018
FXHW01 KWNH 120800
PMDHI

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
359 AM EDT Sun May 12 2024

Valid 00Z Mon May 13 2024 - 00Z Mon May 20 2024

...Heavy rain threat mid-late week and possibly extending into the
weekend...

In the short term, latest guidance has changed some for the
details of the compact upper low hovering just north of the main
islands--having it weaken in place before shearing out versus
tracking more steadily eastward.  Still, weakening of the low
should offer a brief lull in organized heavy rainfall potential
early in the week.  The surface trough initially over the state
should drift westward, allowing for a period of light to moderate
southeasterly background flow.  This should lead to a mix of
windward and sea breeze focus for showers.

From midweek onward, latest guidance has improved its clustering
for the unseasonably deep/south upper low expected to settle
north-northwest of the state.  Dynamical guidance and
ECMWF-initialized machine learning models agree that closest
proximity of this upper low should be around Wednesday-Thursday
with a position just west of 30N 160W.  This represents a
gravitation toward what was the eastern side of yesterday`s
envelope as represented by the 00Z/11 GFS run.  This clustering
now suggests that the associated surface front will help to focus
an axis of heavy rainfall over the central islands mid-late week
as deep moisture flows northward.  By Friday-Sunday the guidance
shows the upper low wobbling northwest to some degree while
ridging builds southeast of the state--leading to a gradual
westward shift in the heavy rainfall axis.  Differences in
specifics are well within typical spread/error for 5-7 days out in
time with no pronounced clustering, favoring an intermediate
solution where discrepancies arise.  Indicating the abundant
moisture that may be available for heavy rainfall, the 00Z
GEFS/12Z ECens means show precipitable water values reaching up to
3-4 stdevs above normal for a time late this week and/or the
weekend, while the 00Z GFS/ECMWF show some higher deviations for
portions of that period.  Continue to monitor forecasts for
further trends, as minor adjustments in specifics will be
important in resolving effects at any particular location.

Rausch

$$