High Surf in Hawaii, December 19 to 21, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
407 AM HST Tue Dec 19 2017

.SYNOPSIS...
Deep tropical moisture will remain over the central and eastern
islands today through Wednesday night, resulting in periods of
locally heavy rain and thunderstorms. A cold front will shift
slowly down the island chain tonight through Wednesday night,
with a cooler and drier airmass filling in following the frontal
passage. Cool and rather dry weather is then expected to continue
through Friday. Another weak front is expected to move into Kauai
Friday night, then stall out over the central and eastern islands
over the weekend. Another cold front may approach the state from
the northwest early next week.

&&

.DISCUSSION...
Currently at the surface, a cold front is located around 75 miles
north-northwest of Kauai, with a weak trough of low pressure over
the central portion of the island chain. Aloft, an upper level
trough is located around 300 miles northwest of Kauai. Infrared
satellite imagery shows mostly cloudy to overcast conditions
across the entire state early this morning. Meanwhile, radar
imagery shows some showers and isolated thunderstorms over Maui
County and the Big Island, with quite a bit of additional shower
and thunderstorm activity to the south of the island chain. Main
short term concerns revolve around the potential for heavy
rainfall and thunderstorms over the next couple days.

Today through Wednesday night,
The cold front will approach from the northwest this morning,
move through Kauai this afternoon, then slowly progress
southeastward down the island chain tonight through Wednesday
night. Meanwhile, surface troughing will remain nearly stationary
in the vicinity of Maui County through tonight, before edging
slowly eastward Wednesday and Wednesday night as it merges with
the cold front. The front still appears on track to exit east of
the Big Island by daybreak Thursday.

Deep moisture with precipitable water values between 1.5 and 1.9
inches will linger over Maui County and the Big Island today
through Wednesday before shifting east of the state by daybreak
Thursday. This in combination with strong forcing for ascent
provided by the approaching upper level trough and entrance region
jet dynamics will keep conditions unsettled, with periods of
locally heavy rainfall and isolated thunderstorms expected. The
main flash flood threat is expected to focus over Maui County and
the Big Island, where the Flash Flood Watch remains in effect
through Wednesday night. Oahu will be right along the northwestern
periphery of the deeper moisture, with the main threat for heavy
rainfall and Flash Flooding expected to remain off to the south
and east. Only a small deviation in the axis of this deeper
moisture could lead to a significant increase in rainfall coverage
and intensity over Oahu however, so the Flash Flood Watch remains
in effect through late tonight here. Kauai will be well displaced
from the deepest moisture. Here showers are expected to increase
especially for north facing slopes and coasts this afternoon and
tonight in association with the cold front, but heavy rainfall
and flooding are not anticipated.

Locally breezy northerly winds will fill in behind the cold front
as it progresses down the island chain from northwest to
southeast, along with cooler and drier conditions. Showers will
favor north facing slopes and coasts following frontal passage.

Deep layer moisture and cold temperatures aloft will bring
periods of snow to the Big Island summits through Wednesday
night. As a result, a Winter Storm Warning remains in effect
through late Wednesday night. Strong winds at the summits will
add to the hazardous conditions with blizzard conditions possible
at times beginning later today through Wednesday night.

Thursday through Friday night,
The cold front will continue to shift eastward and away from the
state on Thursday, with high pressure then settling southward and
over the island chain Thursday night and Friday. The next cold
front is then expected to approach from the northwest Friday
evening, and push southward into Kauai after midnight Friday.
Cool and fairly dry conditions along with locally breezy northerly
winds are expected on Thursday, with some lingering shower
activity expected over the Big Island through the morning hours
before the upper level trough axis shifts east of the state. Cool
and fairly dry weather will follow statewide Thursday night and
Friday, but steep low level lapse rates behind the cold front are
expected to keep scattered showers moving into north facing slopes
and coasts, with a few isolated showers possibly spilling over
into leeward areas from time to time. Shower coverage will
increase after midnight Friday primarily for windward sections of
Kauai in association with the next cold front.

