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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 121322

Area Forecast Discussion
National Weather Service Honolulu HI
322 AM HST Tue Dec 12 2017

A cold front will move down the island chain today through
Wednesday. Breezy north winds will bring cooler and drier
conditions across the state behind the front tonight through
Thursday, with unsettled weather possible from Friday through the
upcoming weekend.


The short-term guidance remains in good agreement through midweek
and brings a cold front (currently 100 miles northwest of Kauai)
over Kauai early today, then east of the state by Wednesday night.
A better source of moisture associated with this boundary
combined with upper height falls and isentropic ascent will
translate to decent rainfall coverage across the state as this
boundary moves through.

Hi-res model reflectivities depict the main band of showers along
the front reaching Kauai by noon today, Oahu through the
afternoon hours, Maui County this evening, then the Big Island
tonight into Wednesday. Although periods of heavy rainfall can't
be ruled out through this period, any widespread flooding
potential remains low due to the front quickly advancing down the
island chain. Another shot of cool and dry air will fill in (PWs
down toward half of an inch; dewpoints into the mid/upper 50s)
behind this front tonight through Wednesday. Winds will shift out
of the north-northeast and become breezy Wednesday through
Thursday (nearing wind advisory levels for some of the typically
windier areas by Wednesday night/Thursday).

Thursday through the weekend, forecast uncertainty remains high due
to some notable differences shown between model solutions that have
been consistent from run-to-run over the past few days. In general,
all solutions indicate a shortwave trough will develop into a cutoff
low near or just east of the islands on Thursday, then drift
westward over the state Friday through Saturday before becoming
absorbed as another trough approaches late in the weekend. The
main difference is shown between the GFS and ECMWF, where the GFS
suggests the bulk of the moisture hanging up east of the state
(dry slot with PWs hanging around an inch through Saturday over
the islands). The ECMWF, however, remains the wetter solution and
depicts a deeper source of moisture along with breezy northeast
winds through this time. If this ECMWF scenario evolves, island
weather would become very active, with the potential for heavy
rain and thunderstorms, as well as snowfall and high winds at the
Big Island summits. The current forecast package through this
period reflects a modest increase in rainfall chances with a
slight chance of snow on the Big Island summits as a compromise
between the two possible scenarios. The chance of thunderstorms
may become included in later packages as confidence rises over the
upcoming couple of days.


A few clouds and showers will linger over Oahu and Molokai this
morning. A cold front approaching the Hawaiian Islands from the
northwest will reach Kauai by 18Z and then move down the island
chain to the Big Island by Wednesday afternoon. There will be
widespread MVFR and localized IFR conditions along the frontal
band as it moves through the northern islands, the front will
weaken with areas of MVFR once it reaches Maui and the Big Island.

No AIRMETs currently in effect. AIRMETs for mountain obscuration
are expected later this morning as the cold front moves into
Kauai and Oahu.


The incoming north-northwest swell has been building on schedule
during the night at Buoy 51101. This swell is expected to arrive
in the islands during the day, then peak tonight into Wednesday.
It is expected to be larger than the last swell, and will produce
warning-level surf on north and west facing shores of the smaller
islands. Surf on the north facing shores of the Big Island is
expected to reach advisory levels in some areas, but with a lot of
local variations due to the expected 320-330 degree direction of
this swell. PacIOOS run-up forecasts for the Oahu north shore
indicate possible overwash of some lower roadways near the coast
during the peak of this next event. A High Surf Warning has been
issued for most north and west facing shores of the smaller
islands for today through Thursday. A High Surf Advisory has also
been issued for the north facing shores of the Big Island for
tonight through Thursday.

This new swell will also bring seas above 10 feet across most of
the marine area, beginning late this morning for the far
northwest waters, and spreading across the rest of the area
this afternoon through Wednesday morning. A Small Craft Advisory
(SCA) has been posted for most of the marine area. Seas are
forecast to remain above 10 feet for most waters through Friday.

The next cold front is forecast to move down the island chain
today through Wednesday morning, with northerly winds increasing
behind the front. Winds behind this front will likely reach SCA
levels across much of the area by Wednesday afternoon or Wednesday
evening. These strong winds are forecast to continue on Thursday,
then decrease from east to west across the area Friday into

See the latest Oahu Surf Discussion (SRDHFO) for additional details
on surf and swell.


High Surf Warning from 6 AM this morning to 6 PM HST Thursday
for Niihau-Kauai Windward-Kauai Leeward-Waianae Coast-Oahu North
Shore-Oahu Koolau-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala.

High Surf Advisory from 6 PM this evening to 6 PM HST Thursday
for Big Island North and East.

Small Craft Advisory from 6 PM this evening to 6 AM HST Friday
for Maui County Leeward Waters-Pailolo Channel-Alenuihaha
Channel-Big Island Windward Waters.

Small Craft Advisory from 6 AM this morning to 6 AM HST Friday
for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward
Waters-Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-
Kaiwi Channel-Maui County Windward Waters.




Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office