Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Mon Dec 20 2021

Valid 00Z Tue Dec 21 2021 - 00Z Tue Dec 28 2021

...Potential for a wetter pattern this week...

Early in the forecast period the guidance continues to show a wet
trade pattern as strong Pacific high pressure supports brisk winds
while the combination of a front dropping south into/through the
state and an upper low that weakens as it tracks just south of the
area should enhance rainfall.  There is fairly good agreement that
highest rainfall totals are likely to be over windward portions of
the Big Island.  This enhanced rainfall should extend through at
least Wednesday or Thursday.

From late week onward the forecast continues to depend on the
complexities of flow aloft to the south of the strong upper high
over the mid-latitude Pacific.  The degree of upper troughing to
the northeast of the state will determine whether the moisture
axis along the old front can stay south of the main islands while
the character of height falls ahead of a developing upper
trough/low well west of the state will decide how much if any
increase of moisture may reach the western islands toward
Sunday-Monday.   

Over the past day the GFS has trended toward other models that
lately have had enough upper troughing to the northeast to support
a drier trend over the state after Thursday as the front/moisture
axis stay suppressed just to the south.  The 00Z GFS has also
backed away some from some prior runs with height falls/moisture
arriving from the west late in the period.  The 00Z GEFS mean is
now the one holdout, with a weaker upper trough that allows for
moisture to linger over the state and is also on the aggressive
side of guidance with late period height falls/moisture in the
west.

Thus the best signal from guidance is for a drier trend to some
extent after Thursday while trades may weaken slightly with time. 
Even assuming the best moisture remains to the south and west of
the state, weak impulses aloft could still provide some localized
rainfall enhancement at times. 

Rausch