Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
714 AM EST Sun Dec 10 2017

Valid 00Z Mon Dec 11 2017 - 00Z Mon Dec 18 2017

An active weather pattern will remain in place across the Central
Pacific over the next week. Cold front pushing across Hawai'i
today is expected to wash out as it reaches the southern islands
tonight.  A couple days of relatively quiet weather will follow
before another upper shortwave amplifies north of the state
Mon-Tue. Models and ensembles continue to show good agreement on
the initial amplification of this system. Guidance also agrees on
showing the wave rapidly amplifying and then an upper low cutting
off near or even over Hawai'i by Thu. After cutting off, guidance
generally show the feature retrograding westward, with the ECMWF
continuing to be the most aggressive with this, although the GFS
has moved toward the ECMWF solution. Expect the frontal boundary
to stall south/east of Hawai'i as the upper low cuts off.
Confidence is increasing that the upper low retrogression, will
allow higher Pwats (1.30-1.70") to be advected north across the
state by late in the week. As a result, the ECMWF and GFS both
show the potential for heavy precipitation by late in the week,
especially across the eastern islands where 20-30 kt easterly
low-level flow will take hold. ECENS probabilities for heavier
rainfall amounts late in the week have increased compared to
yesterday, with a 25-35 percent chance of exceeding 1 inch across
portions of the Big Island and Maui during the 48-hour period
ending 12Z Sat, and a 10-20 percent chance of exceeding 2 inches
during that time frame. Given the model trends and improving
consensus, confidence is increasing in the potential for at least
locally heavy rainfall across portions of the Big Island and Maui
by late in the week.

By next weekend, models/ensembles are in agreement that the upper
low will become absorbed into an amplifying full-latitude
shortwave axis extending from just west of Hawai'i to near 50 deg
north. This will keep southeasterly moist low-level flow in place
(especially across the eastern islands), with numerous showers
possible through the weekend

Ryan