High Surf in Hawaii, April 13 to 14, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
346 PM HST Fri Apr 13 2018

.SYNOPSIS...
High pressure passing north of the islands this weekend will
increase the trades back to windy levels. An upper level low
developing to the west of the islands, and an increase in moisture
spreading up the chain from the southeast, will keep the trades
quite wet through the weekend, along with the possibility for a
few thundershowers. The upper level low will move away early next
week, but lingering moisture over the islands will keep the
trades somewhat wet. Wind speeds will moderate somewhat after the
weekend with a weaker high passing to the north about Tuesday.

&&

.DISCUSSION...
The 00z soundings and recent surface observations showed the
destabilization now in progress over the islands, with increasing
boundary layer moisture pushing in from the southeast, and
decreasing mid-level temperatures pushing in from the northwest.
The colder air aloft is from an upper trough to our NW, digging W
of the islands. All of this spells yet another wet period coming
up soon for the main Hawaiian Islands

The trough aloft will leave behind a weak low in the middle and
upper levels of the atmosphere this weekend to the NW of the
islands. The models are quite bullish on instability over the
weekend, and we may see what we colloquially refer to as
"thundertrades," a rather rare weather pattern here where some of
the tradewind showers get tall enough to produce a few rumbles of
thunder, along with rather intense (though usually brief)
downpours. We have added a slight chance of thunder to most
windward and mauka sections starting late tonight and continuing
through the weekend. Interior and leeward sections of the Big
Island sheltered from the trades should see diurnal circulations
allow for mainly afternoon and evening thundershowers as well, as
we have seen the past few days. Just like with ordinary trade wind
days, the precise timing and locations of the heavier rain...or
the lulls in the rain...is difficult to pin down, since the
synoptic forcing is rather weak.

The low begins to move away on Sunday, and weak ridging will begin
to rebuild over the islands. This should hold through the middle
of next week. However, lingering moisture will continue to keep
trades somewhat wet, although with more subdued rain intensities.
The models disagree somewhat about how much moisture will be
available for trade wind showers, with the ECMWF trying a little
harder to advect drier air back over the islands. There are no
strong signals in the model trends right now, though.

Although the trades have moderated a bit today, a new, strong
high will pass about 1200 miles due N of the main Hawaiian Islands
on Sat, which will ramp the trades back up to windy levels over
the weekend. This high will continue eastward and weaken into next
week. Another weaker high will pass farther to the N of the
islands about Monday night or Tuesday, with trades at breezy to
locally windy levels. After that, the trades should moderate a bit
as a subtropical ridge is left behind N of the islands through
the middle of next week.

&&

.AVIATION...
With rain falling on the Big Island and expected on Kauai,
mountain obscurations possible into tomorrow therefore MVFR
conditions for these areas. AIRMET SIERRA is in place as a result.
With rain expected for north through east sections tonight and
tomorrow, there may be other mountain obscurations there as well.
Mostly VFR conditions across Hawaii with the exception of
previously mentioned areas in Kauai and the Big Island. MVFR
conditions may spread to north through east sections of Oahu
tomorrow.

NE trade winds through the day into tomorrow. With an increase in
the pressure gradient, wind speeds will increase today in most
areas. AIRMET TANGO for low level turbulence is in place as a
result and should continue into tomorrow.

&&

.MARINE...
Trade winds will strengthen through the weekend as a strong high
pressure area, currently northwest of the state, moves into
position due north of the islands. A Small Craft Advisory (SCA)
is currently in effect over the windier areas around the Big
Island and Maui as the trades begin to ramp back up. The winds
will strengthen further over the weekend with widespread SCA winds
across all of the coastal waters, and near-gale force winds in
the Alenuihaha Channel and the other windy zones. The northwest
swell building tonight will add to wave heights, so the SCA now
covers all of the coastal waters for winds, seas, or both through
Sunday.

A new northwest swell has begun to arrive locally and is expected
to build quickly tonight and peak Saturday morning, producing
surf well above the High Surf Advisory (HSA) threshold along
north and west facing shores tonight through Saturday. Therefore,
an HSA is now in effect for most north and west facing shores
through Saturday night. The swell will gradually subside through
Monday. A small north swell is expected through mid-week with a
moderate northwest swell building Thursday night. As trade winds
strengthen this weekend, surf along east facing shores is expected
to read advisory level as well and an HSA will likely be issued.
The elevated east-side surf is expected to persist into the first
half of next week. Surf will remain small along south facing
shores, but new swells from the southern hemisphere will produce a
modest increase starting as early as Sunday.

There is a good chance of thunderstorms over the leeward coastal
waters tonight through Sunday, due to the digging upper trough and
convergence zones extending downwind from the islands. These
storms could produce intense downpours with significant visibility
reductions, along with frequent cloud to surface lightning.

See the Collaborative Surf Discussion for additional details on
surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Sunday for N facing shores of
Niihau, Kauai, Oahu, Molokai, Maui, and the Big Island, and for
west facing shores of Niihau, Kauai, Oahu, and Molokai.

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

Small Craft Advisory until 6 PM HST Sunday 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.

&&

R Ballard/Chevalier/TS