Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
348 AM EDT Tue Jul 20 2021

Valid 00Z Wed Jul 21 2021 - 00Z Wed Jul 28 2021

Strong high pressure to the north/northwest of the state will
support brisk to strong trades over the next few days.  Expect
windward/mountain-focused showers to be on the light side through
midweek with an upper ridge passing overhead.  Then the pattern
should trend wetter with the contribution of moisture from the
remnants of Felicia and Guillermo.  Guidance has maintained good
agreement and continuity in showing the highest precipitable water
values associated with Felicia's remnants passing to the south of
the state Thursday-Friday but with some of the moisture extending
at least up to the Big Island.  Then there is a decent consensus
for moisture from Guillermo to pass through the region around
Sunday-Monday.  The 12Z ECMWF mean seems to be lagging the trend
somewhat, but the overall ECMWF/ECMWF mean trends over the past
couple days have generally been toward the majority GFS/GEFS idea
of taking the core of greatest deep moisture to the north of the
islands.  However an axis of enhanced moisture should still extend
southward through the state.  Also the models seem to be improving
their definition of a potential surface trough that could
accompany the enhanced moisture.  Such a trough would support
lighter trades and reduce the windward focus for rainfall.  By
early next week guidance is suggesting that energy aloft--from
some combination of Guillermo's remnants and/or what could drop
down from the westerlies--may develop a trough/upper low just
northwest of the state.  Details of this evolution are very
uncertain for now but merit watching for how such a feature may
keep moisture over the islands.

Rausch