Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
729 AM EST Thu Dec 29 2016

Valid 00Z Fri Dec 30 2016 - 00Z Fri Jan 06 2017

A very strong 500 hPa positive anomaly just south of the northern
Aleutians forced by an amplified trough near 160E teleconnects to
a deep cyclone under the ridge's base somewhere to the north of
Hawai'i.  While separation is normally 20 degrees latitude, the
breadth/strength of the positive anomaly is forcing greater
separation than usual.  The negative anomaly under its base could
be composed of multiple closed lows, which is advertised nicely by
the global guidance over the next five days.  As usual, model
disparity increases beyond day 5/January 3, with the GEFS mean
supporting the GFS solution and the ECMWF ensemble mean supporting
the ECMWF solution with the merging system north of the 50th
state.  Differences here are linked to the evolution of flow
pattern the system is embedded within, which transitions from a
Rex-like block to something more Omega-like.  Although the 06z GFS
looks too strong with ridging in the northeast Pacific per the
ensemble mean solutions, believe its solution is closest to
correct near Hawai'i, though it is probably a touch (5 degrees)
too far to the east-southeast with the merged closed cyclone.  The
00z Canadian allows it to escape to the east, which doesn't fit
the flow pattern, while the 00z ECMWF has a pair of closed
cyclones embedded within the ridge near the 165th meridian west,
which is probably one closed low too many (its system near the
Aleutians appears to be the odd man out here).

With the deep moisture offset east of Hawai'i, a generally quiet
pattern is expected with windward focused showers and no more than
modest trades from the northeast.   Believe a cold front/shear
line will drag out of the Papahanaumokuakea Marine National
Monument into the northwestern main islands around Wednesday,
which could increase rain chances mid next week.

Roth