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TROPICAL STORM DARBY DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP052016
900 AM MDT TUE JUL 12 2016

Recent microwave data indicate that the depression has a well-
defined circulation with the center located to the northeast of the
strongest convection.  Subjective Dvorak intensity estimates are
T2.0 from TAFB and T3.0 from SAB, and the latest ADT estimate is
right at the tropical storm threshold.  A consensus of these values
supports upgrading the depression to a tropical storm, with the
initial intensity set at 35 kt.  After starting off May and June
very quietly, the eastern North Pacific season has already caught
up to where it should be climatologically in terms of named storms.

Darby appears to have turned westward with an initial motion of
270/9 kt.  A subtropical ridge extending westward from northern
Mexico is expected to strengthen during the next three days, which
will steer Darby westward, or even a little south of due west,
during that time.  The model fields continue to show differences in
the strength of the ridge at the end of the forecast period.  The
ECMWF maintains a stronger ridge, with Darby possibly continuing a
westward motion, while the GFS erodes the ridge and allows Darby to
gain some latitude.  The track model envelope has again shifted
southward on this cycle, and the updated NHC track forecast has
been nudged in that direction close to a consensus of the GFS and
ECMWF.

Darby will be moving over a warm pool of SSTs around 29.5 degrees
Celsius during the next 24 hours, and the northeasterly vertical
shear affecting the cyclone should abate a bit.  Depending on
its exact track, Darby could move over the cold wake left behind by
Hurricanes Blas and Celia in a couple of days.  Some shear could
also persist for a few days, therefore only steady strengthening is
expected through 72 hours.  Colder water should then cause
weakening on days 4 and 5.  There is still a lot of spread among
the intensity models, so for now no significant changes are being
made to the previous NHC intensity forecast.  This scenario is a
little above the intensity consensus during the first three days
and then close to the consensus on days 4 and 5.

FORECAST POSITIONS AND MAX WINDS

INIT  12/1500Z 16.0N 108.5W   35 KT  40 MPH
 12H  13/0000Z 15.9N 110.0W   40 KT  45 MPH
 24H  13/1200Z 15.7N 112.3W   50 KT  60 MPH
 36H  14/0000Z 15.5N 114.5W   60 KT  70 MPH
 48H  14/1200Z 15.4N 116.8W   65 KT  75 MPH
 72H  15/1200Z 15.9N 121.1W   75 KT  85 MPH
 96H  16/1200Z 16.5N 125.0W   65 KT  75 MPH
120H  17/1200Z 17.5N 128.5W   55 KT  65 MPH

$$
Forecaster Berg

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