Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
412 AM EDT Thu May 12 2022

Valid 00Z Fri May 13 2022 - 00Z Fri May 20 2022

Today's guidance continues to agree upon a significant pattern
change over the course of the period, leading to a wetter regime
by the middle of next week.  Through the weekend expect trades to
weaken somewhat from initially brisk levels and take on a little
more of a southeasterly component as eastern Pacific high pressure
weakens and a wavy front sharpens to the west of the state.  The
combination of upper ridging and somewhat below normal
precipitable water values should keep most of the windward-focused
showers on the light side.  From late weekend into early next week
the trades may be light enough to allow for some sea/land breeze
influence on shower activity.  Models and means all show a central
Pacific upper trough with embedded low (part of a Bering Sea into
Pacific Rex block pattern) approaching from the northwest, likely
reaching its greatest amplitude around early Wednesday before it
starts to lift out.  This system will likely result in stronger
and more southerly winds--especially over the western islands--by
next Wednesday/Thursday.  Latest GFS/ECMWF runs and their ensemble
means all show a significant increase of moisture by midweek. 
However around Wednesday-early Thursday the GFS still extends the
upper trough a little farther southeast versus other guidance so
it is quickest to increase moisture and extends the axis of
heaviest rainfall farther east, closer to the western main
islands.  The 00Z GEFS mean is more similar to the ECMWF with the
upper trough and the current GFS behavior can be an occasional
bias of the model, though differences remain well within typical
predictability/error ranges for forecasts several days out in
time.  A compromise halfway or slightly more in the ECMWF
direction still looks like the most reasonable approach at this
time.

Rausch