Flash Flooding and Heavy Rain in Hawaii, February 10 to 12, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
848 PM HST Fri Feb 10 2017

.SYNOPSIS...
A strong cold front will approach from the west and reach Kauai
toward mid Saturday morning, and continue eastward on to Oahu and
Maui Saturday afternoon. The front is expected to stall and dissipate
over Maui County Saturday night and Sunday. Breezy southwesterly
winds are expected along and ahead of the front with frequent showers
including a chance of thunderstorms. Some of these shower and storms
will have heavy rainfall. The wind dies down and the showers lessens
after the front dissipates. Another cold front, although weaker, is
forecast to bring a shot of showers to the smaller islands next week
Tuesday and Wednesday.

&&

.DISCUSSION...
A cold front is located 210 miles northwest of Kauai this evening,
just west of buoy 51001 and 51101 as of 7 pm HST. Over Kauai, breezy
and gusty low level southwest winds of 30 knots are being noted on
the derived radar winds from the Kauai NEXRAD. The southerly winds
will be strengthening over the other smaller islands at the progresses.
Satellite imagery shows lots of layered clouds covering the smaller
islands. Radar imagery shows one batch of showers moving through
Kauai with more stratiform type rains following it. There also a
thunderstorm about 110 miles southwest of Port Allen Kauai that we
will be watching closely as the storm moves northeastward at around
25 mph. Conditions will worsen further especially Kauai County with
increasing showers and thunderstorms for the rest of the night.

A measure of of the instability of the atmosphere for severe weather
or CAPE, is expected to climb further more the rest of tonight across
Kauai and Oahu. Although we do not expect a thunderstorm to become
severe over land, there is a good bet to support strong thunderstorms
over the surrounding waters of these two counties the rest of tonight,
and may spread to Maui County on Saturday. The CAPE values starts
lowering Saturday evening as the dynamics starts to lift northward.

Thus far, the speed of the front is about 14 mph, and is a bit slower
that what models were saying. Latest GFS has the front reaching Kauai
around mid morning Saturday, passing through Lihue at 11 am HST, and
Oahu early Saturday evening around 8 pm HST. The dynamics lifts north
rapidly and the front is slated to gradually fall apart across Maui
County Saturday night into Sunday.

Currently, there have been passing showers for the other smaller
islands so far this evening, but more is expected for the rest of the
night, especially across Oahu. Some of the showers are falling from
the thick layered clouds running well ahead of front. We will be
watching closely for the Big Island summits where these layered
clouds may pose a threat for precip earlier than expected. The weather
there, other than for the warning level winds, will hold up overnight,
but may settle in sooner than Saturday night with short spurt of
possible snow showers. But other than that, the Big Island is on tap
to stay dry.

We will also be looking at a possible wind advisory for Maui's
Haleakala summit for Saturday.

The weather pattern will be evolving into a giant cyclonic gyre
across the entire North Pacific Ocean over the weekend, where the
main Hawaiian Islands will be under a persistent southwesterly wind
flow to until the next front arrives which is about Tuesday night
at the earliest. A moderate to locally strong southwest wind flow
will reestablish across the area by Sunday night with a bit of
strengthening Tuesday a few hours before the arrival of the next
front.

&&

.AVIATION...
A cold front will approach the island chain from the west tonight,
then move through Kauai Saturday morning and through Oahu late in
the afternoon or early in the evening on Saturday. A band of heavy
showers and embedded thunderstorms will spread from west to east
down the island chain, bringing widespread MVFR cigs/vsbys and
periods of IFR conditions to most of the islands. Prevailing VFR
conditions are expected across the Big Island through 12/06z.

AIRMET Sierra is in effect for mountain obscuration across Kauai
and Oahu. Conditions are expected to continue through at least
Saturday morning. The AIRMET may need to be expanded to include
Molokai, Lanai and Maui later tonight.

AIRMET Tango is in effect for moderate mid and upper level
turbulence across the entire island chain. These conditions are
expected to remain in place through the day on Saturday. The
AIRMET is also in effect for moderate low level turbulence over
and north thru east of the terrain on Kauai and Oahu. The AIRMET
for low level turb may need to be expanded to include Molokai,
Lanai and Maui later tonight.

AIRMET Zulu is in effect for moderate icing between 140 and FL260
for all islands except the Big Island. The Airmet is expected to
remain in place into Saturday morning for these areas, and may
need to be expanded to include the Big Island later tonight.

&&

.MARINE...
Marine conditions will continue to deteriorate through the evening
and overnight hours as a cold front approaches and moves into the
area. The forecast has been updated this evening to include the
thunderstorms that have recently begun to develop over the Kauai
waters, which should continue to increase in coverage and expand
eastward toward the Oahu and Maui County waters through the
overnight hours into Saturday. Frequent lightning, gusty winds in
excess of 34 knots and reduced visibilities in and around any heavy
showers or thunderstorms will be possible.

In addition to the deteriorating weather expected as the front moves
into the area, a moderate west-northwest swell (300 deg) associated
with a storm-force low that developed east of Japan earlier this
week will continue to rise at the buoys this evening and peak late
tonight through Saturday before trending down. This source will
support advisory-level surf along north and west facing shores
through Saturday night before trending down. This swell combined
with advisory-level winds out of the southwest generating short-
period/choppy seas locally has led to rough boating conditions,
which will persist through Saturday before trending down.

Conditions will steadily begin to improve as the front stalls and
dissipates later in the weekend and the west-northwest swell eases.
Local winds are forecast to shift out of the west across the Kauai
and Oahu waters behind the front at light to moderate speeds through
Saturday night then trend moderate to locally strong southwest winds
on Sunday.

For the outlook next week, the active period across the northwest
Pacific is forecast to continue. The latest ensemble and
deterministic wave model guidance remain in decent agreement with
the large scale pattern and depicts a very large west-northwest (310
deg) swell filling in late Monday and persisting through mid week
before trending down. Fresh to strong southwest winds will remain
likely Monday through Tuesday as another cold front moves into the
area.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch starting at midnight HST tonight through Saturday
afternoon for Niihau Kauai Oahu.

High Surf Advisory until 6 AM HST Sunday for north and west facing
shores of Niihau Kauai Oahu Molokai, and north facing shores of
Maui, and west facing shores of the Big island.

High Wind Warning until 6 AM HST Sunday for Big Island Summits.

Small Craft Advisory until 6 AM HST Sunday for waters west of
Molokai and the windward waters of Maui County.

&&

Discussion...Lau
Aviation...Jelsema
Marine...Gibbs