Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
349 AM EDT Mon Mar 20 2023

Valid 00Z Tue Mar 21 2023 - 00Z Tue Mar 28 2023

Guidance is consistent with the overall forecast scenario through
the period, but with persistent spread for some of the details
over the next several days.  An upper low is about to close off
northwest of the state and will support northward-moving surface
low pressure over the coming days.  This evolution will transition
light southerly winds to moderate southeasterly flow, while the
axis of moisture along and east of the trough anchored by the
surface low should reach at least into the western islands around
mid-late week.  The guidance distribution for the east-west spread
of this moisture/surface trough axis has been persistent lately,
with the 00Z ECMWF/UKMET and 12Z ECMWF mean on the western side
versus the farthest east GFS.  At least the 00Z GFS has backed
away a bit from the even farther east 18Z run.  The GEFS mean is
again between the GFS and farther west solutions, with the 00Z CMC
serving as a compromise.  Within this spread, the past day of
ECMWF runs has been nudging eastward gradually so there is now
better clustering that supports some increase in rainfall over the
western islands.  An average among the 00Z GEFS mean, CMC, and
ECMWF would provide reasonable continuity from recent days, with a
period of higher rainfall totals over the western islands around
Wednesday-Friday and lesser increases farther east.

Today's models/means are a lot closer together than 24 hours ago
for another upper low that drops into place farther west from late
week onward.  As this occurs, the leading upper low will continue
northward and weaken, allowing for more typical trades across the
state by Saturday-Monday.  The second upper low should be far
enough west to support an upper ridge building over the area
during the weekend, albeit with some uncertainty as to its
persistence through Monday.  This ridge and somewhat drier air low
level air moving in with the trades should lead to a lighter trend
for the generally windward focused showers.

Rausch