Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
651 AM EST Fri Dec 15 2017

Valid 00Z Sat Dec 16 2017 - 00Z Sat Dec 23 2017

...Heavy rain threat next week...

An upper low will slowly drift west-northwestward into Saturday,
passing just north of Maui, and then on Sunday the upper low is
forecast to lift northward away from the state, allowing brief
ridging late Sunday and Sunday night.

On Monday, an upper trough approaches from the northwest, which
then crosses the western islands Tue and slows as it crosses the
eastern islands Wed and Thu. 
There was improvement in the timing of the upper trough among the
models and ensembles with good clustering among the ecmwf/ecmwf
ensemble mean and gfs and gefs mean. The primary differences are
the phasing, with both the gfs and ecmwf showing occasional closed
lows embedded within the upper trough.

Scattered showers are expected Friday and Saturday across the
islands, with a little more coverage in the western islands in
advance of the trough.
The heaviest rain on the east side of the upper low will remain
displaced to the northeast of the region.  Less coverage is
expected Sunday with the upper trough departing. Shower coverage
increases again Tuesday and Wednesday with the upper trough
approaching.  With the fetch of deep tropical moisture, there is
potential
for locally heavy downpours. Rainfall coverage/amounts will
decrease once the upper trough passes late next week.

Petersen