Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
821 AM EDT Wed Apr 10 2019

Valid 00Z Thu Apr 11 2019 - 00Z Thu Apr 18 2019

...Heavy rain possible latter half of the week into the weekend...

Today's guidance agrees well for the large scale evolution through
the period.  Strong trades will prevail from now through the
weekend as eastern Pacific high pressure retrogrades and merges
with a fast moving western-central Pacific high.  Trades will
likely slacken Monday-Wednesday as the high returns eastward and a
cold front approaches from the west.  Meanwhile an upper low that
has closed off the east of the state will drift westward close to
the Big Island late this week and should open up into an elongated
trough during the weekend.  The trough axis may remain near the
Big Island into early next week before shifting eastward in
response to upstream flow.

Model/ensemble consensus is best between now and Friday night. 
This is when there is above average confidence in the threat for
heavy windward-focused rainfall that may occasionally stray to
leeward areas.  The Big Island and vicinity will see the best
combination of low heights aloft and high precipitable water
values, with a few thunderstorms and localized flooding possible. 
Guidance differences remain more pronounced during the weekend as
GFS/GEFS mean runs continue to bring a strong push of drier air
into the region from the northeast while the ECMWF/ECMWF mean keep
more moisture in place, extending the period of significant
rainfall potential.  The ensemble means reflect their operational
counterparts but in slightly less extreme fashion.  Note that for
the elongated upper trough the 00Z UKMET/CMC show heights at least
as high as the GFS, leaving the 00Z ECMWF and some of its members
on the stronger side of the spectrum.  However differences appear
somewhat more subtle than yesterday so an intermediate solution
appears best for the purposes of a single deterministic forecast.

During the early/middle part of next week, guidance agrees upon
lighter rainfall as the upper trough finally moves eastward though
a pocket of slightly higher moisture may reach the islands from
the east around Tuesday-Wednesday.  By next Wednesday the most
prominent model difference involves the 00Z CMC becoming more
amplified than other guidance with the upper trough passing by to
the north, leading to greater southeastward progress of the
leading front.  A non-CMC consensus looks reasonable for this part
of the forecast.  Also in that time frame the lighter trend for
trades may promote some sea/land breeze focus for showers.

Rausch