Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
341 AM EST Sat Dec 19 2020

Valid 00Z Sun Dec 20 2020 - 00Z Sun Dec 27 2020

...Wet pattern to subside after the weekend...

Expect the potential for some locally heavy rainfall to extend
into Sunday, especially over the eastern islands, from the
combination of a nearby upper trough/embedded low(s) just to the
east along with abundant deep moisture (precipitable water values
up to 1.50-1.75 inches).  After Sunday there will be a notable
drying trend as the upper trough and deep moisture depart to the
east/southeast, while upper ridging builds overhead.  Brisk trade
winds during the weekend will trend weaker into next week as the
supporting surface ridge continues into the eastern Pacific and a
front approaches from the northwest.  Thus rainfall should
transition from primarily windward focus toward some sea/land
breeze focus by next Wednesday-Thursday.  As the cold front
reaches the area late in the week, guidance diverges somewhat on
accompanying moisture and the progression of trailing high
pressure.  This affects how much rainfall there will be along the
front along with the onset and strength of northeasterly winds
behind the front.  Currently would recommend an intermediate
solution given the split in guidance.  The 00Z GFS/GEFS mean/CMC
form a modest majority for faster progression of the
front/trailing high.  Meanwhile the GFS/GEFS mean are on the lower
side of the spread with moisture/rainfall but the 12Z ECMWF mean
suggests precipitable water values may not reach quite as high as
seen in the 00Z ECMWF run.

Rausch