High Surf in Hawaii, May 3 to 5, 2022

Area Forecast Discussion
National Weather Service Honolulu HI
342 AM HST Tue May 3 2022

.SYNOPSIS...
Breezy trade winds will be rather wet for most of this week, with
some locally heavy downpours, and possibly a thunderstorm, the next
couple of days. Most of the showers will focus over windward
communities, but a few showers will occasionally drift to leeward
areas. Thickening high clouds are expected statewide from Wednesday
night through Friday. Fewer clouds and showers, and lighter winds,
are anticipated by the weekend.

&&

.DISCUSSION...
A ridge of high pressure well N of the islands will support breezy
to locally strong trade winds this week. A front approaching from
the NW by the end of the week will force the ridge southward,
leading to some weakening of the winds, while also veering them to
the ESE. Light to moderate trade winds are then expected to continue
into early next week.

Of greater significance to island weather the next couple of days is
a closed low aloft that is currently just N of Kauai, and its
associated trough aloft, now over the island chain. This low and
trough will keep the island atmosphere destabilized this week as it
moves slowly toward the WSW. It has already led to 8 to 10 inches of
rain over the last 48 hours in windward portions of Maui and the Big
Island, where water levels in streams and rivers are elevated.

The core of the 500 mb low (with temperatures as low as -16C) will
pass directly over Kauai and Oahu today and tonight, acting to
thermodynamically destabilize the atmosphere to the point that a few
thunderstorms are likely to develop, and they are included in the
forecast. Deep moisture reaching the Big Island Summits, where
temperatures are below freezing, has led to a wintry mix of
precipitation, and a Winter Weather Advisory remains in effect
through today. As the cold core of the mid-level low drifts away to
the W late Wednesday and Thursday, mid-level temperatures will warm,
reducing the amount of thermodynamically-induced instability, and
the potential for winter weather over the Big Island Summits.

Although thermodynamic instability is expected to diminish, latest
GFS and ECMWF guidance indicate periods of large-scale dynamic lift
supported by divergence aloft E of the trough axis on Wednesday, and
again on Friday, as the trough aloft becomes negatively tilted.
While confidence in the timing is not high, confidence is fairly
high that the island atmosphere will remain relatively unstable for
most of the week, especially so over Kauai and Oahu today and
tonight. Although high clouds over Maui County and the Big Island
may diminish somewhat tonight and Wednesday, thicker high clouds are
expected to blanket island skies from Wednesday night through Friday.

With trade winds persisting in the low-levels, the threat of flash
flooding is somewhat reduced as most showers will move quickly
westward. The potential for heavy rain to become terrain-anchored
can't be entirely ruled out, especially if the trough aloft works
it's way closer to the surface. While a Flash Flood Watch is not
deemed necessary at this time, it may be required later in the week,
when model guidance depicts a vertical wind profile that may be more
conducive for terrain-anchored cells to develop.

A trend toward increased atmospheric stability is expected over the
weekend as the mid- and upper-level low weakens and lifts NE of the
islands. A mid-level ridge will build over the area from the SW, and
PWAT will trend toward normal, so just a few trade wind showers are
expected. In the meantime, expect a rather wet pattern, especially
for windward areas.

&&

.AVIATION...
With high pressure persisting north of the islands, trade winds will
remain moderate to breezy, and locally strong, through the day.
Also, a nearby upper low and trough will continue to destabilize the
atmosphere. Showers and isolated thunderstorms, with accompanying
MVFR and IFR conditions, are likely until these features depart the
area, with the eastern part of the state being most susceptible to
heavier precipitation.

AIRMET Sierra for mountain obscuration remains in effect for all
windward zones.

AIRMET Tango for lee turbulence below 090 remains in effect for the
entire area.

&&

.MARINE...
High pressure north of the state will continue to produce strong
trade winds across the area over the next several days. The Small
Craft Advisory (SCA) has been extended for all Hawaiian Coastal
Waters through tonight. The combination of strong trades and a late-
season northwest swell will keep SCA conditions in place across many
areas through the middle of the week. In addition, an upper low over
the area has destabilized the local airmass, producing isolated
thunderstorms. This low is expected to linger over the state through
the middle of the week.

A long-period northwest swell will continue to fill in across
exposed north and west waters today, then get a reinforcement late
tonight into Wednesday. With the reinforcement arrival, surf heights
could reach low-end advisory levels, then lower gradually through
the remainder of the week.

The current long-period south swell is expected to hold today. An
even larger south swell will fill in later today, peak Thursday then
slowly lower through rest of the week. A High Surf Advisory is in
effect through Thursday 6 PM HST for all south facing shores as
these south swells maintain elevated surf.

Trade winds over and upwind of the state will continue to produce
above normal, short-period surf along east facing shores through
this next week. Surf heights are expected lower during the second
half of the week as the trade winds weaken some.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Thursday for South facing shores
of all islands.

Winter Weather Advisory until 6 PM HST this evening for Big Island
Summits.

Small Craft Advisory until 6 AM HST Wednesday for all Hawaiian
waters.

&&

DISCUSSION...Birchard
AVIATION...Kinel
MARINE...VA