Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
749 AM EDT Wed Sep 27 2017

Valid 00Z Thu Sep 28 2017 - 00Z Thu Oct 05 2017

Surface ridging to the northeast of the islands will strengthen
slightly during Thursday into Friday, resulting in a minor
increase in the trades, with any shower activity favoring windward
terrain.  With the approach of an amplifying central Pacific
mid-upper trough northwest of the state, trades will begin to
weaken and turn more southeasterly by late Friday into Saturday.

Beyond the middle of the weekend...the forecast becomes
increasingly uncertain.  While they all agree on the
abovementioned trough amplification around 170W late
Friday-Saturday, the models have shown little run to run
continuity in its evolution.  In part, they have struggled with
the degree of phasing expected with northern stream energy moving
through the Bering Sea, and in part due to  differences handling
short wave energy moving through the northwestern Pacific (which
is impacting the eastward progression of this low over time). 
Right now...the 06Z GFS is on the western end of solutions due to
much weaker upstream energy (and thus a stronger upper ridge to
its north/west).  The 00Z ECMWF is much different from its
previous solution, with the upper low initially farther north than
the GFS, but maintaining a strong trough extending southward along
165W Sunday...with a low eventually closing off and moving
eastward north of the islands early next week.  The Canadian and
UKMET offer a compromise solution between the GFS/ECMWF.  The
ECMWF ensemble mean generally supports this compromise solution,
keeping the main trough/closed low off to the northwest of the
islands, and this is currently our preference at this time.

Based on this scenario...would expect that a surface front will
approach and possibly reach the western part of the island chain
early next week and bring a risk of widespread shower activity. 
Farther east, the low level flow should remain southeast to
south-southeast and relatively light, so shower activity will
generally be sea/land-breeze driven versus the standard trade
regime.

Klein