WTPA43 PHFO 221447
TCDCP3

TROPICAL STORM DARBY DISCUSSION NUMBER  44
NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI   EP052016
500 AM HST FRI JUL 22 2016
 
After decreasing through much of the night, deep convection around
Darby has once again flared along the northern quadrant. Exposed
low cloud bands across the southern semicircle show a reasonably
well organized system, with good outflow to the northeast.
Objective Dvorak satellite intensity estimates remain unchanged at
3.0/45 knots from all three analysis centers (HFO, JTWC and SAB).
An 0718 UTC QSCAT pass showed a broad swath of 45 knot winds across
the northern semicircle, with a small handful of 50 knot barbs
thrown in. We will drop the initial intensity to 50 kt for this
advisory based on this pass and eagerly await 53rd Weather
Reconnaissance Squadron data, scheduled for arrival later this
morning, to clarify Darby intensity.

Initial motion is 270/10 kt, partially derived by rebesting the
previous three best track points. While last evening's QSCAT pass
depicted a center likely too far south, the overall impression from
a growing body of microwave imagery was that Darby was actually
several tenths of a degree south of where we though it was. Darby
was relocated for this advisory. A 1223 UTC SSMI pass, received too
late to use for this advisory, hints that Darby may need to be
relocated westward later. Track guidance is tightly packed depicting
a gently curving path along or just east of the Hawaiian Islands
between 36 and 72 hours. The exception is GFDL, which wants to
divert Darby almost due northward beyond 24 hours. Our forecast
track fits in the envelope, closely following TVCN consensus.
Adjustments were made to account for relocation. However, that
portion of the track closest to the Hawaiian Islands changed very
little. Darby continues to move along the southern flank of a ridge
centered north of Hawaii and will continue to do so through 36
hours. Afterwards, a deep low will dig south and weaken the ridge,
slowing this system down and allowing Darby to curve gently
northwestward from 36 to 72 hours, then northward beyond 72 hours.

Intensity guidance varies from SHIPS, which rapidly weakens Darby
after 48 hours, to GHMI, which maintains current strength
throughout. The effects of shear and slightly warmer SSTS are
expected to essentially cancel each other out through 48 hours, with
the weakening effects of shear becoming dominant afterwards as Darby
gains latitude. Any weakening effects that land interaction would
have on Darby intensity have been ignored so far. Our intensity
forecast depicts Darby as weaker at all tau than previous
advisories, with this system finally becoming a depression on day
5. 
 
Interests outside of the watch and 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  22/1500Z 18.6N 149.2W   50 KT  60 MPH
 12H  23/0000Z 18.7N 150.9W   45 KT  50 MPH
 24H  23/1200Z 19.0N 152.8W   45 KT  50 MPH
 36H  24/0000Z 19.7N 154.4W   45 KT  50 MPH
 48H  24/1200Z 20.7N 155.7W   45 KT  50 MPH
 72H  25/1200Z 23.0N 158.2W   40 KT  45 MPH
 96H  26/1200Z 26.5N 160.5W   35 KT  40 MPH
120H  27/1200Z 31.3N 161.6W   30 KT  35 MPH
 
$$
Forecaster Powell