High Surf in Hawaii, April 18 to 20, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
941 AM HST Sat Apr 18 2020

.SYNOPSIS...
Moderate trade winds will gradually decrease today and shift out
of the southeast tomorrow, leading to a land and sea breeze
pattern over most islands Sunday through Tuesday. The atmosphere
will remain stable, focusing meager trade wind showers over
windward slopes through tonight, then give way to a rather dry
land and sea breeze pattern Sunday through Tuesday. East to east-
southeast winds may strengthen by the middle of next week.

&&

.DISCUSSION...
The stable trade wind flow is on a decline. The surface ridge
driving the trades currently sits several hundred miles north of
the state and is being weakened and pushed southward by an
approaching front. Trades are down slightly from yesterday and
will continue to slowly ease today. Rainfall has been and will
continue to be meager today as precipitable water in the trade
wind flow remains below April normal and a ridge aloft maintains
very stable conditions.

As the surface ridge edges closer on Sunday, winds across the
region will ease further and shift out of the southeast for at
least northern portions of the island chain. This will lead to a
dominant land and sea breeze pattern across all areas except for
windward Big Island and east Maui. The persistent ridge aloft will
maintain stable conditions, and although interior and leeward
areas will see increase in afternoon clouds, we are not expecting
much shower activity.

Little change in the stable and rather dry land and sea breeze
pattern is due Monday and Tuesday, as the surface ridge parks
just north of the islands and a front stalls a couple hundred
miles north of Kauai. Windward and southeast slopes of the Big
Island and perhaps east Maui will have higher, though still low,
chances of modest showers. Elsewhere, afternoon sea breezes will
produce a few showers with clearing expected at night.

East to east-southeast winds will likely increase Wednesday and
Thursday as high pressure far northeast of the islands
strengthens. Due to subtle, though ongoing, differences in the
GFS and ECMWF solutions, it is too early to tell if a land and
sea breeze pattern will hang on or a trade wind shower pattern
returns. Regardless, both models show a firm mid level ridge
persisting, which suggests showers will not be very active. An
approaching upper level trough late next week could trigger
increased shower activity.

&&

.AVIATION...
High pressure centered northeast of the state will allow for VFR
conditions across the islands today. Expect light trades through
this afternoon with isolated showers mainly over areas with
elevated terrain. Sea breezes may develop along some leeward
areas before diminishing by this evening.

No AIRMETs are currently in effect.

&&

.MARINE...
High pressure far north-northeast of the area will drift east
over the next few days as a frontal boundary approaches from the
northwest. Light to locally moderate trade winds today will
gradually diminish and veer to a more east-southeasterly direction
Sunday and Monday. These lighter winds are expected to persist
through the first half of next week as the high remains far
northeast of the area and the frontal boundary lingers northwest
of Kauai.

Data from buoys 51001 and 51101 indicate the new long period
northwest swell is gradually rising, though it is running behind
model guidance by several hours. Thus surf will be a bit slower to
arrive along north and west facing shorelines. However, a High
Surf Advisory will remain posted as we expect surf to gradually
rise today especially along Kauai's shorelines. A Small Craft
Advisory is also posted for waters exposed to this swell as seas
are expected to reach at least 10 feet across those areas. This
swell will linger through Sunday then lower gradually Sunday night
through Monday night. Another northwest swell, with a shorter
period, is expected to gradually fill in on Tuesday, then linger
during the Wednesday through Friday time period. Surf heights are
expected to be near advisory levels during the peak of this swell.

Small, mainly background south swells are expected through Sunday
night. A new larger, and longer period south swell is expected to
arrive on Monday, peak Monday night and Tuesday, then lower
gradually Wednesday and Thursday. Surf heights may reach High Surf
Advisory levels during the peak of this swell. Another smaller
south swell is expected toward the end of next week.

Due to the lack of significant trade winds, surf will remain
small along east facing shores over the next several days. Trade
winds may pick up toward the second half of next week resulting in
a slight boost in surf heights.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from noon today to 6 AM HST Sunday for
Waianae Coast-Oahu North Shore-Oahu Koolau-Molokai.

High Surf Advisory until 6 AM HST Sunday for Niihau-Kauai
Windward-Kauai Leeward.

Small Craft Advisory from 6 PM this evening to 6 AM HST Sunday
for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward
Waters-Kauai Channel-Oahu Windward Waters-Kaiwi Channel-Maui
County Windward Waters.

&&

DISCUSSION...Wroe
AVIATION...Thomas
MARINE...Burke