Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
707 AM EST Wed Dec 24 2014

Valid 00Z Thu Dec 25 2014 - 00Z Thu Jan 01 2015

Model and ensemble forecasts show reasonable agreement through the
period with generally modest detail and timing differences.  A
north-south elongated trough/upper low initially over the islands
will lift northward over the next couple days, pulling along
developing surface low pressure that should pass just east of the
Big Island today.  This evolution should promote a drier trend
from the locally heavy rainfall initially affecting the state.

Immediately behind this system another bundle of energy will
amplify into a trough whose southern periphery should pass over
the islands around Fri-Sat.  Upon passage of this trough expect a
further drying trend, with precipitable water values possibly
decreasing below 0.75 inch as seen in latest GFS/ECMWF runs. 
Meanwhile weak high pressure tracking north of the area should
lead to a period of more typical trade flow from the weekend into
early next week.

A third amplified trough will evolve over the central Pacific late
in the period with the southern part again reaching the islands,
and supporting the approach of a cold front.  By next Wed the
primary differences that arise are with amplitude/timing of the
trough aloft.  Recent verification toward the slower guidance
along with strength of the downstream ridge seem to favor the
cluster that includes 00z/06z parallel GFS runs... 00z GEFS
mean... and 00z ECMWF/ECMWF mean that are all slower and/or more
amplified aloft than the operational 00z/06z GFS.  This cluster
would likewise be slower with the associated cold front and
trailing high pressure.

Rausch