Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
726 AM EST Fri Dec 08 2017

Valid 00Z Sat Dec 09 2017 - 00Z Sat Dec 16 2017

A persistent area of negative 500 hPa height anomalies (-3 to -4
standard deviations) north of Hawai'i will support the passage of
a couple cold fronts through the Islands over the next week. First
shortwave amplifies north of Hawai'i on Saturday. Models/ensembles
show generally good agreement surrounding this system. The
associated surface front should reach the northern Islands on
Saturday and the GFS and ECMWF now both show the front stalling
and dissipating across Hawaii Saturday night into Sunday. A
relatively dry air mass will be in place across the state ahead of
the front (Pwats near or < 1 inch), with modest moisture (Pwats
near 1.25 inch) pooling along the front. Thus, expect shower
activity to be generally limited to northward facing terrain given
the northerly low-level winds along the front. A second shortwave
amplifies north of the state on Tuesday, with guidance once again
showing relatively minor differences with respect to the
timing/amplitude. The associated surface front reaches Hawai'i
early Tuesday morning and crosses the state on Tuesday.
Significant uncertainty remains on whether the front will clear
the state or potentially stall as a wave develops near the state.
Ensemble support is growing for a 500-hPa low to close off just
north of Hawai'i by late in the week, which would support some
sort of stalling/wave development along the front, perhaps just
south/east of Hawai'i. The ECMWF has backed off the idea of
stalling the front directly over Hawai'i Wed-Thu and now resembles
the GFS on pushing the front through the Islands on Thursday. The
ECMWF differs from the GFS after that, however, developing a wave
along the front east of the Islands that retrogrades back across
the state on Friday, while the GFS continues to push the front
quickly through Hawai'i allowing much drier air to move in.
Ensemble spread surrounding this wave evolution is quite large,
although many ECENS members do show a frontal wave developing in
the vicinity. Nonetheless, higher Pwats associated with this
second front (1.50-1.80 inch) will result in greater precip
coverage and the potential for locally heavy rainfall. ECMWF
ensemble probabilities continue to show 10-20 percent
probabilities of rainfall totals exceeding 1-2 inches across the
eastern Islands associated with this system. At this time,
significant uncertainty remains surrounding the exact evolution of
this frontal boundary as it reaches/crosses Hawai'i next week, and
the potential for some heavy rainfall across the eastern Islands
remains.

Low-level easterlies will be suppressed south of Hawai'i through
the next week, the result of the negative height anomalies across
the central Pacific and the frequent frontal incursions into the
region. Surface flow across Hawai'i will switch between
northerly/southerly components ahead of/behind each front as it
passes through the state.

Ryan