High Surf in Hawaii, November 15 to 17, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
828 PM HST Thu Nov 15 2018

.SYNOPSIS...
Surface high pressure building northwest of the area will cause
the trade winds to strengthen across the islands through early
next week. An upper-level trough passing near the state may cause
increasing instability and wetter conditions as moisture
associated with a weakening frontal band moves down across the
area through this weekend. The frontal band is expected to push
south of the Big Island Sunday night. The trailing end of another
front may move over the islands around the middle of next week.

&&

.DISCUSSION...
A weak north-northeast to south-southwest oriented surface trough
appears to be just north of the central Hawaiian Islands early
this evening. The leading edge of a band of broken to overcast low
clouds and numerous showers assoicated with a weakening frontal
boundary is located about 250 miles north-northwest of Lihue. This
leading edge has been moving toward the south-southeast at about
10 mph during the past six hours. The surface trough and the front
continue to keep the pressure gradient relatively relaxed across
the region early this evening. This is producing light winds
across most areas for now. However, a 1026 mb surface high near
34N 168w, or nearly 1000 miles northwest of Lihue, is moving
toward the east at about 15 mph. This feature is expected to
gradually cause the pressure gradient to tighten across the state
later tonight and Friday as it moves closer to the area.

Aloft, a middle tropospheric trough is evident near Kauai, while
a broad upper tropospheric trough is digging down toward the
region from the northwest. Even though the trough aloft near Kauai
is producing some instability, the mid-layers of the atmosphere
are quite dry. There are also high clouds streaming over the area
from the southwest ahead of the digging upper-level trough. This
continues to produce mostly cloudy skies the islands. However,
rainfall remains minimal across the state early this evening.

The forecast guidance continues to indicate the trades will become
locally breezy starting Friday as the surface high builds
eastward. At the same time, showers are forecast to increase over
windward sections. Note that even though the models indicate
unstable conditions will develop due to the troughing aloft, the
most recent forecast has removed any mention of thunderstorms and
heavy rainfall. We will be monitoring the situation overnight in
case there need to be updates to the short-term forecast.

As we head in to this weekend, the surface high will likely push
the weakening frontal band down toward the islands. Based on the
latest guidance, we expect the remnant band of low clouds and
showers will reach Kauai late Friday night, and then Oahu Saturday
morning. After that, it may move over Maui County Saturday
evening and the Big Island Saturday night. The band is expected to
move southeast of the Big Island Sunday night. Numerous showers
can be expected over windward areas as this band passes. In
addition, the strong trades will likely transport scattered
showers over many leeward sections of the smaller islands.

The surface high will gradually weaken early next week. At the
same time, a new front is forecast to approach the region from the
northwest due to a strong new high building behind this front.
The trailing end of the front will likely move over the islands
during the middle of next week, which will produce a wet trade
wind weather pattern.

&&

.AVIATION...
High pressure will build northwest of the state resulting in
strengthening trade winds. High level clouds along the subtropical
jet will gradually shift eastward as an upper level trough moves
over the islands. These high clouds range from FL200-300, but may
thicken up later tonight along the western end of the band.
Moderate turbulence and light icing are both expected within this
cloud band. AIRMET Tango remains posted for tempo moderate
turbulence in FL200-300 Kauai to Maui.

The atmosphere will become more unstable later tonight as the
upper trough approaches. Combined with increasing trade winds,
this will cause showers to become more frequent over windward
sections, with areas of MVFR conditions and mountain obscuration
possible. AIRMET SIERRA may be required for mountain obscuration
later tonight.

No other AIRMETS are in effect.

&&

.MARINE...
Trades are forecast to increase across the island waters and hold in
the fresh to strong category through the weekend as high pressure
builds north of the state in the wake of a front. Trades may reach
the strong category across most waters Monday night through
Wednesday with near gales across the typically windier waters
between Maui County and the Big Island. Seas are forecast to reach
the advisory level (10 ft) across the northwest waters late tonight
through Saturday due to a combination of a new northwest swell
filling in and locally generated wind seas. As a result, the
small craft advisory currently in place will remain up and will
likely need to be expanded to other areas in later forecast
packages.

Surf along north and west facing shores will quickly rise overnight
as a new northwest swell fills in from a system that reached storm-
force category Tuesday night into Wednesday as it tracked east of
the Date Line around 40N. Buoy observations are already
reflecting this new source within the 14-18 second bands (320-330
deg) at the northwest offshore buoys and the nearshore PacIOOS
Hanalei buoy. Surf is forecast to reach the advisory thresholds
for north and west facing shores of the smaller islands by
daybreak Friday morning, then hold through Friday night before
slowly easing over the weekend. Expect this source to gradually
shift out of a more northerly direction through the weekend as it
fades.

A reinforcement out of the northwest is expected to fill in late
Monday, peak Monday night into Tuesday, then fade through midweek.
This source will be from a storm-force low that was depicted in
the latest analysis over the far northwest Pacific near the
western Aleutians. ASCAT showed a decent sized area of strong- to
gale-force westerly winds associated with it focused at the area
within the 300-320 directional band. Guidance remains in decent
agreement and shows this feature tracking eastward to the Date
Line come Saturday. Surf should remain below the advisory levels
along north and west facing shores as it peaks early next week.

Surf along east facing shores will gradually rise over the weekend
and early next week as the trades increase into the fresh to strong
category, especially next Tuesday as trades reach the strong
category across most waters locally. Limited fetch upstream of the
state, however, should keep the surf below advisory levels for east
facing shores through this time.

Small surf will continue along south facing shores with mainly
background southern Pacific pulses moving through.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Saturday for north and west facing
shores of Niihau, Kauai, Oahu, and Molokai, and for north facing
shores of Maui.

Small Craft Advisory until 6 PM HST Saturday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha
Channel-Big Island Southeast Waters.

&&

DISCUSSION...Houston
AVIATION...Powell
MARINE...Gibbs