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Hawaiian Islands Synoptic Discussion and Guidance

431
FXHW60 PHFO 161404
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
404 AM HST Fri Nov 16 2018

.SYNOPSIS...
Surface high pressure building far north-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 reach the islands around the
middle of next week.

&&

.DISCUSSION...
A 1026 mb surface high near 34N 166w, or about 950 miles north-
northwest of Lihue, is moving toward the east-southeast at about
15 mph. This feature is causing the pressure gradient to tighten
across the state early this morning, which is causing the
northeasterly trade winds to gradually strengthen. In addition,
the surface high is pushing a weakening frontal boundary down
toward the state. The diffuse leading edge of the band of broken
to overcast low clouds and numerous showers associated with the
front is located slightly less than 200 miles north-northwest of
Lihue. This leading edge has been moving toward the south-
southeast at 5 to 10 mph during the past six hours.

Aloft, a middle tropospheric trough appears to be near the
western end of the island chain, while a broad upper tropospheric
trough is digging down toward the region from the northwest. Even
though the trough aloft near the islands is producing some
instability, the mid-layers of the atmosphere remain relatively
dry. Low clouds and showers being transported into the islands
by the strengthening low-level trade wind flow are affecting
windward sides of most islands. A few of these showers are also
passing across some leeward sections of the smaller islands.
In addition to the low clouds, high clouds continue to stream
over the area from the southwest ahead of the digging upper-level
trough.

The forecast guidance continues to indicate the trades will become
locally breezy by mid to late morning as the surface high builds
toward the east-southeast. At the same time, trade showers are
forecast to continue over windward sections. The slight
instability due to the troughs aloft may produce some brief heavy
rainfall. However, the showers will likely be fast morning, so no
significant flooding is forecast at this time. We will need to
monitor radar and rain gages in case training of echoes
unexpectedly occurs. This scenario, if it occurs later today or
tonight, might require the issuance of a Flood Advisory or two
for localized ponding of water on roads or elevated flow in some
streams and drainage systems over some windward sections.

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 tonight, 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 and mauka 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 new high building behind this front. The
trailing end of the front will likely reach the islands around
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 today 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 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 today.

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 nearing the
advisory level (10 ft) across the northwest waters this morning
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 has been extended through
Sunday for the typically windier waters.

Surf along north and west facing shores will quickly rise this
morning 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 14-16 second bands
(320-330 deg) at the northwest offshore buoys and the nearshore
PacIOOS Hanalei buoy. The swell ended up coming in a couple of
feet higher than predicted at the offshore northwest buoys this
morning, which should be reflected at the exposed nearshore buoys
today. Surf is forecast to reach and exceed the advisory
thresholds for north and west facing shores of the smaller islands
today, then hold through tonight 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 associated with this feature should
remain below the advisory levels along north and west facing
shores as it fills in and 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 Sunday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory until 6 PM HST Saturday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters.

&&

$$

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

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office