Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
245 AM EST Sat Dec 18 2021

Valid 00Z Sun Dec 19 2021 - 00Z Sun Dec 26 2021

...Potential for a wetter pattern next week...

Models and ensembles continue to show a stronger trend for trades
during the weekend into next week as strengthening central Pacific
high pressure extends its influence southward, leading to greater
windward/mountain focus for rainfall.  The combination of a front
dropping southward on the southern periphery of this high, along
with an upper low initially northwest of the state and then
opening up as it tracks just south of the islands next week,
should increase rainfall somewhat. 

Forecast details from about midweek onward remain uncertain as the
state remains under a general weakness aloft underneath a very
strong central Pacific upper ridge while a band of enhanced
moisture along the old frontal boundary may lift northward.  Since
yesterday the ECMWF/ECMWF mean have adjusted northward from their
previously suppressed solutions while the 00Z GFS/GEFS mean have
also adjusted northward from 24 hours ago.  The CMC still keeps
the moisture south of the islands but is now more in the minority.
 Even with the southern scenario decreasing in probability, there
will be a fine line between the band of moisture lingering over
the islands for a period of time (18Z GFS and past two ECMWF runs)
and lifting a little farther northward, leading to a more brief
period of enhanced rainfall potential (00Z GFS).  Important
details aloft within this part of the pattern have low
predictability several days out in time so confidence is low in a
specific scenario for now.  At this point the main theme versus
yesterday is a little more potential for a period of enhanced
rainfall mid-late week but low confidence for intensity or
duration.      

Rausch