Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
720 AM EST Thu Dec 21 2017

Valid 00Z Fri Dec 22 2017 - 00Z Fri Dec 29 2017

The threat for locally heavy rainfall will quickly diminish after
Thursday as an upper trough, likely forming a closed low by
tonight, progresses east of the islands Friday onward.  The
gradient between leading cold front and trailing high pressure
will support a period of northeasterly winds over the state with
terrain-focused shower activity, likely extending through the
weekend.  Locations over/near the Big Island may see somewhat
higher rainfall totals relative to other areas corresponding to
relatively higher precipitable water values.  Moisture should not
be abundant in absolute terms though.

From about Sunday onward guidance diverges with respect to
mid-latitude Pacific systems and influence of trailing fronts on
the state.  For low pressure tracking well north as of Sunday,
GFS/GEFS guidance is southward and weaker than most other
guidance.  Then the GFS is much stronger/northward than other
solutions with the next developing system.  The trailing trough in
the GFS and to some extent GEFS passes through the islands on the
faster side of the guidance around Tuesday.  On the other hand 00z
ECMWF shortwave details lead to a slower evolution that brings up
more moisture from the south Wednesday-Thursday versus the current
majority or the previous ECMWF run.  With minimal confidence, for
the portion of the forecast from Sunday onward will recommend a
compromise between the 00z GEFS and ECMWF means with a slight tilt
toward the ECMWF mean.  By next Thursday this would lead to a
pattern between the extremes, the GFS that builds decent high
pressure north of the islands and the 00z ECMWF that would have
moist flow from the south/southeast. 

Rausch