Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 AM EST Thu Nov 23 2023

Valid 00Z Fri Nov 24 2023 - 00Z Fri Dec 01 2023

Expect moderate trades and windward/mountain focused showers late
this week to be mostly in the lighter half of the spectrum as an
area of enhanced moisture drifting across the area departs.  A
front approaching from the northwest may help to weaken the trades
and turn winds more southeasterly by Saturday, with sea breezes
having an influence on shower activity.  Regarding this front,
guidance has made a significant adjustment over the past day in
exactly how the supporting trough energy splits during the
weekend.  Consensus now shows a less amplified trough that passes
by to the north, so into Monday the front's moisture band stays
north of the main islands (00Z GFS/ECMWF/CMC) or gets no farther
south than the northern islands (00Z GEFS/12Z ECens means). 
Still, high pressure north of the front should provide a brief
return of moderate trades.  Meanwhile the southern trough energy
that drops into a closed low west of the state should open up and
advance eastward by early next week, starting the process of
drawing more moisture northward.

Then guidance is consistent in principle for rapid
amplification/sharpening of upstream trough energy just west of
the state during Tuesday-Wednesday and persisting into Thursday,
with an embedded upper low likely to form.  This would help to
develop surface troughing/low pressure and more
southerly/southeasterly low level flow over the islands.  The
latest GFS runs are much closer to the average of other guidance
aloft today, though it pulls its surface reflection somewhat
westward of other solutions and this may play a role in its
somewhat lower QPF immediately over the islands compared to most
other guidance.  The 00Z CMC/CMCens form a slight minority cluster
that keeps the core of the trough northwest of the islands versus
the GFS/ECMWF and their means that are close to due west.  With
the ECMWF exhibiting some east-west waffling for the upper
trough/low (00Z run closer to the western islands and east of the
prior run and 12Z ECens/00Z GEFS), a multi-run average of
ECMWF/GFS runs and their means aloft looks most reasonable.  Then
would favor excluding the western GFS for the surface reflection. 
This general evolution should further increase moisture and
rainfall going into and beyond midweek, as indicated by guidance
bringing precipitable water values up to two or more standard
deviations above normal by Thursday.   

Rausch