319
FXHW01 KWNH 110759
PMDHI

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
358 AM EDT Sat May 11 2024

Valid 00Z Sun May 12 2024 - 00Z Sun May 19 2024

...Wet pattern for Hawaii, especially this weekend and mid-late
next week...

The forecast remains on track for a compact upper low to drift
over or near the northwestern and central islands this weekend,
finally departing to the east and opening up early next week.  An
associated surface trough, supporting light background flow and
more sea/land breeze influence, should give way to a brief period
of southeasterly trades and more terrain focus after Sunday.
These features along with ample deep moisture will support areas
of heavy rain and thunderstorms, especially during Saturday.
Departure of the upper low should reduce the heavy
rain/thunderstorm threat somewhat Sunday into early next week, but
lingering anomalous moisture may continue to support locally heavy
rainfall especially over and near the Big Island.

The 00Z GEFS mean is not there yet, but latest GFS runs have
finally adjusted to the prior and ongoing majority cluster of
dynamical and machine learning (ML) models from midweek onward.
This majority solution depicts an unseasonably deep upper low
closing off northwest of the state by Wednesday and then not
moving too much through the rest of the week.  Associated surface
low pressure should form near the upper center with a trailing
front/trough extending southward.  This evolution would support a
northward surge of moisture across the state with the best focus
for heavy rainfall likely being over the western islands.  By
Friday-Saturday some east-west spread develops for the upper low
position, with the 00Z ECMWF/12Z ECens mean drifting a little west
of the 00Z GFS/CMC (with the CMCens mean in the middle).  00Z ML
models generally favor an intermediate position at that time.
These differences are well within typical guidance
errors/predictability for several days out in time, but will play
an important role in determining the exact axis of heavy rainfall.
 The eastern side of the envelope would prolong the duration of
the heavy rainfall threat while the western side would allow for
enough of a westward drift to push the heaviest rainfall slightly
west of the main islands by around the end of the week.  An
intermediate solution would be a reasonable starting point for a
single deterministic forecast at this time.

Rausch

$$