Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
720 AM EST Fri Dec 22 2017

Valid 00Z Sat Dec 23 2017 - 00Z Sat Dec 30 2017

There is still some spread among the guidance regarding a front
likely to affect the state around the middle of next week but
differences are a little less extreme than yesterday, while
preceding evolution looks more agreeable than seen previously.

Initially expect a band of drier air to drift south across the
state behind a departing wavy front and upper shortwave/compact
low.  Then on Saturday a weakening front with only modest
accompanying rainfall will reach about as far south as the
northwestern islands, followed in another day or so by a weak
front that approaches but may not quite reach the northwestern
islands.  Progression of surface highs to the north, with trailing
ones weaker than the first, will lead to brisk northerly winds
becoming gradually less strong and directed from the northeast and
east-northeast over the course of the weekend.  The trend toward
easterly trades may bring in enough moisture to support a modest
increase in windward-focused shower activity Sunday onward.

During early-mid week most guidance agrees in principle that a
sharpening upper trough should approach/reach the area, but with
differences in shape that affect the progression of the associated
front.  As noted earlier the guidance spread for frontal timing
has narrowed, by way of the GFS slowing down by about a day over
the past 24 hours and the ECMWF speeding up somewhat.  Now the 00z
CMC is on the flat/fast side (not supported by its mean) while the
00z ECMWF/ECMWF mean are slower side by a small margin.  The 00z
GEFS mean shows a slower progression than GFS runs and in fact
toward the end of the period holds onto somewhat higher
precipitable water values over the state than the 00z ECMWF due to
the latter being more amplified with the southern part of upper
trough.  Recent trends/consensus still favor progression slower
than the GFS, near an ECMWF/GEFS mean/ECMWF mean compromise.  This
front will provide some enhancement to rainfall but with specifics
determined by timing that still has a fair degree of uncertainty.

Rausch