Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
714 AM EST Sun Dec 03 2017

Valid 00Z Mon Dec 04 2017 - 00Z Mon Dec 11 2017

A strong upper-level ridge centered well to the west of Hawai'i
will have downstream impacts across the north/central Pacific with
persistent negative height anomalies and southward penetration of
the westerlies toward Hawai'i. A shortwave is forecast to amplify
north of the Islands Mon-Tue, pushing a cold front southward
across the state. Models and ensembles show relatively low spread
surrounding this feature, and confidence is above average early in
the forecast period. While Pwat values will already be relatively
low (0.80-1.00 inch), the passage of the front by Tue will bring
even drier air to the state, with forecast Pwats forecast to drop
to near or even below 0.50 inch. Gusty northerly to northeasterly
winds of 20-25 kt will likely also accompany and follow the
frontal passage. The lack of preexisting moisture ahead of the
front will likely preclude any significant shower activity
associated with its passage. Another shortwave is forecast to
amplify north of Hawai'i by Wed, with a rather strong surface
cyclone for 30-35 deg N accompanying it. Models and ensembles are
also in relatively good agreement on this scenario with some
differences surrounding the intensity of the system (00Z UKMET was
strongest while the CMC was weakest). At this time the preferred
solution is somewhere in the middle of the spread, toward a blend
of the 00Z ECENS/GEFS/NAEFS. The 06Z GFS seems to be in this
vicinity while the 00z ECMWF is a bit faster than consensus. A
wave along the front may slow the progression toward Hawai'i but
another cold front should be approaching the Islands by late in
the week, with an associated increase in moisture increasing
precipitation chances.

Ryan