High Surf in Hawaii, April 24 to 25, 2022

Area Forecast Discussion
National Weather Service Honolulu HI
331 AM HST Sun Apr 24 2022

.SYNOPSIS...
Expect breezy trade wind conditions to continue into the middle
of the week as high pressure persists to our north. Cloud cover
and scattered showers will favor windward and mountain areas, but
leeward areas my also get wet from time to time given the strength
of the trades. Trades may decrease around the middle of the week
as a weakness develops in the subtropical ridge.

&&

.DISCUSSION...
A broad area of overcast low clouds lies across the islands this
morning and will likely linger through this afternoon. Radar
shows isolated to scattered showers within this cloud cover,
favoring windward areas but, with the breezy trades, blowing
leeward from time to time. Overnight soundings continue to show a
neutral airmass with a strong inversion near 6500 feet, likely
capping deep convection and keeping total rainfall light. Beneath
the inversion, misty showers will continue to occur across almost
all of the islands until the the overcast cloud area passes west
of the islands later today or this evening.

High pressure northeast of the state will continue to drive
moderate to locally strong trade winds into the middle of the
week, with little variation in speed. Scattered light showers will
mainly affect windward and mountain areas of the islands. Models
continue to show the potential for a decrease in trade winds
around the middle of the week due to a weak surface trough moving
eastward north of the state, creating a weakness in the
subtropical ridge.

Models show that trades will stay around through the end of the
week, but differ on wind strength. We will continue to use a
model blend for winds in the extended period, keeping the
slightly weakened trades in place into next weekend. Models also
show an upper trough over or near the state toward the end of
the week, but differ in its strength and location. This could at
least weaken the strong inversion and allow for more vertical
cloud development, though confidence remains low on trough
location and specific impacts at this time.

&&

.AVIATION...
Breezy easterly trade winds will continue into Monday as a strong
ridge of high pressure remains in place north of the islands.
This moist trade wind pattern will continue to force a majority of
showers and low level clouds toward favored windward and mauka
areas. The radar trends this morning support the idea of rain
showers stretching across windward slopes and in channels between
mainly the central and eastern islands, in weakly organized
convergent low level flow. Additionally, sea breeze clouds and
isolated showers are expected once again today across leeward Big
Island. A mix of VFR/MVFR cigs continue across TAF locations this
morning, though cigs will continue to deteriorate toward more
widespread MVFR conds as the band of moisture north of the islands
becomes concentrated along windward zones. AIRMET Sierra
continues for tempo mtn obsc across most wind ward zones through
this afternoon.

AIRMET Tango remains in effect for moderate low level turbulence
below 8000 feet over and downwind of the terrain for all islands.
These turbulent conditions are expected through today.

&&

.MARINE...
The large, long-period northwest swell (330 degrees) is expected
to peak this morning. A High Surf Advisory remains in effect for
exposed north and west facing shores of the smaller islands
through tonight. Expect the surf to then gradually lower through
Tuesday, and remain small through much of the remainder of the
week. A moderate, long period south swell (170-180 degrees) will
boost surf to slightly above seasonal averages today, before
gradually lowering during the start of the new work week.

High pressure to the northeast of the islands continues to bring
locally strong trade winds to the region, and this is expected to
continue over the next several days. These winds will continue to
produce moderate, choppy surf along east facing shores. A surface
trough approaching from the northwest will first enter the
offshore waters around Wednesday, and help to weaken the pressure
gradient over coastal and offshore waters during the middle to
latter half of the week leading to a decrease in wind speeds.

A Small Craft Advisory (SCA) is in effect for all Hawaiian coastal
waters through today for a combination of the locally strong
trade winds and seas generated by the northwest swell mentioned
above. As the northwest swell declines tonight, the SCA will be
scaled back to the typically windier waters near Maui County and
the Big Island. The SCA for the winds may need to be extended into
the middle of the week, however will hold off on that extension
to monitor the trend of the winds in the model with the
approaching trough mentioned above.

&&

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

Small Craft Advisory until 6 PM HST Monday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory until 6 PM HST this evening for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-Kaiwi
Channel-Maui County Windward Waters-Maui County Leeward Waters-
Big Island Windward Waters.

&&

DISCUSSION...Powell
AVIATION...SWR
MARINE...M Ballard