Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EST Thu Feb 16 2023

Valid 00Z Fri Feb 17 2023 - 00Z Fri Feb 24 2023

...Heavy rainfall/flooding threat especially during
Thursday-Saturday within a longer term wet pattern...

Guidance is agreeable and consistent with the short-term pattern
evolution that will favor a period of heavy rainfall and flooding
potential Thursday-Saturday.  Models/ensembles suggest the overall
pattern may support a continued threat for heavy rainfall well
into next week but with enough spread and run-to-run variability
for exact position of the supporting southeast-northwest moisture
band to temper confidence in specifics from Sunday onward.

The forecast through Saturday has held up well over the past day
or so with respect to model clustering and continuity.  Expect the
upper low initially just west of the state as of Thursday to drift
south-southeast thereafter and eventually shear out.  The
deep-layer southeasterly flow between the upper low and ridge off
to the northeast will bring up abundant moisture into the islands,
with models continuing to highlight Friday as the most likely day
when extremely heavy rain may fall over southeastward-facing
terrain of the Big Island.  In the Thursday-Friday period the GFS
continues to show more tendency than other guidance for a surface
wave to brush the state, though now the GEFS mean is hinting at
such a feature as well.  For now continue to have fairly low
confidence in that aspect of the GFS until more guidance trends in
that direction.

From Sunday through next Thursday, models and ensembles maintain a
decent signal for shortwave energy approaching from the west to
form another trough/upper low, which combined with the persistent
upper ridge northeast of the state, would maintain the
southeasterly flow and southeast-northwest oriented axis of
enhanced moisture.  In a continuation of what had been a multi-day
issue in the GFS for the first upper low, latest GFS runs show a
deeper/farther eastward extent of the second feature relative to
most other guidance (now including the GEFS mean, which at times
had leaned somewhat closer to the GFS).  As a result the GFS now
is a more pronounced extreme in keeping the moisture band over the
islands on average through this time frame, as opposed to the band
shifting somewhat westward.  Thus prefer an average of non-GFS
models and the GEFS/ECMWF means.  This would support a gradual
lowering of rainfall totals with time, but with enough moisture
still present to produce a heavy rainfall threat over some
locations (especially the western islands and Big Island) into
midweek.

Rausch