Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
822 AM EDT Mon Oct 01 2018

Valid 00Z Tue Oct 02 2018 - 00Z Tue Oct 09 2018

Near the start of the forecast period expect light to moderate
trades to become more southeasterly as high pressure initially
north of the islands weakens and lifts northeast.  Another high
building farther northeast over the Pacific will become a
prominent feature from midweek onward and flow around this high
should maintain the southeasterly winds into the start of next
week.  Expect fairly light shower activity into midweek with below
normal precipitable water values over most of the area.  From late
week onward some moisture will likely move in from the southeast
and promote somewhat higher rainfall totals, especially over
windward terrain on the Big Island.  At times winds may be light
enough for sea breeze boundaries to provide a focus for showers.

Over the coming day Hurricane Walaka (currently well southwest of
the main islands) will turn northward near 170W longitude and by
Thursday-Friday likely interact with an upper low that closes off
to the north.  The overall system will then eject northeastward
Friday onward under the influence of upstream troughing.  Consult
Central Pacific Hurricane Center products for the latest
information on Walaka.  As Walaka tracks away to the north,
guidance continues to show a north-south band of enhanced
moisture/rainfall that may reach close to Ni'ihau and Kauai in the
late week/weekend time frame.  While solutions have been
displaying some spread and run to run variability, latest
GFS/ECMWF runs are closer together than in previous days and their
average provides a reasonable intermediate starting point for a
deterministic forecast.  As has been the case previously the 00Z
GEFS mean brings the moisture axis farther east.  The ECMWF mean
stays farther west than the operational run.  The 00Z CMC/CMC mean
are stronger than other guidance with the upper ridge
over/northeast of the state as of late week--thus holding moisture
even farther west--so there is still a fair degree of uncertainty
over the exact longitude of the moisture axis.  Regardless of
exactly how far east this axis reaches, there is good agreement
that low level flow will begin to push the enhanced moisture back
to the west by Sunday-Monday.

Rausch