High Surf Advisory 20, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
346 AM HST Tue Feb 14 2017

.SYNOPSIS...
A cold front will approach from the northwest today, with a few
showers possible over mainly leeward areas in the southwesterly
flow ahead of the front. The front will bring an increase in
showers as it pushes into the western islands tonight and into
the central islands on Wednesday. The front will then dissipate
Wednesday night, with weak high pressure building just north of
the islands Thursday through Friday, bringing drier conditions
back to the state. The next cold front will push down the island
chain over the weekend, with breezy to windy trades building in
behind the front as strong high pressure builds north of the
island chain.

&&

.DISCUSSION...
Currently at the surface, a cold front is located around 325
miles west-northwest of Kauai, with an east to west oriented high
pressure ridge axis located around 200 miles south of the Big
Island. This is resulting in moderate to locally breezy
southwesterly winds across the island chain early this morning.
Infrared satellite imagery shows partly cloudy skies in place in
most areas, with some enhanced cloud cover mainly over leeward
areas of the Kauai and Oahu in the convergent southwesterly flow
in advance of the front. Meanwhile, radar imagery shows some
isolated to scattered showers moving into leeward sections of the
smaller islands, with rain free conditions over windward locales.
Main short term concern for today revolves around the need for a
Wind Advisory across portions of the smaller islands.

Today,
The cold front will continue to approach the state from the
northwest, with a breezy southwesterly flow expected to continue
across the island chain. A sharp low level inversion is evident
around 3.5 kft in the 12Z PHLI sounding along with 30 to 35 knot
west-southwesterly boundary layer flow. The very stable
conditions and strong boundary layer flow should help to
accelerate winds downslope across the windward areas of Oahu and
Kauai as well as funnel winds through the central valley of Maui
today. As a result, a Wind Advisory has been issued through 6 pm
this evening for these areas. As for rain chances, the airmass is
expected to remain quite stable through this evening, and this
should limit the shower activity across the area. Isolated to
scattered showers will be possible in the convergent southwesterly
flow in advance of the front, but the stable conditions should
limit the shower activity primarily to leeward areas.

Tonight through Wednesday night, The cold front will be in a
weakening state as it moves into Kauai tonight and approaches Oahu
by early Wednesday morning. The front will continue to push
eastward and into Maui County by late Wednesday afternoon,
dissipating over Maui County by late Wednesday night. A weakening
band of showers will move through the western and central islands
in association with the front, but with the lack of any
significant forcing mechanism for ascent, the showers should
remain on the light side.

Thursday through Friday,
A drier and more stable airmass will re-build over the state for
the end of the work week, as weak high pressure builds just north
of the island chain. This will allow light to moderate trade winds
to return Thursday and Thursday night, before weakening and
becoming variable on Friday in advance of the next cold front.
Shower chances are expected to remain low through the period, with
the best chances for rainfall expected across windward sections
of Maui and the Big Island.

Friday night through next Monday,
A cold front will approach from the northwest Friday night, then
move quickly southeastward down the island chain Saturday and
Saturday night, before stalling out near or just south of the Big
Island Sunday into early next week. A band of showers will likely
accompany the front as it moves down the island chain, with
windward and mauka areas having the best chances for rainfall,
particularly over Maui and the Big Island late in the weekend into
early next week. The more notable change in the weather across the
state will be the return of strong trade winds beginning as early
as late Saturday afternoon across Kauai. The strong trades will
then spread down the island chain Saturday night and continue
through early next week, as strong high pressure builds north of
the island chain.

&&

.AVIATION...
An extremely low inversion height will bring continued stable and
dry weather conditions to the Hawaiian Islands over the next 12
hours. Strong west to southwesterly winds below this inversion
will require AIRMET Tango for moderate turbulence today. Increasing
clouds are moving into the northwestern side of the state ahead of
an approaching cold front. A few areas may see periods of MVFR
ceilings due to low inversion capped clouds. The frontal band will
move through the islands from northwest to southeast Tuesday night
and Wednesday with scattered MVFR and isolated IFR conditions in
low clouds and rain showers.

AIRMET Tango issued for Tempo Moderate Turbulence below 050 for
all islands except the Big Island. Conditions will continue beyond
1600Z.

&&

.MARINE...
A very large west-northwest swell will gradually fill in across
the Hawaiian waters this morning, peak this afternoon into tonight
before steadily trending down Wednesday through Friday. Surf will
rise well above the warning threshold of 25 ft along the north
facing shores later today through tonight at its peak. Wave run-up
and minor overwash onto exposed coastal roadways could become a
possibility at and around the peak, which is supported by the
latest PacIOOS wave run-up guidance for the North Shore. The swell
direction associated with this source has enough west in it to
impact the leeward coast of the Big Island, mostly from Kona to
South Point. This will likely begin to fill in along this coast by
this evening and generate advisory-level surf tonight through
Wednesday. As a result, a High Surf Advisory will likely be added
in later forecast packages to include Kona to South Point.

In addition to the warning-level surf discussed, fresh winds out of
the southwest are expected today as a cold front approaches. These
winds will generate short-period choppy seas across the waters,
especially over the waters that are typically protected during a
normal trade wind pattern, such as Maalaea Bay and the leeward
areas.

Conditions will improve Thursday through Friday as the winds and
seas trend down. Another large west-northwest swell will fill in
over the weekend and likely generate advisory-level surf along north
and west facing shores once again. Strong trade winds will return
Saturday afternoon through Sunday in the wake of a cold front as
high pressure builds north of the state, which will support Small
Craft Advisory conditions over portions of the waters.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST Wednesday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

Wind Advisory until 6 PM HST this evening for Kauai Windward-
Oahu Koolau-Olomana-Maui Central Valley.

Small Craft Advisory until 6 PM HST Wednesday 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-
Pailolo Channel-Alenuihaha Channel-Big Island Windward Waters-
Big Island Southeast Waters.

&&

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Gibbs