High Surf in Hawaii, April 22 to 24, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
354 AM HST Wed Apr 22 2020

.SYNOPSIS...
A surface ridge over Kauai earlier in the evening has started it
northward movement. This move will allow the trade winds to build
across the smaller islands from the east, and gradually
strengthen in time. All islands will be under a light to moderate
trade wind flow by tonight. The ridge far north of the islands
weakens late in the weekend, leading to a slight decrease in
strength of the trades. Expect mainly moderate trades to hold
through the middle of next week. Trade showers will be favoring
the windward and mountain areas of the larger islands.

&&

.DISCUSSION...
The east to west oriented surface ridge over Kauai earlier in the
evening has started its northward movement, allowing trade winds to
move in. This can be seen on satellite and radar where some low
clouds and isolated showers are threatening to move on to the
eastern shoreline of Kauai from the Kauai Channel. Even clouds and
showers off the Na Pali coast have started to move west instead
of north. The trade wind flow west of Maui is still forecast to be
weak for most of today, favoring an onshore sea breeze flow along
the western, leeward areas of the smaller islands. A shower or two
is likely from the resultant sea breeze and daytime heating. The
showers may not hang around too long though, as the trades kick in
late this afternoon.

The low level inversion is low, between 6 and 7k, as noted by this
morning's raob run from both Lihue and Hilo. The GFS indicate that
this inversion buckle slightly on Friday, raising the inversion
to 10k feet as a upper level trough nears Kauai from the west. The
trough's eastward movement slows down over the central islands on
Saturday. It finally moves east of the Big Island Monday evening,
but not before it deepens with a low over the Big Island early
Sunday morning. Models are not showing much effects from the
through. But past upper level troughs have been known to kick up
some scattered showers and even a thunderstorm over the higher
terrain and leeward slopes of the Big Island. The surrounding
temperate of the trough is a minus 9 to 10 degrees C at 500 mb.
At this temp, it can set off a brief thunderstorm at best. So, we
will be keeping a close eye on it. Scattered showers are in the
forecast for the interior and lee slopes of the Big Island this
weekend.

The upper trough will be accompanied by a sheet of high cirrus
clouds, coming in as early as tonight. It will be at its thickest
Friday, then gradually thins out on Saturday. The thickness of
the cirrus may limit the afternoon shower development across the
higher terrain and Kona slopes of the Big Island.

The strength of the trades is forecast to reach locally strong by
Saturday morning, but it may occur sooner, as early as Thursday,
The locally strong trades sort of ebb, coming and going away due
to diurnal effects between Thursday and Friday. We will be keeping
an eye on it as well. The strong trades, though, is forecast to
back off Sunday night as the ridge north of the islands is being
tested by front. Mainly moderate trades are expected to hold
thereafter through the first half of next week. Otherwise, trade
showers will grace the islands, favoring the windward and
mountain areas. The frequency of these showers, although somewhat
less than normal, will increase overnight, then back off by the
afternoon.

&&

.AVIATION...
A dissipating front northwest of the state will help maintain
light southeasterly flow across the islands this morning. Land
breezes this morning will transition to sea breezes by this
afternoon. Mainly VFR conditions expected with the exception of
increased cloud cover and isolated light showers over interior
portions of the islands.

No AIRMETs are currently in effect.

&&

.MARINE...
A stationary front northwest of Kauai will gradually dissipate
today through Friday, with surface ridging building far to the
northeast and northwest of the island chain. This will bring a
gradual return of trade winds from east to west across the marine
area, with the trades strengthening to moderate to fresh levels
Thursday and Friday. Locally strong trades are expected Friday
night through the weekend, and a Small Craft Advisory may be
required for the typically windy areas around Maui and the Big
Island.

A series of northwest swells will continue to move through island
waters through the remainder of the week. The incoming northwest
swell will continue to build today, bringing advisory level surf
to north and west facing shores. This swell will begin to fade
late tonight and early Thursday, but will be quickly reinforced
by a slightly larger northwest swell Thursday afternoon through
Friday. The latest guidance, along with upstream buoy
observations running a bit above predicted levels, suggests that
advisory level surf should hold in place today through Friday, and
the High Surf Advisory has been adjusted accordingly. This
secondary northwest swell will gradually decline Friday night
through the weekend, with only small surf expected early next
week.

The current south swell will hold steady today and tonight, then
lower Thursday and Friday, with only background southerly swells
expected over the weekend through early next week. Surf will
remain small along east facing shores today, then gradually
increase Thursday into the weekend as trades strengthen over and
upstream of the islands.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from 6 AM this morning to 6 PM HST this
evening for Niihau-Kauai Windward-Kauai Leeward-Waianae Coast-
Oahu North Shore-Oahu Koolau-Molokai-Maui Windward West-Maui
Central Valley-Windward Haleakala.

&&

H Lau/Thomas/Jelsema