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Tropical Storm DARBY Forecast Discussion

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WTPZ45 KNHC 201435

800 AM PDT WED JUL 20 2016

Darby continues to be relatively steady state, with convective cloud
tops of -50C to -60C near the center.  An AMSR2 pass at 1017Z showed
that the low-level center is located southwest of the mid-level
center seen in geostationary imagery.  The initial intensity is set
to 50 kt based on a blend of the latest Dvorak estimates of T3.5/55
kt from TAFB and T3.0/45 kt from SAB.  Darby will move over slightly
warmer waters in the next couple of days, but this should be
counteracted by moderate southwesterly shear and a generally dry
atmospheric environment.  The SHIPS and LGEM models show steady
weakening, while the dynamical models are showing some
restrengthening through 48 hours.  Given this, the official forecast
continues to show little change in intensity during the first 48
hours.  The shear increases late in the period while SSTs will cool
below 26C along the track, which should result in some slow
weakening.  The new NHC forecast is close to the latest intensity
consensus aid.

The initial motion estimate is 270/10.  Darby is expected to move a
little south of due west for the next 48 hours under the influence
of a mid-level ridge building southeastward from north of the
Hawaiian Islands.  After that time, a break in the ridge develops as
an upper-level low retrogrades westward well north of Hawaii, which
should cause Darby to turn sharply northwestward by day 4 and then
north-northwestward on day 5.  Most of model guidance remains in
good agreement on this track scenario, however, there are still some
forward speed differences late in the period.  The ECMWF and UKMET
are faster by days 4 and 5, while the GFS and GEFS ensemble mean are
slower.  Also, the latest HWRF is well south and west of the rest of
the guidance envelope.  The new NHC track forecast is near the
previous one through 48 hours and has been shifted a little to the
left after that time, and lies between the GFS and ECMWF solutions
and their respective ensemble means.

The next advisory on Darby will be issued by the Central Pacific
Hurricane Center in Honolulu, Hawaii.


INIT  20/1500Z 19.9N 139.8W   50 KT  60 MPH
 12H  21/0000Z 19.7N 141.6W   50 KT  60 MPH
 24H  21/1200Z 19.4N 144.0W   50 KT  60 MPH
 36H  22/0000Z 19.0N 146.4W   50 KT  60 MPH
 48H  22/1200Z 18.8N 148.6W   50 KT  60 MPH
 72H  23/1200Z 19.0N 152.0W   45 KT  50 MPH
 96H  24/1200Z 20.5N 153.5W   45 KT  50 MPH
120H  25/1200Z 24.0N 155.0W   40 KT  45 MPH

Forecaster Brennan