Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
804 AM EDT Mon Aug 28 2017

Valid 00Z Tue Aug 29 2017 - 00Z Tue Sep 05 2017

The models are in good agreement through about Day 3 handling the
main features affecting the Hawaiian islands...with solutions
diverging thereafter...primarily with respect to the amplifying
pattern in the polar stream. 

At the start of the forecast period...an area of moisture
extending northeastward from a weak tropical wave southwest of the
islands will interact with a weak mid-level shear axis to support
some enhanced shower activity across the state through Tuesday. 
On Wednesday into Thursday...trades should return to near normal
levels as high pressure builds to the northeast in response to a
strengthening upper ridge in the eastern Pacific.  At the same
time, drier air will advect in from the northeast, leading to
generally dry conditions statewide.

Later this week through the weekend, generally favored a blend of
the Canadian and ECMWF solutions over the GFS which dig a stronger
short wave trough southward over a an amplified ridge in the west
Pacific. Its associated surface front will set up well northwest
of the islands and should result in trades weakening a bit as well
as veering around to a more southeasterly direction.  This will
likely result in an increase in low level moisture, but overall
the models maintain slightly below average PWs, so not expecting a
large increase in shower activity through Saturday.  By
Sunday-Monday, again favored a blend of the Canadian and ECMWF
solutions which significantly amplify a trough in the polar stream
and have a pretty well-defined surface front approaching the
northwest part of the island chain.  As a result...trades should
weaken considerably, with winds turning more southerly. 
Increasing tropical moisture should support the chance for diurnal
showers...particularly across the southern part of the state. 
Given the divergence of model solutions (likely a result of
potential tropical development in the West Pacific), confidence is
low in any details for the forecast this weekend into early next
week.

Klein