Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
315 AM EST Wed Dec 22 2021

Valid 00Z Thu Dec 23 2021 - 00Z Thu Dec 30 2021

The complex upper flow pattern to the south of a strong upper high
over the mid-latitude Pacific continues to play a role in guidance
spread and run-to-run variability for some forecast details over
the state.  There is agreement upon moderate to brisk trades for
most of the period, supported by flow around the southern
periphery of Pacific high pressure, leading to windward/mountain
focus for rainfall with some activity occasionally reaching
leeward locations.  Heavier rainfall initially associated with
moisture along an old front (focusing over the Big Island) should
trend lighter after midweek as upper troughing to the northeast
helps push the moisture band to the south.  However there is still
some uncertainty over exact timing of this trend.  Then an upper
low west of the state should reach as close as 170-175W longitude
by the start of next week and then drop southward.  Flow on the
east side of this feature will likely increase moisture from
Sunday onward while the specifics remain in question.

Guidance is still mixed over whether the most rapid decline in
rainfall will be by Thursday or Friday.  The 00Z ECMWF made a
significant adjustment toward the GFS that has been showing an
earlier drying trend.  The UKMET is a member of this cluster but
the CMC is slower with the lighter trend.  Not surprisingly the
GEFS mean sides with the GFS cluster.  The 12Z ECMWF mean held
onto a longer duration of enhanced rainfall, consistent with its
operational counterpart from that cycle, and it will be
interesting to see how much the 00Z version will trend toward the
quicker trend in the operational run.

Ahead of the upper low approaching from the west, continuity is
good with the idea of moisture reaching the western islands by
Sunday-Monday.  Then in contrast to the nearly even split in
guidance yesterday, there is now greater clustering toward the
idea that flow ahead of this upper low will be more dominant than
upper troughing northeast of the state--leading to a period of
higher precipitable water values extending into midweek.  The
improved clustering came via GFS/GEFS trends to the ECMWF/ECMWF
mean.  The 00Z CMC still has stronger northeast troughing that
suppresses the moisture but the CMC solution is very much in the
minority among 00Z GEFS/CMC and 12Z ECMWF ensemble members. 
Currently the guidance as a whole suggests it will be a close call
as to whether the heaviest bands of rainfall directly associated
with the upper low/surface system get as far east as the far
western main islands while the increased moisture over the state
would enhance rainfall within the established trade pattern.

Rausch