Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
340 AM EST Fri Dec 20 2019

Valid 00Z Sat Dec 21 2019 - 00Z Sat Dec 28 2019

...High winds possible Friday into Saturday...

A cold front is forecast to pass through Hawaii on Friday,
stalling south of the state over the weekend. Strong surface high
pressure north of Hawaii along with an area of low pressure well
west of Hawaii is expected to result in an enhanced surface
pressure gradient across the region Friday into Saturday,
resulting in the potential for strong trade winds, with high wind
gusts possible (perhaps gusting as high as 60 mph for some areas).
These winds should gradually relax somewhat by Sun-Mon as the
surface high weakens and moves east.  A second area of high
pressure north of the state (albeit weaker than the first) could
produce another breezy/gusty period by Tue/Wed. By the middle to
end of next week, a cutoff low initially well west of Hawaii
should get picked up by a strong north Pacific trough, and begin
heading eastward. Models/ensemble show general consensus on this
idea. A plume of moisture should accompany the system eastward,
crossing Hawaii by next Wed-Fri. Model solutions differ
significantly on just how deep the moisture in the plume will be,
with the GFS showing deeper moisture (PWs over 2 inches) across
the western islands than the ECMWF (PWs in the 1.70-2 inch range).
At this point in time, ensemble probabilities for heavy rainfall
are relatively low, but the western islands should see the best
chance for at least locally heavy rains by the middle to end of
next week.

Ryan