Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
347 AM EDT Thu Jul 22 2021

Valid 00Z Fri Jul 23 2021 - 00Z Fri Jul 30 2021

Today's guidance continues to show a wetter period from late this
week through much of next week with multiple areas of enhanced
moisture contributing to the increased rainfall.  Forecasts for
the moisture associated with Felicia Thursday-Friday have been
fairly stable in recent days but there is still uncertainty for
specifics of Guillermo's moisture Sunday into the first part of
next work week.  For the latter, consensus had been leaning toward
the GFS idea of taking the core of highest precipitable water
values to the north.  However the 00Z GEFS mean is providing some
support for ECMWF/ECMWF runs that have been carrying the greatest
moisture farther south into the islands (though with less extreme
maximum PWATs than seen in the 00Z ECMWF).  The UKMET/CMC suggest
some blend of the possible range of solutions.  Accompanying this
moisture surge, guidance still shows an upper trough developing to
the north/northwest of the state while a surface trough crossing
the islands around Sunday-early Tuesday should temporarily weaken
the trades and reduce the windward/mountain focus for rainfall. 
Toward Wednesday-Thursday the guidance varies considerably for
another area of moisture that could reach the area.  The 00Z GFS
shows the most moisture reaching the islands while the 00Z ECMWF
misses to the south.  00Z GEFS/12Z ECMWF means reflect less
extreme ideas of their operational counterparts and an average of
the two would be a reasonable starting point at this time.  Modest
strengthening of eastern Pacific high pressure should lead to a
rebound in trades around or after midweek.

Rausch