Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
718 AM EST Thu Dec 07 2017

Valid 00Z Fri Dec 08 2017 - 00Z Fri Dec 15 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, with the
exception of the CMC which is less amplified and quicker to
progress the wave east. The associated surface front should reach
the northern Islands on Saturday and then sweep across the
remaining Islands Saturday night. 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 north-northwesterly low-level
winds along the front. In the wake of the first front, another
round of very dry air should move across the state, with Pwats
decreasing to well below 1 inch Sunday into Monday. A second
shortwave amplifies north of the state on Tuesday, with guidance
once again showing decent confidence in the timing/amplitude. The
associated surface front reaches Hawai'i starting Monday night and
crossing the Islands on Tuesday. Models solutions begin to differ
by late next week, with the ECMWF developing a wave along the
front which causes the front to stall for a time across the
eastern Islands. Higher Pwats associated with this second front
(1.50-1.80 inch) will result in greater precip coverage, so a
potentially stalling front as shown by the ECMWF would result in
the potential for heavy rainfall across the eastern Islands on
Wednesday. The GFS differs from the ECMWF and pushes the front
through the state quickly, with dry air once again invading the
state by next Thursday. ECMWF ensemble probabilities show 10-20
percent probabilities of rainfall totals exceeding 1 inch across
the eastern Islands associated with this system.

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
north-northwesterly and south-southwesterly in ahead of/behind
each front as it passes through the state.

Ryan