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TROPICAL STORM DARBY DISCUSSION NUMBER  49
NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI   EP052016
1100 AM HST SAT JUL 23 2016
 
The 53rd Weather Reconnaisance Squadron has been flying through
Darby all morning, providing valuable data as to Darby's strength
and size. Maximum flight level winds near 50 kt indicate that Darby
has weakened slightly since their visit last night, and the initial
intensity for this advisory has been reduced to 40 kt. 

The initial motion is estimated to be 280/09 kt, with the
poorly-defined center of Darby located on the north side of the deep
convection. Track guidance is fairly well clustered this time
around, and continues to indicate that the center of Darby will move
over the Big Island later today as it moves toward the west. A
weakness in the mid-level ridge is near and north of Darby due to a
deep-layer low centered well to the north of the Hawaiian Islands,
with Darby expected to move north of the ridge axis over the
next day or two. This will result in a gradual turn toward the
northwest, as well as a gradual increase in forward speed. The
official track forecast is close to the previous and a tightly
clustered guidance suite, and offers no significant changes. 

As seen in water vapor imagery, Darby is embedded in an environment
characterized by dry mid- and upper-levels, well captured by special
soundings from Lihue and Hilo this morning. Additionally, vertical
wind shear on the order of 10 to 15 kt is taking its toll on the
system, and deep convection has been struggling to organize over
the center. The updated intensity forecast indicates that Darby will
remain a tropical storm through 24 hours, after which time
increasing shear and gradually cooling water temperatures will lead
to a slow but steady weakening, with Darby becoming a remnant low at
the end of the forecast period, close to guidance provided by
SHIPS/LGEM. The intensity forecast assumes that Darby will be able
to remain some organization as it interacts with the terrain of the
Big Island, but no longer considers an alternative track scenario. 
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  23/2100Z 18.9N 154.8W   40 KT  45 MPH
 12H  24/0600Z 19.4N 156.0W   35 KT  40 MPH
 24H  24/1800Z 20.2N 157.3W   35 KT  40 MPH
 36H  25/0600Z 21.3N 158.6W   30 KT  35 MPH
 48H  25/1800Z 22.5N 160.0W   30 KT  35 MPH
 72H  26/1800Z 25.4N 163.0W   25 KT  30 MPH
 96H  27/1800Z 28.7N 165.6W   25 KT  30 MPH
120H  28/1800Z 33.0N 167.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
 
$$
Forecaster Birchard
 
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