High Surf in Hawaii, March 21 to 23, 2022

Area Forecast Discussion
National Weather Service Honolulu HI
403 PM HST Mon Mar 21 2022

.SYNOPSIS...
A front approaching from the northwest will stall and weaken near
Kauai on Tuesday. The front will bring increased shower activity
to the state, with Kauai expecting to receive the bulk of the
showers. Breezy to locally windy trade winds will fill back in on
Tuesday as a new high builds in to our north. Moisture associated
with the remnant frontal boundary will allow a wet trade wind
pattern through Friday, especially over the west end of the island
chain.

&&

.DISCUSSION...
High pressure systems are located far to the northeast and
northwest of Hawaii. A cold front is located between the two highs
passing within 120 miles northwest of Kauai. Visible and infrared
satellite shows the front as a 150 mile wide band of layered
clouds slowly approaching the state this afternoon. Water vapor
imagery shows an upper trough behind the front pushing southeast
towards Hawaii.

Light winds across the state earlier today will transition to
breezy trade winds for most areas tonight. Sea breezes that set up
earlier allowed for interior clouds and briefly heavy showers to
develop, but these should quickly dissipate by sunset.

Clouds and showers associated with the front will reach Kauai
this evening, with the focus shifting to windward and mauka areas,
though many of these showers will reach leeward sides as the winds
ramp up. The front is expected to stall and dissipate across
Kauai Tuesday into Wednesday and bring persistent showers to the
Garden Isle. There is a small chance of flooding on Kauai given
the fetch of trade winds, incoming moisture and general instability
with the upper trough moving over the area. Stream gage guidance
predicts that water levels will approach, but stay just below
minor flood threshold. Do not think a Flash Flood Watch will be
necessary for Kauai at this time, but will continue to monitor the
situation. Added a slight chance of thunderstorms and locally
heavy rainfall to the forecast for Kauai for Tuesday and Tuesday
night.

The other islands, particularly Oahu, will also see a boost in
showers (windward focused) as the trade winds build in. Trade
winds will also become strong by Tuesday, which will continue into
this weekend as the front slowly dissipates. This will create
blustery conditions as it couples with the incoming showers. A
possible Wind Advisory may be issued for some of the windier areas
around the state as early as Tuesday night. Guidance does
indicate that the moisture axis may shift a bit to the south later
in the week, potentially increasing rainfall chances over
windward areas of Maui and the Big Island.

&&

.AVIATION...
This afternoon, sea breeze convergence over the islands,
especially Oahu, has resulted in a few showers over interior
sections. MVFR visibility and ceilings are possible within these
showers with briefly heavy rain, but VFR conditions continue to
prevail at most TAF sites. Meanwhile, a frontal boundary north of
the main Hawaiian Islands will approach Kauai tonight into
tomorrow. This will increase low clouds and showers tonight and
tomorrow for Kauai and, to a lesser extent, Oahu. Periods of heavy
rain and isolated thunder cannot be ruled out with this activity.
Additionally, much stronger northeast winds are expected
beginning at Kauai by the end of today before spreading eastward
across the rest of the state overnight.

Although no AIRMETs are currently in effect, AIRMET Sierra for
mountain obscuration will likely be needed tonight or early
tomorrow morning across Kauai and possibly Oahu as the front
approaches the western islands and low clouds/showers increase.
AIRMET Tango for low-level turbulence will also eventually be
needed as the stronger trade winds increase across the western
half of the state tonight.

&&

.MARINE...
Northeast to east trade winds will increase tonight as strong high
pressure far northwest of the area pushes a front toward Kauai. A
shallow surface trough near Molokai separates gentle to moderate
northerly winds around Kauai and Oahu from east to southeast winds
around the Big Island and Maui this afternoon. The trough will
dissipate during the evening, and northeast to east trade winds
will strengthen, especially around Kauai tonight. The front will
stall near Kauai on Tuesday and gradually dissipate through
Thursday or Friday as strong high pressure builds northwest of the
state. Trade winds will ramp up to fresh to strong levels
Tuesday, especially along and just north of the front, and persist
through at least Thursday. A Small Craft Advisory (SCA) has been
issued for all waters, and thunderstorms could develop along the
front.

A northwest swell will continue to build tonight. The swell has
come in line with guidance, and the NOAA buoys northwest of Kauai
have consistently measured swell heights at 9 to 10 ft at 14 to 15
seconds through much of the day, prompting the issuance of a High
Surf Advisory for most north and west facing shores from Kauai to
Maui. North shore surf has already topped HSA levels on Kauai and
will spread to Oahu and Maui County through the evening. The
swell should peak tonight and be reinforced by a pulse of north
swell on Tuesday. On the Big Island, effects from the initial
northwest will be limited to a slight increase in surf along west
facing shores early Tuesday, but north facing shores will
experience a more significant rise as the north swell fills in
late Tuesday and Wednesday, though advisory level surf is not
expected. The north swell will gradually decline Wednesday
through Friday.

Rises are expected on east and south facing shores as well. Wind
swell will build as the trade winds increase Tuesday and
Wednesday, but the islands will lie near the eastern edge of the
fetch of strong trade winds, which will limit the growth of the
local seas. The latest guidance shows the rough trade wind swell
could push east shore surf on Kauai near the HSA level as early as
Tuesday, but seas are projected to build to around seasonal
average elsewhere. That said, some wrapping north swell could
lead to elevated east shore surf, as well as surges in Kahului and
Hilo Harbors. The trade wind swell will persist through at least
Thursday, and the combined seas from the trade wind swell and the
northwest to north swell will likely require an SCA Tuesday
through Thursday. In addition, background south shore surf will be
interrupted by a small south-southwest swell arriving late Tuesday
afternoon and peaking Wednesday and Thursday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Tuesday for north and west
facing shores of Niihau, Kauai, Oahu, Molokai and for north facing
shores of Maui.

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

Small Craft Advisory from 6 AM Tuesday to 6 PM HST Wednesday for
all remaining Hawaiian coastal waters.

&&

DISCUSSION...Foster
AVIATION...Vaughan
MARINE...Wroe