Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
728 AM EST Sat Dec 09 2017

Valid 00Z Sun Dec 10 2017 - 00Z Sun Dec 17 2017

An active weather pattern will remain in place across the Central
Pacific over the next week, with a couple shortwaves amplifying
north of Hawai'i, and cold fronts affecting the state during the
period. The first shortwave will skirt north of the state today
and tonight, pushing a weak cold front across the Islands. The air
mass ahead of the front continues to be relatively dry, and models
show only modest moisture pooling along the boundary (Pwats near
1.20"). As a result, isolated to scattered shower activity will
accompany the frontal passage, primarily focused on north facing
terrain given the northerly winds associated with the frontal
passage. A couple days of relatively quiet weather will follow the
first front before a second 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
has also come into better agreement in showing the wave rapidly
amplifying and then an upper low cutting off near or even over
Hawai'i by Thu. After cutting off, there is some disagreement
among the guidance on if and how quickly the feature would
retrograde westward, with the 00Z ECMWF the most aggressive in
this, while the GFS/CMC are a bit farther east.
ECENS/GEFS/NAEFS/CMC ensemble means are all in excellent agreement
in showing a position fairly close to what is shown by the 06Z
GFS, thus will prefer the GFS handling of this feature at this
time. The degree to which the upper low retrogrades will have
major implications for the amount of precipitation that occurs
across Hawai'i. Expect the frontal boundary to stall south/east of
Hawai'i as the upper low cuts off, with some potential for drier
air to overspread the state. If the ECMWF solution were to verify
and the upper low to retrograde more quickly, this would allow
higher Pwats to be advected north across the state more quickly.
This is shown in the model QPF, with the ECMWF solution showing
more potential for convection across Hawai'i late next week
relative to the GFS. ECENS probabilities continue to show only a
10-15 percent chance for QPF exceeding 1 inch over a 48-hour
period ending at 00Z Sat, suggesting that the deterministic ECMWF
solution is not necessarily the most likely one at this time
(although it still cannot be ruled out). Regardless, expect
another period of shower activity Tue night/Wed as the front
crosses the state, with a more uncertain forecast after that time,
largely depending on the exact evolution and track of the upper
low.

Ryan