High Surf in Hawaii, March 9 to 10, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
344 AM HST Sat Mar 9 2019

.SYNOPSIS...
Breezy to locally windy trades will prevail through the weekend
as strong high pressure settles northeast of the state. Low clouds
moving along in the trade wind flow will focus rainfall over
windward areas, but some showers will spread leeward at times. An
increase in showers is expected tonight through Monday morning as
a band of moisture moves through. Trade winds are expected to
gradually ease Monday through late next week, while continuing to
deliver passing clouds and showers.

&&

.DISCUSSION...
Currently at the surface, a 1032 mb high centered around 1000
miles northeast of Honolulu, is driving breezy trade winds across
the island chain early this morning. Infrared satellite imagery
shows partly to mostly cloudy conditions over windward areas,
with partly cloudy skies in most leeward locales. Additionally,
some jet stream cirrus is beginning to spread northward over the
Big Island, but appears fairly thin at the moment. Radar imagery
shows scattered showers moving into windward areas, with some of
the showers reaching leeward areas at times as well. The shower
coverage is currently the greatest over Molokai and over the
coastal waters east of Oahu. Main short term focus revolves
around the strong trades and rainfall trends.

Model solutions are in excellent agreement with the large scale
synoptic pattern through the forecast period. High pressure
north-northeast of the state will slowly shift eastward today,
before becoming nearly stationary around 1000 miles northeast of
the island chain tonight through early Monday. The area of high
pressure will begin to track eastward again later Monday, then
accelerate towards the west coast of the continental United States
Monday night through Wednesday as a cold front passes by to the
north of the islands. The front will stall out and dissipate
around 300 to 400 miles north of Kauai Wednesday night, with a
surface ridge axis settling southward to a position around 100 to
150 miles north of Kauai by Friday.

Breezy to locally windy conditions are expected to prevail through
through Monday, although the trade wind speeds should edge
slightly lower on Monday. Winds have dropped below advisory
levels this morning and should remain below this threshold through
the day, therefore the Wind Advisory has been cancelled. There
are a couple periods where we could see a slight increase in the
trade wind speeds however, tonight and again Sunday night, and
during those times some of the typically windier zones across Maui
County and the Big Island that accelerate the winds most
effectively could potentially reach low end Wind Advisory levels.
Will let the day shift take a look at the latest model guidance
and make a decision on the need for a Wind Advisory headline with
the afternoon forecast package. The trades will trend downward
Monday and Monday night, then hold in the moderate range Tuesday
through Thursday, before becoming light by Friday.

Fairly stable conditions remain in place across the island chain
early this morning, with a strong inversion noted in the morning
soundings between 5.5 and 7 kft. Precipitable water values remain
well below normal for this time of year in the 0.75 and 1.0
inch range. A disorganized band of low clouds and showers is
developing to the north of Maui County and east of Oahu early this
morning, and this should lead to an increase in shower coverage
across Oahu and Kauai this morning, with the band holding over
Kauai through the afternoon hours as Oahu attempts to dry out.
Across Maui County and the Big Island, trade wind shower coverage
should not be as great, and we should see a typical diurnal
downward trend here in the afternoon as well. A more pronounced
band of moisture is expected to push into the islands tonight and
linger through Monday morning. This should bring periods of
fairly wet weather to windward areas, with more regular leeward
spillover. The showers will likely be most prevalent at night into
the morning hours.

High clouds are also expected to spread northward into the central
islands, and perhaps as far west as Oahu, today through Sunday as
a strong jet stream lifts northward over the state. The thickest
high cloud cover is expected over Maui County and the Big Island
this afternoon through early Sunday.

Fairly typical trade wind weather is expected statewide from late
Monday morning through the remainder of the work week, with some
decrease in shower coverage likely toward the end of the week as
upper level heights build and the surface ridge axis settles
southward and closer to the island chain. Showers will continue
to favor windward and mauka areas with a few showers reaching
leeward communities from time to time.

&&

.AVIATION...
Strong high pressure north of the islands will persist through
today and beyond. Thus, trade winds will again be breezy to
occasionally strong. Passing low clouds and showers, with
accompanying MVFR conditions, will be concentrated over mauka and
windward coastal areas on the individual isles. And as has been
the case, some of these elements will move over leeward sections
of the smaller isles and West Maui from time to time due to the
strength of the trade flow.

An AIRMET remains in effect for TEMPO moderate low level
turbulence to the lee of island terrain because of the robust
trade winds. There is also a possibility that an AIRMET for
mountain obscuration may be needed later this morning for much of
the state.

&&

.MARINE...
A slow-moving surface high with a central pressure near 1033 mb
will sit about 850 nm NE of the islands through the weekend,
supporting strong to near gale force trade winds. The trades will
drive an increase in short-period wind waves that will combine
with an increasing long-period NW swell to bring elevated seas,
and a Small Craft Advisory (SCA) is posted through Monday for all
waters. Winds and seas will gradually trend down early next week
as the high moves away, and a front washes out N of the islands. A
new, but weak, high building NW and N of the islands on Tuesday
and Wednesday will support light to moderate trade winds. Winds
and seas are expected to be below SCA levels in all zones by that
time.

A new long-period NW swell has been building at buoys 51101/51001
NW of Kauai overnight, and observed wave heights are a little
larger than forecast. A High Surf Advisory remains in effect for
exposed north and west facing shores until Noon Sunday, as the
swell is expected to peak tonight, then gradually diminish Sunday
into Monday. A slightly smaller NW swell arriving Monday through
Wednesday will produce surf below advisory levels. Yet another NW
swell arriving Thursday may peak near advisory levels before
diminishing on Friday.

Rough and choppy surf along E facing shores will gradually
build to advisory levels Sunday and Monday as the upstream fetch
of trade winds increases. Surf along E facing shores is expected
to diminish Tuesday and Wednesday as winds diminish. A small long-
period south swell will build slightly into Sunday, peak below
advisory levels, then diminish Monday and Tuesday.

The recently updated Oahu Surf Discussion (SRDHFO) contains details
on the source of the swells arriving in the islands over the next
several days.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until noon HST Sunday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Windward Haleakala.

Small Craft Advisory until 6 PM HST Monday for all Hawaiian
waters-

&&

DISCUSSION...Jelsema
AVIATION...Kinel
MARINE...Birchard