Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
730 AM EST Tue Dec 13 2016

Valid 00Z Wed Dec 14 2016 - 00Z Wed Dec 21 2016

...Heavy rainfall threat commencing this weekend...

The 06Z unified surface analysis depicted the tail-end of a
baroclinic zone sweeping south of the island chain with only few
widely scattered showers falling in the post-frontal environment.
A high amplified synoptic flow regime over the mid-latitudes will
allow for an additional area of troughing to form to the northwest
by mid-week. All models agree on a compact upper low with likely
sub-558 dm mid-level heights being located just south of 30N and
east of 160W by late Thursday night. Given the tightening pressure
gradient, moderate to occasionally strong northwesterly flow is
expected within the cool sector of this system. It appears like
the model consensus is keeping the frontal precipitation on the
lighter side except where orographics come into play. During the
weekend, this powerful synoptic low will dip southward with some
solutions carrying a near 1000-mb surface cyclone toward the
northern Hawai`ian islands. This increase in low-level convergence
combined with strengthening lift under the upper low will
definitely increase the coverage of showers over the region. This
should be particularly the case along an old frontal zone which
stalls to the south of the Big Island. Moderate to heavy rainfall
is likely over the southern islands although some of the activity
may remain to the south. Most models show wet conditions
prevailing over the weekend and into early next week.

Rubin-Oster