High Surf in Hawaii, December 29 to 31, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
338 AM HST Tue Dec 29 2020

.SYNOPSIS...
Showers will increase today as a band of moisture moves in from
the east. These showers will be short-lived as a drier air mass
begins to move in tonight. Breezy trades should return by
Thursday, with widespread windy conditions expected around New
Years Day. Showers will likely increase, especially across
windward areas, as trades strengthen.

&&

.DISCUSSION...
Locally breezy southeast flow near the Big Island is blocked from
reaching the smaller islands, resulting in either weak southeast
flow or light and variable winds from Maui westward this morning.
High pressure far northeast of the islands has been pushed
eastward by an impinging frontal boundary far to our north and
northwest. The result places the islands along the southwest
periphery of the subtropical high. Overnight soundings show our
airmass is stable and quite dry, with PW amounts between 0.9 and
1.2 inches. Satellite loop shows clear skies across the islands
this morning, with a ragged band of broken low clouds 120 miles
east of the islands slowly drifting westward. These clouds are
expected to move in across the state starting this afternoon.
Light to moderate showers should be on the increase as winds
slowly swing back to northeasterlies during this time. Most of
these showers should be focused over windward areas and only light
rainfall amounts are expected.

A drier air mass is expected to move in tonight into Wednesday,
likely causing showers to decrease. Starting Thursday, trade winds
will increase again, with breezy conditions expected state wide.
Showers will ride in on the strengthened trades, with showers
expected to focus over windward and mountain areas. Trades are
expected to become even stronger on New Years Day, with 20 to 30
mph wind speeds possible. Beyond Friday, a ridge of high pressure
will remain far north of the state and we should continue to see
breezy trade wind weather in the extended forecast.

&&

.AVIATION...
A front far north of the area has been slowing and shifting winds
around the state. Winds near the Big Island remain from the east,
but they veer to the southeast moving up the chain to Kauai. This
regime will result in nighttime land breezes and daytime sea
breezes developing over many locales. VFR will prevail over most
of the land area, though a showery cloud band with brief or TEMPO
MVFR conditions may begin to affect mainly windward sections from
the Big Island to Molokai by the afternoon hours today.

No AIRMETs are in effect.

&&

.MARINE...
A cold front entering the far northwest offshore waters will
weaken and become stationary today. This front will push a ridge
slightly closer to the islands, producing moderate east to
southeast winds over windward waters through mid week. Due to
proximity to the ridge axis and island shadowing from the
southeast winds, winds around Kauai will be light and variable.
High pressure building in north of the island chain Thursday and
Friday will reestablish and strengthen the trades into the fresh
to locally strong category.

Recent observations at the Waimea nearshore buoy show a large,
long-period northwest swell has arrived in the local waters and will
continue to fill in through the morning. This swell will push
surf to warning level for north and west facing shores of Niihau
through Oahu, and advisory level for north and west Molokai and
north Maui shores as it peaks tonight into early Wednesday. This
swell will gradually lower into Thursday. A reinforcing, similar-
size northwest swell is forecast to fill in Thursday with peak
surf Thursday night into Friday, followed by a gradual lowering
later Friday. These northwest swells will cause seas to exceed the
10 foot Small Craft Advisory (SCA) threshold over exposed waters.
Thus, an SCA is in effect for all windward waters and island
channels through at least Wednesday morning. See the High Surf
Advisory, High Surf Warning, and Small Craft advisories for
additional details. These may all have to be extended and expanded
throughout the week. These swells, combined with high astronomical
tides, may cause some coastal water inundation along low-lying
areas, especially during early morning high tides.

East shore surf will slowly decline as the trades ease, especially
for the western half of the state. However, due to a fairly long
upstream fetch, waves may remain elevated for the eastern half of
the state. East shore surf will rise again everywhere toward the
end of the week with the strengthening trades.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Wednesday for Molokai-Maui
Windward West-Maui Central Valley-Windward Haleakala.

High Surf Warning until 6 AM HST Wednesday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau.

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

&&

DISCUSSION...Powell
AVIATION...Kinel
MARINE...TS