Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
714 AM EST Tue Dec 25 2018

Valid 00Z Wed Dec 26 2018 - 00Z Wed Jan 02 2019

Expect a fairly wet pattern to extend into the weekend with the
potential for areas of heavy rainfall and some thunder.  The
primary uncertainty is exactly how long this wet pattern persists.

Over the next couple days the trade winds will relax as high
pressure to the north of the state progresses rapidly eastward and
merges with another high over the eastern Pacific.  This will
allow for more sea/land breeze influence on shower activity, with
precipitable water values forecast to be above average for the
time of year.  Meanwhile an upper low just southeast of the state
and additional weak shortwave energy passing over the area may
enhance rainfall to some degree.

Late this week there is relative agreement among the guidance that
a sharpening upper trough will push a cold front into the state,
with corresponding focused rainfall that may be heavy at times. 
However in the Friday-Saturday time frame the model/ensemble
guidance diverges considerably regarding evolution of energy
within the upper trough--leading to differences in how long the
wet pattern persists.  ECMWF runs have differed in the details but
generally keep more upper trough/low energy nearby, holding the
front/surface troughing over the main islands for a longer time
than most other solutions.  The 00Z ECMWF ensemble mean seems to
provide a reasonable intermediate scenario among the full array of
guidance, dropping upper level energy over the area somewhat
faster than the 00Z ECMWF but slower than the GFS or GEFS mean. 
This would have trades resuming in the Sunday-Tuesday time frame
with rainfall likely decreasing in intensity as precipitable water
values decrease to within a standard deviation on either side of
normal.

Rausch