 50 kt. Note that this is higher than the subjective Dvorak
estimates of 45 kt from PHFO and SAB, and 30 kt from JTWC. The next
aircraft mission into Darby is scheduled for this evening.
Darby is estimated to be moving at 280/11 kt to the south of a
ridge. This ridge is forecast to weaken due to a low pressure system
digging southward to the north of cyclone. This weakness is expected
to decrease Darby’s forward motion over the next day, and increase
the amount of vertical shear affecting the system this weekend. The
trusted dynamical models have remained largely consistent today,
bringing Darby over the Big Island on Saturday. Thus, the current
forecast has been nudged southward closer to the dynamical consensus
with a landfall on the Big Island, followed by a path over Maui
County and near Oahu. The latter part of this path assumes that
Darby survives its impact on the Big Island which is not a certainty.
The intensity forecast rationale remains the same since the last
package. The main factors affecting the intensity forecast include
marginal sea surface temperatures, the amount and timing of vertical
wind shear, and the effects of any potential interactions with the
Hawaiian Islands. Sea surface temperatures will remain marginal near
26.5C over the next couple of days but vertical shear is expected to
increase as the previously mentioned upper level trough digs farther
south. This shear increase is not expected to become strong
until later this weekend. The forecast calls for only slow
weakening with Darby maintaining tropical storm strength through
the weekend. This is consistent with the previous package but
slightly lower than the intensity consensus. Interactions with the
Big Island may cause significant disruptions to Darby so the
intensity forecast confidence is not high at this time.
The expected movement of Darby, as well as the latest wind
probability guidance, warrants the issuance of a Tropical Storm
Watch for the island of Oahu with this package.
Interests on Kauai and the Papahanaumokuakea Marine National
Monument should continue to monitor the progress of Darby. 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 23/0300Z 18.7N 152.1W 50 KT 60 MPH
12H 23/1200Z 18.9N 153.5W 50 KT 60 MPH
24H 24/0000Z 19.4N 154.7W 45 KT 50 MPH
36H 24/1200Z 20.2N 155.8W 45 KT 50 MPH
48H 25/0000Z 21.2N 157.1W 40 KT 45 MPH
72H 26/0000Z 23.5N 160.0W 40 KT 45 MPH
96H 27/0000Z 27.0N 163.0W 35 KT 40 MPH
120H 28/0000Z 31.0N 165.6W 30 KT 35 MPH