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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 110647

Area Forecast Discussion
National Weather Service Honolulu HI
847 PM HST Sun Dec 10 2017

A cold front located around 600 miles northwest of Kauai is forecast
to approach the state through the day Monday, then move down the
island chain Tuesday morning through Wednesday. Winds will remain
light into Monday, then shift out of the southwest and strengthen
Monday night ahead of the front. Breezy north winds will fill in
behind the front Tuesday through Thursday, with unsettled weather
possible from Friday into the following weekend.


Satellite imagery and observations showed a band of clouds
associated with a dissipated front that had stalled over the state
today beginning to retreat northward. Showers within this band
remain light with the best coverage located over the windward waters
between Molokai and Oahu. Overnight, this cloud band will continue
to lift northward as a stronger shortwave trough and its attendant
cold front approach the region. Ahead of this front, winds will
remain light with the background flow shifting from the east
tonight to the southwest late Monday, likely giving way to land
and sea breezes.

The latest hi-res model guidance remains in good agreement Monday
night through midweek and brings this next cold front over Kauai
early Tuesday, then east of the state by Wednesday night. A better
source of moisture associated with this next boundary combined with
upper height falls and isentropic ascent will likely translate to
decent rainfall coverage across the state. Model reflectivities
depict pre-frontal showers developing over the northwest islands
late Monday through Monday night, followed by the main band of
showers along the front that should reach Kauai by noon Tuesday,
Oahu through the afternoon hours, then the rest of the islands
Tuesday night into Wednesday. Another shot of cool and dry air
will fill in (PWs down toward half of an inch; dewpoints back into
the mid/upper 50s) behind this front Tuesday through Wednesday.
Winds will shift out of the north-northeast and become breezy
Wednesday through Thursday.

After Thursday however, forecast uncertainty increases dramatically
as forecast models have been exhibiting a fair amount of run-to-run
variability, due to the shortwave diving SE toward the islands by
mid-week. The 12Z GFS/ECMWF/CMC indciate that the shortwave will
develop into a cutoff low just E of the islands on Thursday, and
then drift W over the islands Friday and Saturday before gradually
dissipating Sunday and Monday. If this scenario develops, 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. However, if the NAVGEM is correct (and now the 18Z
GFS) the cutoff would remain E of the islands, and the impacts on
island weather would be significantly less. With confidence in the
large scale weather pattern so diminished, the details of the
forecast are highly uncertain, and no significant changes to the
longer range forecast are expected at this time.


Cloud and shower bands are breaking off from an old frontal
boundary over Maui county and moving West-Northwest tonight ahead
of another cold front approaches the Hawaiian Islands from the
north. These showers will continue to bring periods of MVFR
conditions to Molokai and Oahu tonight through Monday morning.
Low cloud and shower bands will bring periods of MVFR conditions
to Kauai by Monday afternoon. The next cold front will move in
from the Northwest reaching Kauai by Tuesday morning.

VFR will prevail at the terminals on Kauai, southwestern Oahu,
Lanai, Maui and the Big Island tonight and tomorrow morning.
Expect lingering MVFR conditions over Molokai and Northeastern
Oahu this evening.

AIRMET Sierra for Mountain Obscuration remains in effect for
Molokai and Oahu.


The current large north-northwest swell peaked across the coastal
waters during the afternoon and early evening hours, and is now
beginning to decline. Warning level surf will continue overnight on
the north and west facing shores of the smaller islands exposed
to the surf, with advisory level surf continuing on the north
facing shores of the Big Island. This swell will continue to
decline through Monday night. Expect the high surf warning to be
downgraded to an advisory by Monday morning. Surf along the north
facing shores of the Big Island should drop below advisory levels
by Monday morning.

This swell continues to produce seas at or above 10 feet across
exposed waters, and a Small Craft Advisory (SCA) remains posted
for most areas. Seas are expected to lower overnight from west to
east. Some windward/eastern waters may need to have the SCA
extended into Monday morning, depending on how quickly the seas
drop overnight. Will make that decision with the early morning
forecast issuance.

The next north-northwest swell is expected to arrive Tuesday, then
peak Tuesday night into Wednesday. This swell is forecast to be
even larger than the current swell, and is expected to produce
warning-level surf on north and west facing shores of the smaller
islands. Surf on the north facing shores of the Big Island will
probably reach advisory levels in some areas, but with a lot of
local variations due to the expected 330 degree direction of this
swell. This swell will also bring seas back above 10 feet across
most of the marine area, beginning as early as midday Tuesday for
the far northwest waters, and spreading across the rest of the
area Tuesday night into Wednesday morning.

The next cold front is forecast to move down the island chain
Tuesday 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 night.

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


High Surf Warning until 6 AM HST Monday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward

High Surf Advisory until 6 AM HST Monday for Big Island North
and East.

Small Craft Advisory until 6 AM HST Monday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Alenuihaha
Channel-Big Island Windward Waters.




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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