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Hawaii Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 0758Z Apr 10, 2024)
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Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
358 AM EDT Wed Apr 10 2024

Valid 00Z Thu Apr 11 2024 - 00Z Thu Apr 18 2024

Latest guidance is consistent in showing an upper low closing off
to the northwest of the state during the day Wednesday, followed
by a north-northeast drift by the weekend when it should open up
as energy to the north/northwest reinforces mean troughing with
another embedded low.  There is a fair amount of spread for the
exact details of the southern part of the trough from the weekend
into next week. The initial upper low will support a corresponding
surface low and a front/convergence axis that extends southward
just to the west of the main islands.  This pattern will lift
abundant moisture northward, with even the ensemble means showing
precipitable water values reaching at least three standard
deviations above climatology for a time between Thursday and the
weekend.  Dynamical and machine-learning guidance is fairly
similar in principle with the surface feature helping to focus an
axis of potentially heavy rainfall that may affect Kauai and
vicinity Thursday onward, possibly nudging westward a bit for a
time.  Then gradual progression of the upper trough should begin
to push the front eastward, with enhanced precipitation extending
into the western half or so of the islands by Sunday-Monday. 
After that time, some moisture/rainfall may reach farther eastward
but with lesser totals as precipitable water values decrease. 
More pronounced drying may reach into the western islands.

As for guidance comparisons, the 00Z CMC strays from most other
guidance with a farther east upper trough and surface reflection
from the weekend onward.  Remaining dynamical guidance and
machine-learning models do not agree well for the exact character
of upper troughing to the north and embedded upper low position
from late Sunday onward.  Recent GFS runs are farthest south with
the second upper low versus other dynamical models/means (though
the 00Z GFS is less extreme than the 18Z run) while recent
machine-learning models have varied considerably for details to
the north of the state.  Therefore confidence in specifics for the
front moving into the islands becomes fairly low after the
weekend, favoring an average of non-CMC models and means as the
most palatable starting point for a deterministic forecast at that
time frame.