Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
301 AM EST Sun Dec 19 2021

Valid 00Z Mon Dec 20 2021 - 00Z Mon Dec 27 2021

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

Through the first half of this week the guidance maintains
continuity with the general evolution.  Trades will be on the
brisk side as strong central Pacific high pressure extends its
influence farther south, providing a windward/mountain focus for
rainfall while a front drops into and just south of the main
islands.  An upper low initially northwest of the state will slide
southeastward and weaken as it tracks just south of the state,
likely helping to enhance rainfall around Tuesday-Wednesday
especially over the Big Island.

After midweek the models and ensemble members are as divergent as
ever for the location of the moisture axis along the old front,
due to important differences in low-predictability flow details
south of the strong upper high over the mid-latitude Pacific.  The
past two ECMWF runs have reverted to a more suppressed solution
due to more influence from upper troughing to the northeast.  The
CMC is also suppressed.  As has been the case recently, the GFS is
farther north with a moisture axis over the islands (but notably,
not reaching north of the state as yesterday's 00Z GFS depicted). 
Individual ensemble members vary considerably though the 00Z
GEFS/12Z ECMWF means generally tilt toward their parent models. 
Teleconnections relative to the upper ridge's positive height
anomaly center still suggest that the ECMWF cluster's upper
troughing northeast of the state could be somewhat overdone. 
However models have a notoriously difficult time in determining
flow details around the southeast side of closed upper highs, so
confidence in any particular scenario remains low.  Thus the
forecast is in a holding pattern of advertising some potential for
enhanced rainfall but also the possibility of a drier trend.

By next Sunday an area of moisture could approach the far western
islands with the slow eastward advancement of a negatively tilted
upper trough.  Timing will depend on the pattern farther east,
with the GFS/GEFS details allowing for the greatest eastward
advance of the western height falls/moisture.

Rausch