High Surf in Hawaii, November 15 to 16, 2017

Area Forecast Discussion...Corrected
National Weather Service Honolulu HI
428 AM HST Wed Nov 15 2017

.SYNOPSIS...
High pressure far to the north of Hawaii will produce locally windy
trades through Thursday before easing somewhat Friday and Saturday.
A weak frontal boundary pushing down across the islands will
continue to bring wet weather to parts of the state today. While
most of the rainfall will be focused along windward facing slopes,
brief showers may spread to some leeward sections of the smaller
islands. Drier weather conditions are expected from Thursday through
Friday, with brief trade showers mainly over windward and mauka
areas. The trades will weaken later this week as a surface low
develops far north of the region.

&&

.DISCUSSION...
A 1034 mb surface high centered near 38N 161W, or about 1150
miles north of Honolulu, has been moving slowly southward early
this morning. This feature continues to push a diffuse frontal
boundary down across Maui County toward the northern Big Island
early this morning. The pressure gradient is becoming tighter
across the state. This is producing locally breezy northeasterly
trade winds across most areas, especially north of the weak
frontal boundary. Low clouds and showers associated with the
boundary are currently spreading down across the central islands,
especially along the windward facing slopes and beaches. Low
clouds and showers are also lingering over windward and mauka
sections of Kauai and Oahu, while the clouds and showers are now
approaching the windward Big Island. In addition, brief showers
are being transported over to some leeward sections of the smaller
islands from Kauai to Maui.

Aloft, a mid-tropospheric trough extends northeast from the
islands. The presence of this feature and the weak frontal boundary
has caused the low-level inversion to be elevated, or non-existent.
This instability combined with moisture along and ahead of the
boundary will maintain wet conditions today across most of the
state today, especially over windward and mauka sections. The
tightening pressure gradient will also produce locally windy trade
winds across the islands, especially after mid-morning.

After the remnant front passes through, the airmass is expected to
become relatively dry, with fewer showers expected for the
smaller islands starting later tonight or Thursday. In addition,
the strong trades and drier air (background dewpoints are expected
to drop to the lower 60s) will make it feel rather cool and
"fall-like" by Hawaii standards. The surface high will likely
begin to weaken as it shifts to a position far northwest of the
region toward the end of the work week. Therefore, we expect the
trades to weaken slightly from late Thursday through Friday.

Unfortunately, the global forecast models continue to have
divergent solutions for the large-scale weather pattern over the
Central Pacific Ocean this weekend. This means that we are
uncertain about the type of sensible weather conditions we might
experience in Hawaii from this weekend into early next week.

The ECMWF model remains the outlier, since it has been rather
consistent in developing a surface low far north of the region on
Thursday. The latest model output indicates this low will move
rapidly toward the south, and will stall north of islands near
Latitude 30N by late Saturday. Assuming this scenario verifies,
it appears that a front may move down across the islands some time
this weekend. This would be followed by a period of northwest to
north winds. Wetter weather conditions might also develop, with
rainfall amounts possibly becoming locally heavy over some areas.

In contrast, the GFS model appears to keep the surface low much
farther northeast away from the islands. This scenario would
likely cause north to northeast low-level flow to develop over the
islands starting some time this weekend. Our confidence in the
weekend forecast and beyond will remain rather low until we see
more definitive model solutions. At the moment, the forecast
beyond Friday essentially follows a blend of climatology and
multi-model output. We expect adjustments will be needed for the
forecast this weekend and early next week as the synoptic pattern
becomes more clear.

&&

.AVIATION...
A band of showery low clouds moving across the islands is
generating mountain obscuration across windward slopes this
morning. AIRMET SIERRA for mountain obscuration above 2000 feet
is in effect for the smaller islands now, but may need to be
expanded to include the Big island later. AIRMET TANGO is also in
effect for mechanical turbulence below 7000 feet over and
immediately south through west of mountains across the smaller
islands.

While VFR conditions dominate at the terminals now, MVFR ceilings
and visibilities are still possible as the showery cloud band
continues to move by.

&&

.MARINE...
Fresh to strong trades associated with 1034 mb high pressure
building southward over the central Pacific following a
dissipating frontal boundary that has moved into the area will
translate to rough boating conditions across the Hawaiian waters
through Thursday. A mix of wind seas, a small west-northwest (300
deg) swell and a, relatively short-period, north-northeast (000-
020 deg) swell has already translated to advisory-level (10 ft)
seas at most exposed nearshore PacIOOS buoys. Observations at
these buoys are running around 1-2 ft higher than predicted. The
local winds and seas should trend down temporarily Friday into the
upcoming weekend as another front approaches.

For surf, guidance supports the current active trend over the
northeast Pacific continuing due to a persistent blocking pattern
that remains established across the central Pacific. This pattern
will continue to feature storm tracks setting up over the Gulf of
Alaska from northwest to southeast, that will drive overlapping
north-northeast swells toward the islands through the rest of the
weekend and next week.

The current north-northeast (000-020 deg) swell is forecast to peak
today, then slowly ease through the second half of the week. This
source combined with the locally strong trade winds will support
rough beach conditions along east facing shores. The high surf
advisory remains in effect for these beaches through early
Thursday morning. A reinforcing north-northeast (010-020 deg)
swell is expected Friday through the upcoming weekend, that will
continue to generate advisory-level surf along east facing
beaches into the weekend.

Surf along north facing shores exposed to north-northeast swells
will remain up through the upcoming weekend, but should remain
below advisory levels each day through Saturday. Although
confidence remains low at this time, more significant northerly
swells will be possible Sunday into Monday and again by midweek.

Surf along south and west facing shores will remain small with
mainly long-period south-southwest swells expected.

&&

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

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

Small Craft Advisory until 6 AM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters.

&&

Discussion...Houston
Aviation...Powell
Marine...Gibbs