High Surf in Hawaii, January 24 to 27, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
850 PM HST Tue Jan 24 2017

.SYNOPSIS...
Locally breezy trade winds will continue into Thursday, with light
and variable winds expected Friday into Saturday. An increase in
northeast winds is expected late Saturday into Sunday as a cold
front moves down the island chain. Cool, locally breezy trade winds
are expected after the front passes Sunday, with these winds
persisting through early next week. While the trade winds will
continue to deliver a few windward showers, a trend toward drier
weather is expected the next couple of days, until the front brings
showers this weekend.

&&

.DISCUSSION...
Showery weather continues to affect windward Big Island this
evening, with a record-setting amount of precipitation for the date
observed at the Hilo airport. A gradually-departing upper level
trough is providing instability, allowing cloud tops to grow up to
20 thousand feet in that area, while the trade winds force air up
the slopes. Meanwhile latest satellite animations indicate a
significant decrease in low cloud cover elsewhere around the island
chain, and over windward waters, with a few showery low clouds
riding in on moderate trade winds. The trough will exit late tonight
and cloud top growth and rainfall intensity are expected to diminish
by morning. An upper level ridge will replace the trough, capping
our cloud tops to a more normal 8 to 12 thousand feet through the
week.

High pressure north of the state will maintain breezy trade winds
into Thursday. Showers will fall mainly over windward and mountain
areas as the trade winds push limited moisture into the windward
slopes. The upper level ridge will maintain a stable atmosphere and
rainfall amounts will be limited.

A front will approach from the NW by the end of the week and winds
will start to weaken ahead of the front late Thursday, becoming
light and variable over the islands Thursday night through Saturday.
A diurnal weather pattern of nighttime land breezes and afternoon
sea breezes will prevail during this period. The sea breezes and
land heating will create some afternoon clouds and showers, but the
aforementioned upper ridge will keep showers at a minimum.

The front is expected reach Kauai Saturday, Oahu and Maui County
Saturday night, and the Big Island Sunday. The front will bring
widespread showers, but moisture along the front will likely be too
shallow for heavy rain. Breezy (but not as strong as last weekend) N
to NE winds behind the front will push moisture into N and E facing
slopes, bringing showers to those upslope areas. A new high will
move E behind the front, maintaining breezy trade winds through the
first half of next week.

&&

.AVIATION...
Evening satellite and radar data along with surface observations
show improving conditions across windward sections of the smaller
islands from West Maui through Kauai, as a drier airmass filters
southward into the state. AIRMET Sierra for mountain obscuration for
these areas will likely be dropped later this evening. Showers and
clouds will remain more prevalent across windward sections of the
Big Island through much of the night, with conditions expected to
improve there by Wednesday morning. AIRMET Sierra for IFR is
currently posted on the windward Big Island, but may give way to an
AIRMET for mountain obscuration later tonight.

VFR conditions are expected to prevail statewide from mid morning
through the afternoon on Wednesday, with only isolated MVFR
conditions possible on the windward slopes. Trade winds are expected
to strengthen slightly on Wednesday, and AIRMET Tango may be
required for moderate turbulence leeward of the mountains.

A few reports of moderate mid-high level turbulence near the islands
have been received this evening, as a mid level trough moves through
the area. AIRMET Tango for moderate turb may be needed overnight.

&&

.MARINE...
The latest observations from the nearshore PacIOOS buoys have begun
to reflect the overdue long-period northwest swell over the past few
hours. The offshore buoys northwest of Kauai have climbed to 12 ft,
which could continue to rise a few more feet this evening before the
peak is realized. The ECMWF-WAVE solution seems to be handling the
timing and magnitude of this swell the best compared to the
WAVEWATCH III and ensemble guidance and depicts a peak late tonight
through Wednesday. This will translate to advisory-level surf along
the exposed north and west facing shores continuing through
Wednesday night. If the swell comes in slightly larger than
anticipated at and around its peak, low-end warning-level surf will
become a possibility, most likely occurring early Wednesday. This
episode will trend down Thursday and Thursday night.

This downward trend will be short-lived due to a the very active
pattern in place across the northwest Pacific. Over the weekend,
back-to-back overlapping large swells will impact the local waters
and exposed beaches. The first episode is expected to fill in Friday
and Friday night (320 deg), then peak Saturday into Saturday night.
Similar to this current swell filling in, the ECMWF-WAVE solution is
depicting a later arrival time than the other wave model solutions
through this time. Advisory-level surf will be expected with this
source beginning as early as late Friday and potentially continuing
into Sunday. The second, much larger, source (330 deg) is expected
to fill in late Sunday and peak Sunday night into Monday in response
to a captured fetch that will near the region Sunday night in the
wake of a cold front. This episode could peak in conjunction with
fresh to strong northerly breezes in the wake of the front locally.

A combination of onshore winds (northerly) and well above warning-
level surf associated with this second source late Sunday into
Monday could lead to some minor overwash onto the typical vulnerable
roadways along the north and west facing shores, especially at and
around the times of high tide. This lines up well with the latest
PacIOOS wave run-up forecast.

Fresh to strong trade winds (highest in the channels) will continue
through Wednesday night before trending down into the light to
moderate range Thursday through early Saturday. Land and sea breeze
conditions can't be ruled out across the smaller islands through
this time Thursday through early Saturday. Strong northerly winds
are expected to fill in late Saturday in the wake of a cold front
and persist through Sunday night before veering toward the east
early next week as high pressure builds north of the state.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Thursday for North and West facing
shores of Niihau Kauai Oahu Molokai and for North facing shores of
Maui.

Small Craft Advisory until 6 AM HST Thursday for all Hawaiian waters.

High Wind Watch from 6 AM HST Wednesday through late Wednesday night
for Big Island Summits.

&&

Discussion...Birchard
Aviation...Jacobson
Marine...Gibbs