WTPA43 PHFO 231459

TCDCP3



TROPICAL STORM DARBY DISCUSSION NUMBER  48

NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI   EP052016

500 AM HST SAT JUL 23 2016



Deep convection associated with Darby has increased once again this

morning, with the bulk of this activity now across the system's

southeast semicircle. Outflow remains best within the northeast

quadrant, but is restricted throughout the south semicircle, thanks

to continued 7 to 10 kt southwest shear. Low cloud swirls east of

the LLCC add confidence to the initial position based heavily on

satellite fixes. However, Darby continues to defy predictions to

gain latitude. Given the continued messy satellite presentation and

the lack of aircraft data for this package, it's possible that this

system is a tenth of a degree or two farther south. The next

forecast package may benefit from land-based weather radar position

estimates. Initial intensity is decreased to 45 kt as a compromise

between continued 35 kt objective Dvorak satellite intensity

estimates and earlier SFMR intensity from reconnaissance aircraft.

The next aircraft reconnaissance mission will be later this morning.



Initial motion is 275/08 kt, representing a gradual slowing over

the past 12 hours. However, this is a 12 hour motion. Darby has not

gained latitude over the past 6 hours and we may find out later

that a small southward component exists when radar estimates become

available. Darby is moving westward along the southern flank of the

subtropical ridge. Global models continue to depict a weakness in

this ridge between 150W and 160W due to low pressure digging in from

the north. All models continue to show Darby slowing and

gradually turning northwest along various curving paths through the

next five days. Track guidance remains tightly packed but shifts

slightly to the left, with all tracks crossing the Big Island.

The right outlier remains GFDL, which takes Darby over Oahu after

passing across leeward waters. The right outlier is the Canadian

model, which takes Darby west of Kauai after its encounter with the

Big Island. The forecast track was tapped to the left again though

day 4 to keep pace with initial motion and to maintain the track

within the guidance envelope. This track, closely following TVCN

consensus, takes Darby to the Big Island in 12 hours.



The intensity forecast continues to be based on the expected toll of

increasing vertical shear on Darby, which gradually overtakes any

sustaining effects from marginal sea surface temperatures. However,

this shear is not expected to become strong until after 18 hours.

The forecast calls for slow weakening with Darby maintaining

tropical storm strength through the weekend. While initial intensity

is decreased to 45 kt, this forecast weakening trend is consistent

with the previous package, is close to IVCN guidance and represents

a compromise between SHIPS and HWRF, which weaken DARBY quickly, and

GHMI, which keeps Darby as a tropical storm through day 5. It is

important to note that weakening due to land interaction has so far

been ignored for that portion of the track beyond the Big Island.



Interests outside of the watch and warning areas in the Hawaiian

Islands 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  23/1500Z 18.8N 153.9W   45 KT  50 MPH

 12H  24/0000Z 19.2N 155.2W   40 KT  45 MPH

 24H  24/1200Z 20.0N 156.5W   40 KT  45 MPH

 36H  25/0000Z 20.9N 157.7W   35 KT  40 MPH

 48H  25/1200Z 22.0N 159.0W   35 KT  40 MPH

 72H  26/1200Z 24.8N 162.1W   35 KT  40 MPH

 96H  27/1200Z 28.2N 164.9W   30 KT  35 MPH

120H  28/1200Z 32.7N 167.0W   30 KT  35 MPH



$$

Forecaster Powell