Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
834 AM EDT Thu Jul 21 2016

Valid 00Z Fri Jul 22 2016 - 00Z Fri Jul 29 2016

The major influence on the islands remains the track and intensity
of Tropical Storm Darby, which is currently forecast by the
Central Pacific Hurricane Center to approach the Big Island on
Saturday and pass to the north northeast on Sunday. 
There is still quite a bit of uncertainty with the ensembles with
a majority of models taking the center to the north of the island
but there are still a minority of solutions that take it closer to
the big island so this could have significant implications for the
forecast.  The most recent gfs model shows an increasing area of
precipitable water values increasing from 1.25 inches to 2 to 2.25
inches plus on Sunday and Monday with an increasing chance of
showers for the Big Island and elsewhere.  The factors coming
together for the path of Darby include a retrograding a digging
trough over the east central Pacific that appears to strongly
influence the path of Darby as it comes closer to the islands. The
approach of this trough appears to break up the more traditional
trade pattern as a large anticyclone to the north drifts westward
across the north central Pacific.

Kocin