Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
716 AM EST Sat Dec 23 2017

Valid 00Z Sun Dec 24 2017 - 00Z Sun Dec 31 2017

This weekend the state will be within a band of below
climatological precipitable water values, leading to relatively
light rainfall where any occurs.  The most favored areas for such
activity would be near a dissipating front near the northwestern
islands and late weekend over the Big Island as moisture drifts in
from the east.  Brisk northeasterly winds should weaken a bit and
turn more easterly.  Toward late Sunday a front will approach from
the northwest but likely stall before reaching the northwestern
islands.

Into Tuesday guidance agrees fairly well that an
amplifying/sharpening upper trough will approach the area,
deepening low pressure to the northwest/north and encouraging a
strengthening flow of moisture from the south.  However from late
Tuesday onward models/ensembles continue to show some divergence
regarding the details of energy within the upper trough and timing
of frontal passage.  As has been the case in recent days the GFS
is on the faster side of the envelope.  The 00z GEFS mean
continues to argue for slower timing than the GFS, leaning closer
to the 00z ECMWF with respect to arrival of lower precipitable
water values by Thursday-Friday.  The 00z ECMWF/CMC ensemble means
are even a little slower than the ECMWF, so the GEFS mean/ECMWF
offer the best intermediate solution.  The combination of moisture
and fairly strong dynamics may support a band of enhanced rainfall
with this midweek front.  In varying ways guidance is suggesting
that upper level energy may close off/linger just east of the
state by next Saturday, potentially maintaining/bringing back some
moisture into the Big Island and vicinity.  The nature of
evolution and distant time frame yield low confidence in this part
of the forecast.

Rausch