WTPA43 PHFO 222107
TCDCP3

TROPICAL STORM DARBY DISCUSSION NUMBER  45
NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI   EP052016
1100 AM HST FRI JUL 22 2016
 
Deep convection associated with Darby was mainly in the
system's northern semicircle with persistent tops colder than
-70C. An SSMIS pass from 1732 UTC indicated the system was tilted
toward the east. The first Air Force Reserve 53rd Weather
Reconnaissance Squadron mission into Darby found maximum SFMR winds
of just over 50 knots but the initial passes had difficulty
determining the location of the low level center. Based on the
recon data thus far, the tropical storm force radius was expanded
slightly in the northern semicircle and Darby has been held at 50
kt for this advisory package.

Darby is estimated to be moving at 270/10 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 Darby. This is expected to
decrease the forward motion over the next day, and increase the
amount of vertical shear on the tropical cyclone this weekend. The
trusted objective aids are consistent with this scenario but have
shifted southward slightly with some solutions indicating landfall
over the Big Island. As a result, the current forecast has been
shifted southward a bit and is between the dynamical consensus and
the previous forecast. Given current guidance trends, a direct
impact on the Big Island and Maui is a distinct possibility this
weekend. 
 
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 be significant until
later this weekend. Thus, the forecast is close to the previous
package which has Darby only slowly weakening and maintains the
cyclone as a tropical storm through the weekend. This is close to
the intensity consensus guidance. Note that interactions with the
main Hawaiian Islands may cause significant disruptions to Darby and
so the intensity forecast confidence is not high at this time.
 
Interests outside of the warning areas 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  22/2100Z 18.5N 151.0W   50 KT  60 MPH
 12H  23/0600Z 18.8N 152.5W   50 KT  60 MPH
 24H  23/1800Z 19.4N 154.4W   45 KT  50 MPH
 36H  24/0600Z 20.4N 155.7W   45 KT  50 MPH
 48H  24/1800Z 21.4N 156.9W   40 KT  45 MPH
 72H  25/1800Z 23.8N 159.3W   40 KT  45 MPH
 96H  26/1800Z 26.0N 161.5W   35 KT  40 MPH
120H  27/1800Z 30.0N 162.5W   30 KT  35 MPH
 
$$
Forecaster Kodama