High Surf in Hawaii, May 1 to 2, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
848 PM HST Fri Apr 30 2021

.SYNOPSIS...
Breezy trade winds will persist through the weekend, focusing
clouds and showers over windward areas. Trades will diminish
early next week as an upper trough approaches from the northwest.

&&

.DISCUSSION...
A band of moisture over the central and western portions of the
state is on the move as stronger trades push into the area.
Associated clouds and showers will focus mainly over windward and
mauka areas through tonight before moving west of the state early
Saturday. Expect drier weather as breezy trades become established
for the balance of the weekend.

Models show an upper trough will amplify northwest of the islands
early next week. The surface pressure gradient will weaken
considerably as the associated front approaches the islands,
causing winds to decrease. This is expected to muster in a land
and sea breeze pattern, setting up the potential for increased
afternoon showers over leeward and interior locations Monday and
Tuesday.

Forecast uncertainty increases by the middle of next week
concerning the development of the upper trough. GFS is on the
wetter side of the solution envelope, while ECMWF shows returning
trades as the upper trough shifts overhead Wednesday through
Friday. This would favor a return to wetter trades of the moderate
or breezy variety during the latter half of next week.

&&

.AVIATION...
Trade winds will become moderate to breezy across the state as a
high pressure ridge north of the islands strengthens. Showers and
accompanying MVFR conditions will mainly affect windward and mauka
sections.

An AIRMET for mountain obscuration remains in effect for Kauai and
Oahu. One for low level turbulence may be needed later tonight or
Saturday morning as trade winds are on the increase.

&&

.MARINE...
A late season west-northwest swell has arrived and surged above
predicted levels this evening over the exposed Kauai and Oahu
waters. The nearshore PacIOOS buoy at Waimea reflects this new
swell with the spectral peak centered around 18 seconds, which
should peak overnight into Saturday. This swell combined with
fresh easterly trade wind driven energy will lead to seas nearing
the small craft advisory threshold of 10 ft over the exposed
coastal waters through Sunday as the swell builds down the island
chain. In addition to the seas, small craft advisory-level winds
will continue over the channel waters between Maui County and the
Big Island, Maalaea Bay, and waters around South Point through
Sunday. Guidance continues to depict the ridge weakening early
next week as an upper trough and front approach from the
northwest. Local trades will respond and drop into the light to
moderate category by Tuesday.

Surf along exposed north and west facing shores of Kauai and Oahu
quickly ramped up this afternoon due to the arrival of the late
west-northwest swell. Expect this source to peak overnight into
Saturday above predicted levels, then slowly ease through the
second half of the weekend. A high surf advisory has been issued
for north and west facing shores of Kauai and Oahu, and may be
required for west facing shores of the Big Island Saturday given
the more westerly direction. Surf along north facing shores of
Maui should remain lower due to the westerly direction and much of
the energy being clipped by Molokai. No other significant swell
events are anticipated for north and west facing shores through
next weekend after this swell moves out.

Surf along south facing shores will remain up through Saturday as
a small south-southwest swell continues to move through. A
downward trend is anticipated Sunday into early next week with
mainly background southerly swell energy expected.

Surf along east facing shores will trend up over the weekend due
to the fresh to strong trades returning locally and upstream. A
downward trend is expected early next week as the trades weaken.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Saturday for Niihau-Kauai
Leeward-Waianae Coast-Oahu North Shore-Kauai North.

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

&&

DISCUSSION...Powell
AVIATION...Kinel
MARINE...Gibbs