Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
715 AM EST Thu Dec 18 2014

Valid 00Z Fri Dec 19 2014 - 00Z Fri Dec 26 2014

The surface high to the north/northeast of the state will continue
to drift eastward, allowing moderate to brisk trades to trend
lighter with time.  By the weekend the departure of this high and
approach of a strong cold front should yield moderately weak
trades from more of a southeasterly orientation.  From now through
the weekend generally windward-focused showers should be on the
light side with guidance showing precipitable water values
reaching their lowest point near or slightly below an inch around
Fri.

Latest models and ensemble means agree in principle that anomalous
amplification of an upper trough/closing of an embedded low will
push the aforementioned cold front across the islands in the late
Sun-Tue time frame, with some increase in moisture and rainfall
potential.  However differences remain with the precise amplitude
and timing of the upper trough.  As was the case 24 hours ago the
parallel GFS runs are showing a slower and more amplified solution
that compares much better to the ECMWF/ECMWF mean versus the
flatter and more progressive GFS/GEFS mean runs.  Canadian
guidance has tended to be much too open/progressive but at least
the 00z CMC mean has adjusted to the GEFS.  The 00z ECMWF
amplification aloft leads to some 500mb height anomalies of 5-5.5
standard deviations below normal just north of the islands by
00z-12z Wed so that exact solution could be a little extreme.  The
00z parallel GFS is similar to the 00z ECMWF at 12z Tue but
becomes flatter thereafter and the 06z parallel GFS is flatter
yet, so for now would recommend the timing of the slower cluster
but with somewhat less extreme amplitude than the 00z ECMWF.  With
passage of the front expect much drier air to spread into the
state Wed-Thu with fairly strong winds taking on more of a
northerly component than usual.

Rausch