Saturday through next Monday,
Model solutions are coming into better agreement with respect to
the handling of the large scale synoptic pattern over the weekend
into early next week. The ECMWF has trended toward the less
progressive GFS solution with respect to the next cold front. As a
result, the cold front is expected to shift southward into the
central islands on Saturday then stall out in the vicinity of Maui
County and the Big Island Saturday night and Sunday, then
dissipate Sunday night. Another cold front is then expected to
approach from the northwest early next week.

Overall, we expect a band of clouds and showers to progress
southward down the island chain in association with the front,
with showers focusing mainly over windward and mauka areas.
Showers are expected to decrease in coverage Sunday night and
Monday as the front dissipates. Light to moderate trade winds are
expected to return over the weekend, before easing with the
approach of the next cold front early next week.

&&

.AVIATION...
A low pressure area north of the state and a cold front moving in
from the northwest will keep light winds and an unstable
environment across the Hawaiian Islands for the next 24 hours.
Tropical moisture and shower bands will continues to move up
across the central islands from the south, with TEMPO MVFR CIG
and SHRA moving across Oahu, Molokai, Lanai and Maui. Isolated
thunderstorm activity will be embedded in the tropical moisture
band. The cold front moving in from the northwest will reach Kauai
with increasing low clouds and showers forecast this afternoon.

AIRMET Sierra issued for Tempo Mountain Obscuration over Molokai.
This AIRMET will likely be expanded later this morning as
additional low clouds and showers move into the islands of Oahu,
Lanai and Maui.

AIRMET Tango remains in effect for Tempo Moderate Turbulence
over the islands between FL280 and FL360 due to jet stream winds.

AIRMET Zulu remains in effect for Tempo Light Rime Icing from
Oahu to the Big Island between 140 and FL220.

&&

.MARINE...
A large shield of light rain with some embedded heavier showers
and a few thunderstorms has lifted northward over the waters
early this morning, mostly from Oahu to the Big Island. The
latest guidance reflects this trend continuing into midweek as a
cold front approaches and moves through. The cold front is
forecast to reach Kauai later today, then continue down the
island chain tonight through Wednesday night. Winds will remain
light through a good portion of the day today ahead of this
boundary, then shift out of the north and strengthen into the
fresh category as it moves down the island chain. Some areas may
near advisory levels due to terrain induced accelerations with
northerly flow, such as the Kaulahahi Channel tonight through
Wednesday. There will be a brief lull in the northerly winds
Thursday night into Friday as another front approaches. Fresh
northeast winds are possible during the weekend as this front
potentially moves down the island chain.

Surf along north facing shores is forecast to remain up through
the week as reinforcing northerly pulses fill in and impact the
area. Observations at the offshore buoys northwest of Kauai ended
up spiking above guidance yesterday evening, then at the Hanalei
and Waimea buoys earlier this morning. Swell direction will
remain out of the north-northwest today, then gradually shift to
the north and north-northeast Wednesday through Friday. The Maui
County windward waters have been added to the small craft advisory
that remains in place through Wednesday. The high surf advisory
currently up for exposed north facing shores will possibly be
expanded in later packages to include the north facing shores of
the Big Island as the swell veers to the north and northeast
Wednesday.

The active pattern will continue into the weekend as a smaller west-
northwest (320-330 deg) fills in Thursday night into Friday,
followed by a large north-northwest (340-350 deg) late Saturday
through Sunday. The latter could result in another round of high
surf and small craft advisories for exposed waters and shores
through the weekend. See the recently updated Collaborative
Nearshore Swell and Wind Forecast for Oahu for more details.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through late tonight for Oahu.

High Surf Advisory until 6 PM HST Wednesday for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Olomana-
Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

Flash Flood Watch through late Wednesday night for Molokai-Lanai-
Kahoolawe-Maui-Kona-South Big Island-Big Island North and East-
Kohala-Big Island Interior.

Winter Storm Warning until 6 AM HST Thursday for Big Island
Summits.

Small Craft Advisory until 6 PM HST Wednesday for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Kaiwi Channel-Maui County
Windward Waters.

&&

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Gibbs