Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
815 AM EDT Sun Jul 22 2018

Valid 00Z Mon Jul 23 2018 - 00Z Mon Jul 30 2018

Guidance still agrees upon a fairly stable moderate to locally
brisk trade wind pattern with patches of enhanced moisture helping
to increase shower activity at times.  Rain will tend to focus
over windward areas but sometimes reach leeward locations as well.
 The primary question marks involve the magnitude of moisture
increases and associated impact on rainfall after midweek.

There has been good consensus with the first pocket of enhanced
moisture progressing from east to west across the area early this
week, followed by a brief gap of somewhat drier air (though with
precipitable water values still likely to be above normal).  Then
solutions diverge somewhat for the next area of moisture to reach
the state in the late Wednesday-Friday time frame.  The 00Z GFS
shows the greatest northward extent of PWATs greater than 2.00
inches and thus heaviest rainfall.  Besides being wetter than
other solutions including the 06Z GFS and 00Z GEFS mean as well as
ECMWF, that GFS run has a curious evolution of an upper impulse
that drifts north-northeast toward the islands--essentially toward
the middle of the ridge covering the area.  Thus would at least
favor a compromise among the non-00Z GFS solutions.  By late
Saturday-Sunday further differences arise with the spread similar
to the preceding episode, 00Z GFS with the greatest moisture and
then less in the 06Z GFS/00Z GEFS mean and finally 00Z ECMWF
driest.  Upper ridging weakens a bit late in the period but still
seems supportive of a non-00Z GFS compromise. 

Rausch