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Hawaii Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0759Z May 13, 2024)
 
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Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
359 AM EDT Mon May 13 2024

Valid 00Z Tue May 14 2024 - 00Z Tue May 21 2024

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

Continue to expect a relative lull in organized heavy rainfall as
the compact upper low just north of the state steadily fills in
place early this week.  There will still be the potential for
localized enhanced rainfall within a regime featuring light to
moderate southeasterly background flow with some sea breeze
influence on showers.

From midweek onward the latest dynamical and machine learning
model guidance has been fairly consistent and agreeable over the
past day in depicting a multi-day wet and stormy pattern due to an
unseasonably deep/south upper low expected to settle
north-northwest of the state.  Consensus maintains the idea that
closest proximity of this upper low should be around
Wednesday-Thursday with a position just west of 30N 160W, followed
by only gradual weakening and north/northwest drift (00Z CMC being
the southeastern exception by next Monday).  Most of the current
guidance shares the general scenario of an associated cold
front/surface trough reaching into the central islands and
focusing heavy rainfall potential over that part of the state
mid-late week as leading southerly flow helps to increase
precipitable water (PWAT) values.  Then during the weekend the
surface boundary and heavy rainfall focus will likely drift back
into the western islands, as the supporting upper pattern shifts
northwestward a bit.  As this occurs, the GFS/ECMWF and their
ensemble means suggest that a further increase in PWATs may occur
with the ensemble means reaching up to 3-4.5 stdevs above
climatology and the operational GFS/ECMWF runs reaching even
higher.  Thus the heaviest rainfall during the event may be over
the western islands during the weekend.  Most guidance shows the
rainfall axis retreating to just west of the western islands by
next Monday.  Continue to monitor forecasts for any potential
adjustments in specific details.

Rausch