High Surf in Hawaii, December 23 to 25, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
343 PM HST Wed Dec 23 2020

.SYNOPSIS...
Trade winds will return on Thursday and strengthen on Friday and
Saturday as high pressure passes north of the state. Showery
weather is expected for Kauai on Friday, and for the island of
Oahu and maybe windward Maui County on Saturday, as a trough aloft
passes through the north part of the state.

&&

.DISCUSSION...
An upper level ridge of high pressure centered across Oahu, is
spreading a dry and stable airmass aloft across the entire State
of Hawaii this afternoon. Precipitable water indicated at Lihue
only 0.79 inch, anything under an inch being well below normal for
this time of year. Sounding shows marine layer is capped below
7000 feet with dry stable air above. Result is only a few clouds
over the mountain slopes, and little if any rain shower activity
statewide.

Upper level ridging will remain in control across our latitude
with dry and stable conditions across the state through Thursday.
A buckle in the mid latitude westerlies is being predicted by the
global models to push an upper level trough across the state on
Friday through Saturday morning, which will slightly destabilize
the airmass as a weak mid level cold front moves near Kauai.
Showery weather will increase for Kauai during the day, and Oahu
during the overnight. On Saturday, the mid/upper trough moves away
to the east, with the trailing mid level cold front and upper
level jet weakening near Oahu. Expecting lingering showers across
Oahu on Saturday, maybe reaching as far south as windward Molokai
and Maui.

At the surface, a high pressure ridge will consolidate far to the
north by Thursday, bringing a return to breezy trade winds to the
islands through the weekend. Some increase in moisture is
expected in the low level trades by Sunday, especially near Big
Island.

Winds will likely decrease and veer southeasterly later on Sunday
and early next week as the subtropical ridge is forced southward
again. A land and sea breeze regime are expected over most areas,
with trade winds holding for the Big Island.

Satellite imagery indicates a plume of volcanic haze emanating
from Halemaumau Crater and generally moving toward the southwest
and affecting the Kau and Kona Districts. Significant impacts to
air quality are not expected, but we are still learning about the
concentrations of particulates associated with this new eruption.

&&

.AVIATION...
The sea breeze, currently affecting many leeward locations, is
expected to diminish by this evening with the loss of day time
heating. A light trade wind pattern will once again return to the
islands tonight as high pressure begins filling in behind a dying
cold front to the north.

During the forecast, isolated period will mainly be limited the
windward slopes and coasts. Otherwise, VFR conditions prevail.

No AIRMETs are currently in effect.

&&

.MARINE...
A southern-moving cold front is pushing a surface ridge axis to
just north of the islands. This has maintained light and variable
winds across the waters surrounding Kauai over the past couple of
days. Slightly stronger gentle to moderate east southeast winds
will occur around Oahu and points eastward the next couple of
days. New and strengthening high pressure will build north of the
state through the remainder of the week. This will produce a tight
enough northeasterly pressure gradient back across the local
waters this evening into Thursday to generate areawide moderate
to locally fresh trades. Trades will further strengthen to
sustained fresh to locally strong magnitudes as high pressure
moves further northeast of the islands Christmas Day and through
the holiday weekend.

A large, long period northwest swell reached the northwestern
offshore water buoys early this morning and will be steadily
lifting surf through Thursday. Timing is for Kauai and Oahu's
north and west-facing shoreline surf to gradually build from this
afternoon into Thursday afternoon. A High Surf Advisory (HSA) is
in effect as surf is forecast to exceed advisory thresholds from
as early as late tonight and peak through Thursday afternoon along
the northern and western exposed shorelines of the smaller islands.
A Small Craft Advisory (SCA) is in effect for high seas that
commenced today around Kauai and, due to the larger northwest
swells traveling down the island chain, has been expanded eastward
this afternoon to cover those waters around Oahu and Maui. The
HSA may also need an extension into late Thursday or Friday to
account for reinforcing northwest swells arriving Thursday night.
A more westerly component to these recent northwest swells could
push surf to 8 foot HSA levels along the Kona coast of the Big
Island during the day Thursday.

An out-of-season south swell is filling in along south-facing
shores today. Since last night, offshore and nearshore buoys have
been picking up on longer 17 to 19 second band energy (that likely
has northwest energy mixed into the south swells). Surf along
southern exposed shorelines will near and may just hit the 8 foot
HSA threshold Thursday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Thursday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

Small Craft Advisory from 6 PM this evening to 6 PM HST Thursday
for Kauai Channel-Oahu Windward Waters-Kaiwi Channel-Maui County
Windward Waters-Alenuihaha Channel-Big Island Windward Waters.

Small Craft Advisory until 6 PM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters.

&&

DISCUSSION...Brenchley
AVIATION...Thomas
MARINE...Blood