WTPA43 PHFO 230250
TCDCP3

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
 
$$
Forecaster Kodama