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TROPICAL STORM DARBY DISCUSSION NUMBER  42
NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI   EP052016
500 PM HST THU JUL 21 2016
 
Darby's satellite signature has improved somewhat since the
previous advisory, with a curved convective band wrapping into the
center from the north, and outflow increasing in the north and
northwest semicircles. Dvorak intensity estimates continue to be
3.0/45 kt from HFO/SAB/GTW, but the initial intensity has been
maintained at 55 kt for this advisory, as earlier intensity
estimates may have been a little generous. Initial reconnaissance
into Darby by the 53rd Weather Reconnaissance Squadron is scheduled
for tomorrow morning, and their data will give us a much better
idea as to Darby's intensity and size.
 
The initial motion for this advisory is estimated to be 270/11 kt,
with Darby currently being steered by a retrograding mid-level ridge
centered well north of Hawaii.  This ridge will be the primary
steering mechanism over the next day or so, after which time a deep
layer low digging southward will weaken the ridge, resulting in a 
reduction in Darby's forward speed.  As the ridge weakens, Darby is
expected to turn toward the northwest, but the turn is not expected
to be a sharp as earlier forecast. As Darby begins to gain latitude,
it will move north of a mid-level ridge axis extending toward Hawaii
from the eastern Pacific, and become embedded with a deep-layer
southerly flow that takes Darby toward the northwest. Latest track 
guidance shifted toward the previous forecast, and the new track
forecast is essentially an update of the previous one. Although
still on the right hand side of a tighter guidance suite, the
official forecast is now very close to the multi-model consensus
TVCN.
 
The intensity forecast is essentially unchanged from the previous,
with light to moderate shear inhibiting strengthening while
Darby traverses slightly warmer water over the next 48 hours.
Gradually cooling water temperatures and increasing shear are
expected to lead to a weakening trend on days 4 and 5, and the
updated forecast is close to FSSE guidance, and a little higher
than the intensity consensus IVCN.  Of note, while the official
track forecast indicates Darby tracking over portions of
the Hawaiian Islands, the intensity forecast is thus far not
indicating significant weakening due to interaction with land due
to uncertainties associated with the track forecast.
 
Interests outside of the watch area 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  22/0300Z 18.9N 147.5W   55 KT  65 MPH
 12H  22/1200Z 18.8N 149.1W   55 KT  65 MPH
 24H  23/0000Z 18.8N 151.1W   55 KT  65 MPH
 36H  23/1200Z 19.2N 152.9W   55 KT  65 MPH
 48H  24/0000Z 19.7N 154.5W   55 KT  65 MPH
 72H  25/0000Z 21.3N 156.9W   50 KT  60 MPH
 96H  26/0000Z 23.6N 159.1W   40 KT  45 MPH
120H  27/0000Z 27.0N 162.0W   35 KT  40 MPH
 
$$
Forecaster Birchard
 
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