Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
301 AM EST Tue Dec 21 2021

Valid 00Z Wed Dec 22 2021 - 00Z Wed Dec 29 2021

...Wet pattern likely to taper off by late week...

Some aspects of the forecast continue to be affected by
low-predictability details within complex flow to the south of a
very strong and persistent upper high over the mid-latitude
Pacific.  The most agreeable theme is for brisk to strong trades
(especially early in the period) around the southern periphery of
surface high pressure corresponding to the upper ridge, leading to
primarily windward/mountain focus for rainfall--though with some
rain occasionally straying to leeward locations.

Through at least midweek the Big Island should see the best
potential for locally heavy rainfall due to the combination of a
weakening upper low tracking just south of the state and a band of
enhanced moisture aligned with an old frontal boundary.  Latest
guidance is gradually showing better agreement and stabilizing
with respect to enough suppression of the pattern to support a
drier trend late this week into at least Christmas Day/Saturday. 
The one key difference at the moment is that the majority cluster
of models/ensemble means would delay the drier trend by about a
day versus the 00Z GFS.

Guidance continues to show another upper low developing west of
the state by early Sunday.  Latest consensus has it reaching
closest to the state (centered near 170-175W longitude) around
late Sunday-Monday and then dropping southward.  Leading moisture
may increase rainfall for a time over western areas around
Sunday-Monday.  By Tuesday guidance once again diverges for
moisture, with the 00Z ECMWF/12Z ECMWF mean's farther east upper
low drawing in more moisture from the south versus the 00Z
GFS/GEFS mean that actually start a drier trend with flow from the
northeast having more influence.  For now would favor a compromise
while monitoring for trends/clustering.

Rausch