High Surf in Hawaii, February 13 to 16, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
841 PM HST Fri Feb 12 2021

.SYNOPSIS...
An upper level low just east of the Big Island will drift west-
southwestward over the next few days, with the center passing
just south the islands this weekend. Expect increasing shower
trends spreading from east to west across the state through
Sunday. Easterly trade winds with localized sea breezes are forecast
into Saturday will weaken with winds becoming more southeasterly
on Sunday, as the high pressure ridge weakens. A light wind
pattern will develop favoring local scale land and sea breezes on
Sunday and Monday. A cold front approaching the state from the
northwest early next week will slow on Tuesday and Wednesday, with
continued unsettled weather trends in the forecast through much
of next week. Trade winds return from Tuesday onward as a new high
pressure system builds in north of the state.

&&

.DISCUSSION...
The satellite picture this evening shows an upper level low
roughly 300 miles east of Hilo Hawaii and a cold front roughly
1000 miles northwest of Kauai. These two weather system will bring
periods of unsettled weather conditions to each island over the
next five to seven days. The upper low will drift into the region
on a west-southwesterly track over the next several days, passing
just south of the main Hawaiian Islands from Sunday through
Tuesday. Meanwhile, the cold front will slowly approach the
islands from the northwest, with unsettled weather potentially
developing over the western islands on Tuesday and Wednesday.

For the weekend, a weakening easterly trade wind pattern will
allow localized sea breezes to develop along western sides of the
western islands through Saturday. A cold core upper low just east
of the Big Island will drift into the region this weekend with the
center passing just south of the main Hawaiian Islands on Sunday
and Monday. Initially stronger winds aloft will develop near the
Big Island tonight with wind speeds reaching Wind Advisory
thresholds for the summits of Mauna Kea and Mauna Loa. A Wind
Advisory remains in effect for both mountain peaks through
Saturday morning. Deeper and more unstable moisture will then move
in from the east over the Big Island by Saturday morning with
elevated Precipitable Water (PW) levels ranging from 1.20 to 1.60
inches spreading westward into the other islands through the
weekend and into the first half of next week. Shower activity will
trend higher on Saturday, mainly over the eastern islands,
as lingering easterly trade winds force this unstable moisture
over windward and mountain slopes. Trade winds will fade on Sunday
as the upper low and approaching cold front weakens the ridge
north of the state. Wind directions will shift towards the
southeast and become light enough for local scale afternoon sea
breezes to develop over the smaller islands. Converging sea
breezes will drive the shower activity towards the interior of
each island into Monday trending towards the afternoon to early
evening hours. One exception to this weather pattern will develop
along the southeast to east slopes of the Big Island, where large
scale southeasterly winds will continue to be strong enough for
periods of upslope showers.

Monday through Wednesday...The upper level low will stall south
of Kauai and Oahu on Monday as the cold front approaches the
islands from the northwest. Light winds on Monday will keep sea
breezes in the forecast for one more day with brief periods of
mountain and interior showers affecting the smaller islands. The
forward motion on the front begins to stall on Tuesday as it
approaches Kauai. Forecast guidance on Wednesday shows moderate
to locally breezy trade winds spreading westward across the state
from Wednesday onward.

The long range forecast model guidance remains in fairly good
agreement through Monday. Global weather models begin to show some
differences on how far this front marches into the western half
of the state by mid week. The latest American (GFS) model brings
the cold frontal band into Kauai and Oahu by Wednesday, while the
European (ECMWF) model stalls the front a few hundred miles
northwest of Kauai. These model differences keep our long range
confidence in a wet weather pattern for the western islands in a
low to medium range this far out. Please continue to monitor the
forecast for next week as this pattern and weather impacts will
likely evolve over time.

&&

.AVIATION...
High pressure northeast of the state will maintain moderate to
locally breezy easterly trade winds around the Big Island and
Maui, while lighter trades allow for localized land and sea
breezes over the western end of the state. Brief MVFR conditions
will be possible along windward slopes, but predominantly VFR
conditions are expected through at least Saturday morning. An
increase in moisture moving in from the east Saturday afternoon
and Saturday night will bring the potential for widespread MVFR
conditions across windward sections of the Big Island and Maui.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain of Maui and the Big Island.

&&

.MARINE...
Strongest winds will hold across the typically windier channels
and waters around Maui County and the Big Island through Saturday
as the easterly trades hold in the fresh to strong category.
Strongest winds will shift to areas along the Hamakua coast, the
northeast Maui coast (from Haiku to Hana), and waters south of
the Big Island Saturday night through the holiday as the winds
shift out of the southeast direction.

For the Kauai and Oahu waters, light to moderate east to southeast
winds will hold through the holiday due to the proximity of the
ridge and an approaching front. Leeward and sheltered waters near
the coasts will continue to experience land and sea breeze
conditions each day.

Although the forecast confidence lowers, local winds could remain
in the light to moderate category (background flow remaining out
of the southeast to southerly direction) with localized land and
sea breeze conditions continuing near the coasts Tuesday into
midweek as an upper disturbance and the tail-end of a frontal
boundary move into the area. In addition to the winds and seas,
increasing moisture combined with these features will drive the
rain chances up later this weekend through the first half of next
week.

A combination of the strong winds over the eastern end of the
state and a large northwest swell building across the waters tonight
will lead to Small Craft Advisory conditions across all Hawaiian
waters through the weekend.

Surf along exposed north and west facing shores is quickly rising
this evening as the large northwest swell begins to build across
the local waters. Observations at the offshore buoys to the
northwest reflect this and appear to be peaking near the predicted
levels within the 16-19 second band around 12-14 ft. This will
result in peak surf heights through the first half of the day
Saturday well above warning levels for the exposed coasts.
Advisory-level surf will be possible along exposed west facing
shores of the Big Island through the day Saturday as the swell
arrives. Although a gradual downward trend is anticipated Saturday
night through Sunday, heights will remain at or around warning
levels.

Impacts associated with this large swell peaking Saturday will
include significant beach erosion with water sweeping across
beaches that typically remain dry. Additionally, some minor
overwash onto the typical vulnerable low-lying coastal roadways
will be possible through the early morning hours Saturday at and
prior to daybreak around the daily peak tide.

Looking ahead, the active trend will continue through the first
half of next week due to a large and persistent gyre that has
setup over the northern Pacific. A combination of small to
moderate, long-period sources arriving Sunday through Tuesday and
the easing northwest swell peaking tomorrow will continue to
produce advisory-level surf through Monday. Surf may dip below
advisory levels Tuesday through midweek.

Surf along east facing shores will trend up over the weekend due
to a decent sized batch of strong breezes upstream across the
eastern Pacific between high pressure and a trough several hundred
miles east-southeast of the Big Island. Downward trend is expected
early next week.

&&

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

Wind Advisory until 6 AM HST Saturday for Big Island Summits.

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

&&

DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...Gibbs