High Surf in Hawaii, December 21 to 31, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
359 AM HST Tue Dec 21 2021

.SYNOPSIS...
A strong surface high pressure system far north of the islands
combined with moist and unstable atmospheric conditions will
produce a windy and wet trade wind pattern during the next couple
of days. Most of the low clouds and showers will favor windward
and mauka areas, but the strong trades will also carry some
showers over to leeward sections of the smaller islands. Clouds
and showers, some with locally heavy rainfall, are also expected
to develop over leeward sections of the Big Island each
afternoon. The latest forecast indicates slightly weaker winds
and a drying trend may develop during the Christmas Holiday
Weekend.

&&

.DISCUSSION...
Winter officially begins in Hawaii ,and the remainder of the
Northern Hemisphere, at 559 AM HST this morning. As we transition
into the new season, we are seeing a blustery and wet trend in
our weather pattern during the next couple of days.

The tight pressure gradient south of a 1039 mb surface high
pressure system located far north of the state is producing
locally windy trades across the region. Loops of water vapor
satellite imagery show the axis of a broad upper-level trough
about 550 miles northeast of Hilo. There is also a small upper-
level low pressure system centered about 450 miles southwest of
Lihue. The close proximity of these features aloft to the state
continues to produce unstable atmospheric conditions across the
island chain. A band of low clouds and showers associated with a
dissipated frontal boundary will continue to bring low clouds and
showers across most windward and mauka areas as it moves slowly
south today. The gusty low-level trade wind flow will also carry
some showers over to leeward sections of the smaller islands
today, especially this morning. Note that we currently do not
anticipate the need for a Wind Advisory later today. However,
some areas will be just below criteria, so we may need to issue
one if the wind observations show stronger sustained speeds and/or
gusts.

The forecast guidance shows the strong trades will persist into
mid-week. The guidance also shows the band of moisture will
continue pushing south across the Big Island into this evening.
This combined with the lingering instability will likely result in
a slight chance of thunderstorms and locally heavy downpours on
the Big Island again this afternoon.

The forecast models indicate the moisture from the remnant
frontal boundary will eventually stall near, or just south of the
Big Island later tonight and Wednesday. This enhanced moisture
combined with continued atmospheric instability may result in a
prolonged period of wet weather for some areas on the Big Island.
The instability will also keep the threat of thunderstorms in the
vicinity of the Big Island around mid-week, especially during the
afternoon hours. The greatest uncertainty at the moment is where
the bulk of the moisture field will end up in relation to the Big
Island? If we see a potential threat for widespread heavy
rainfall, a Flood Watch may be required for the Big Island
starting Wednesday. Elsewhere, there may be a drying trend over
some of the smaller islands around mid-week. There may also be
some slightly cooler nighttime temperatures in some sheltered
locations.

Looking ahead to the long Christmas Holiday Weekend, the latest
forecast guidance continues to show a significant low pressure
system developing west of the state. There is still some
uncertainty in the exact movement of this feature after it
develops, but its presence may cause some weakening of the trade
winds, especially after Friday. We are also seeing trends in the
forecast models now that suggest that the background flow may
become east-southeast. The latest model output seems to be
trending toward keeping the low far enough west so that
significant rainfall may not develop. As a result, we have
introduced a frying trend in the weather over most of the state
for Christmas Day.

&&

.AVIATION...
Breezy trades will persist through the forecast period and likely
generate tempo moderate turbulence over the lee of the mountains.
See AIRMET TANGO below. The aforementioned flow pattern will
concentrate scattered showers along windward portions of the
islands and bring brief periods of MVFR ceilings and visibility.
Isolated showers leeward areas.

Relatively deep moisture and instability aloft will drift towards
the Big Island this afternoon through tomorrow and bring a slight
chance of thunderstorms to areas southeast of Maui. Abundant mid-
upper level clouds will persist elsewhere over the island chain.

AIRMET SIERRA remains in effect for northeast thru east facing
mountain slopes above 2500 feet for Kauai, Oahu, Maui, Molokai,
and the Big Island due to clouds and showers.

AIRMET TANGO remains in effect for all islands over and
immediately south through west of mountains for tempo moderate
turbulence below 9000 feet.

&&

.MARINE...
 nearly stationary, very strong area of high pressure centered
far N of the islands will support fresh to near-gale force trade
winds this week. A bit of a surge in wind speeds is expected
through today as the high pushes the remnants of a front over the
islands from the NE. An ongoing Small Craft Advisory includes all
waters through Thursday due to a combination of winds and seas.
Moisture associated with the frontal remnants may stall near the
Big Island today through Thursday, potentially triggering heavy
showers and/or thunderstorms as the island atmosphere will remain
unstable. A slight chance of thunderstorms is also possible in
other coastal waters south of the isles, especially near Kauai.

While the fetch of trade winds to the E of the islands is
somewhat limited, a steady supply of short- and medium-period
wind waves/swells will be arriving from the N through NE for the
next several days, primarily due to the gradient between the high
and a slow-moving, gale-force low off the U.S. W coast. This will
result in increased seas, with rough and choppy surf along shores
exposed to the N and E. High surf along exposed E facing shores
is expected, and a High Surf Advisory (HSA) is in effect starting
this morning. Surf heights along N facing shores will be similar
to those on E facing shores, but differing criteria between
shorelines means peak surf heights along N facing shores will
remain below HSA heights. Very little swell energy from the NW is
expected this week, with increased potential for long-period NW
swell to arrive late next weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Wednesday for east facing
shores of Kauai, Oahu, Molokai, Maui and the Big Island.

Small Craft Advisory until 6 PM HST Thursday for all Hawaiian
waters.

&&

DISCUSSION...Houston
AVIATION...Thomas
MARINE...Kinel