Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
713 AM EST Sun Dec 24 2017

Valid 00Z Mon Dec 25 2017 - 00Z Mon Jan 01 2018

Expect moderate northeasterly trades from now into Christmas Day,
with only light/scattered windward-focused showers given
relatively low precipitable water values.  There may also be a
band of light rainfall in the vicinity of a weak front that
briefly stalls near Kauai.  Guidance is coming into better
agreement regarding the front passing through the area around
midweek, in association with an upper trough that
amplifies/sharpens just west of the state into Tuesday and crosses
the region Wednesday-Thursday.  Now the guidance spread for
frontal timing is generally determined by small scale shortwave
differences that are typically not well resolved more than a
couple days out in time, favoring a compromise approach.  The
vigorous dynamics aloft and southerly flow of moisture ahead of
the front may produce areas of heavy/convective rainfall.  Shower
activity will trend lighter late week into the weekend.  The
primary forecast uncertainty now involves the potential for some
upper level energy to close off a low over or east of the islands
Friday onward.  Based on the full array of model/ensemble
guidance, an average of the past couple GFS/ECMWF runs provides a
good starting point with a path to the east of the islands but the
GFS runs may be too deep with the upper low.  The 00z CMC is more
open/eastward while some 00z GEFS members and one or two CMC/ECMWF
ensemble members show an upper low track to the west/southwest of
the GFS/ECMWF.  Assuming an upper low somewhat weaker than
depicted in the GFS, by next weekend anticipate less surface
troughing just east of the state and more typical trade flow such
as forecast by the ECMWF and GEFS/ECMWF means.

Rausch