Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
351 AM EDT Sat Mar 18 2023

Valid 00Z Sun Mar 19 2023 - 00Z Sun Mar 26 2023

Most guidance continues to show a wavy front and associated heavy
rainfall staying to the north/northwest of the state over the
coming days.  The model/ensemble mean consensus still shows an
upper low closing off to the northwest by Monday, leading to
surface low pressure development along with a trough to the south
of the low.  Some spread exists in latest guidance for the
eastward extent of the overall trough containing the upper low as
it drifts northeastward, maintaining some uncertainty for weather
specifics over the main islands, but the differences are not as
pronounced as they were 24 hours ago.

Through the weekend expect mostly light southerly flow overall
with showers on the light side and influenced by a combination of
terrain and sea breezes.  The northwestern islands will be closer
to the greater moisture so slightly higher rainfall totals could
be possible there.  As for the evolving upper low/trough and
associated surface reflection next week, the 00Z ECMWF is still on
the western side of the envelope but the model has trended
noticeably farther eastward versus yesterday.  On the other hand,
the 00Z GFS has strayed a bit to the eastern side of the spread. 
By mid-late week this represents a difference of the primary band
of moisture hardly reaching the western islands (ECMWF) versus
extending into the eastern islands (GFS).  At this time a
compromise looks good, with the 00Z GEFS mean and CMC being the
closest pieces of guidance representing that idea.  Based on this
scenario, expect the axis of highest precipitable water values to
reach near the central islands by mid-late week with an increase
of rainfall over the state during this time frame.  Low level
winds should transition to southeasterly but with some possible
disruption near the western islands as the surface trough
approaches/arrives.  The majority of guidance says the upper
trough should weaken sufficiently to allow for a return of more
typical trade flow next weekend.

Rausch