Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
820 AM EDT Thu Oct 12 2017

Valid 00Z Fri Oct 13 2017 - 00Z Fri Oct 20 2017

Trades will become gradually stronger over the next few days as a
surface trough initially over the state moves to the west and
eastern Pacific high pressure drifts southwestward, settling over
a position near 35N latitude.  Expect strongest winds during the
Sunday-Thursday period.  From now into the weekend there will be
the potential for some locally heavy rainfall with the combination
of the early period surface trough, an upper level weakness, and
precipitable water values in the 1.50-2.00 inch range.  Best focus
for rainfall will be windward locations but some activity may
stray into leeward areas as well.  Guidance agrees well that drier
air will move into the region for Monday through at least
Wednesday so expect lighter totals for windward-focused rainfall
in that time frame.  By next Thursday models begin to differ
regarding a possible increase in moisture over the Big Island and
vicinity as sheared upper level energy to the east of the area may
close off a retrograding weak upper low.  Currently GFS runs are
farther west with this upper feature thus bringing more moisture
into the southeastern islands than the 00z ECMWF.  In varying ways
the 00z ECMWF mean and 12z/11 ECMWF offer intermediate solutions
for the upper level feature/lower level moisture increase.  Low
predictability of such a small scale evolution at that time frame
recommends a forecast halfway between the 00z/06z GFS and 00z
ECMWF extremes.

Rausch