High Surf in Hawaii, March 17 to 20, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
838 PM HST Sat Mar 16 2019

.SYNOPSIS...
A cold front will pass through the islands late tonight and
Sunday, providing a boost in shower coverage, and ushering in
cooler breezy northerlies in its wake. Winds will become
northeasterly with rather dry trades early next week as high
pressure to our north moves eastward. Another approaching front
will bring lighter winds during the middle of next week, with
trades and shower activity expected to increase Thursday into next
weekend as the front sags southward toward or into the western
islands.

&&

.DISCUSSION...
Currently at the surface, a cold front is located around 35 miles
north-northwest of Kauai and is moving southeastward at around 20
mph. Infrared satellite imagery shows increasing cloud cover over
windward sections of Kauai, clear to partly cloudy skies across
the rest of the smaller islands, and partly to mostly cloudy
conditions over the Big Island. Radar imagery and rain gauges
indicate dry conditions statewide, with a couple diminishing
showers seen around South Point on the Big Island. Main short
term focus continues to revolve around the monster surf heading
toward the island chain which is addressed in the Marine section
below.

Model solutions are in excellent agreement with the large scale
synoptic features through the majority of the forecast period,
with some minor differences noted late next week through the
upcoming weekend. The cold front will spread rapidly down the
island chain tonight and Sunday. Light to moderate northwesterly
winds have developed in advance of the cold front this evening,
with breezy northerly winds then expected to fill in behind the
front later tonight through Sunday. High pressure will then build
eastward several hundred miles north of the state through early
next week, with moderate to breezy north-northeasterly winds
Sunday night, transitioning over to moderate trades Monday
through Tuesday. A surface ridge will settle southward and closer
to the islands Tuesday night through Wednesday night as a cold
front begins to approach from the northwest. This will ease the
trade winds across the state with localized land and sea breezes
possible. The front is then forecast to slide slowly southward
Thursday through Saturday as a new strong high builds to the north
of the state, with the front possibly reaching the western
islands late in the work week or over the upcoming weekend. This
should increase the trade wind speeds back up into the moderate to
breezy range late next week.

The front moving southward towards the island chain appears
moisture starved, with a 50 mile band of broken low clouds and
showers associated with it. This band will push southward through
the island chain tonight and Sunday, and although the increase in
moisture will be limited, the orographic lift should be enough to
produce scattered showers along north facing slopes and coasts as
the front moves through, although leeward spillover will be
limited. Behind the front a very dry airmass will overspread the
island chain, with only a few windward showers expected Sunday
night and Monday. The returning trades should be enough to moisten
up the boundary layer to give a small boost to windward shower
activity Monday night through Tuesday night, but drier than
normal conditions will continue. As the winds weaken Wednesday
and Wednesday night, showers will favor the interior and mauka
areas during the day and locations near the coast at night,
although drier than normal conditions are expected to persist. The
returning trades will bring a more typical trade wind weather pattern
back to the islands Thursday and Thursday night, with the
potential for a much wetter trade pattern late Friday into next
weekend as a front sags southward toward or into the western
islands.

&&

.AVIATION...
A shallow fast moving cold front will drift into Kauai around 08Z
and Oahu around 10Z tonight spreading low clouds and a few showers
along north and east slopes of both islands. This front will
likely spread low clouds and some showers to north slopes of
Molokai and Maui between 11-16Z on Sunday morning. Low cloud
ceilings will reach the north to northeast slopes of the Big
Island by 18-20Z.

Brief periods of MVFR ceilings are forecast with this frontal
boundary with possible mountain obscurations along the north and
east facing slopes of each island as it passes through the state.

No AIRMETS are currently in effect. AIRMET Sierra for Tempo
Mountain obscuration possible for Kauai and Oahu later this
evening.

&&

.MARINE...
Fresh to strong north-northwest winds will fill in overnight as a
cold front quickly advances down the island chain from west to
east. Winds will shift out of the north Sunday, north-northeast
Sunday night into Monday, then out of the east Monday night into
Tuesday. A ridge of high pressure will build north of the state
in the wake of the front then shift south near the islands and
weaken by midweek as another front passes to the north. The
proximity of the ridge to the islands will lead to a period with
light and variable winds through the midweek time frame. A return
of fresh to strong trade winds will be possible Thursday into
next weekend as high pressure builds north of the area in the wake
of the passing front.

A significant late season northwest swell arriving late tonight
through Sunday will lead to life-threatening surf along north and
west facing shores through Monday. Satellite data earlier today
showed the leading edge of the fetch associated with this swell
within 500 nm northwest of Kauai and a large area of gale- to
near storm-force northwest winds. An altimeter pass showed peak
seas in the 30 to 35 ft range within this area. Seas at the
offshore buoys should notice a sharp spike through the late
evening hours followed with a peak around 20 ft early Sunday
morning.

Surf will rapidly rise through the day Sunday and peak Sunday
night into Monday. A combination of the surf, fresh to strong
onshore winds in the wake of the previously discussed front moving
through will lead to life-threatening conditions at the exposed
beaches Sunday through Monday. Warning-level surf will hold
through Monday then steadily lower into Tuesday. Another large
reinforcement out of the northwest is expected Wednesday that will
likely drive surf back up along north and west facing shores near
warning levels.

Peak monthly tides expected through this time will compound the
situation through the overnight hours Sunday night (around and after
midnight), likely translating to coastal inundation for the
typical low-lying vulnerable areas. The latest PacIOOS run-up and
Haleiwa harbor surge index forecasts reflect this potential, due
to the coinciding peak surf and tides Sunday night. This harbor
surge index suggests that currents within the harbor may be
strong enough to cause damage to boats and infrastructure due to
jostling of boats and docks. The run-up forecasts indicate the
potential for periodic overtopping of nearby roadways.
Additionally, the long duration of large surf and high tides
expected Sunday through midweek, will likely correspond to
significant beach erosion.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning from midnight tonight to 6 PM HST Monday for
Niihau-Kauai Windward-Kauai Leeward-Waianae Coast-Oahu North
Shore-Oahu Koolau-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala-Big Island North and East.

Wind Advisory until 6 AM HST Sunday for Big Island Summits.

Small Craft Advisory from midnight tonight to 6 PM HST Monday
for all Hawaiian waters-

&&

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