Flash Flooding and Winter Storm in Hawaii, February 27 to 28, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
1020 AM HST Mon Feb 27 2017

.SYNOPSIS...
The combination of an approaching upper level trough and a moist,
unstable airmass over the state will lead to an increasing chance
for heavy showers and thunderstorms. This will start with
localized activity this afternoon, but becoming more widespread
tonight and Tuesday as a surface low begins to develop in our
vicinity. The low will shift north of the islands by Thursday and
showers will diminish, though a southerly kona flow will keep
conditions rather humid and voggy for the rest of the week.

&&

.DISCUSSION...
Our trade wind flow is more ESE than usual this morning due to a
weak surface ridge to our E and a broad weak low to our WNW out
near 175W. This has allowed deeper moisture lurking to the S of
the islands to gradually begin advecting up the chain. At upper
levels, there is a rather potent trough around 172W, with a broad
area of relatively cold mid-level air that dominates a rather
large area of the central Pacific.

The upper trough will continue to dig and push slowly E the next
few days, as deeper moisture continues to pull northward. This
will induce a surface trough W of the islands on Tue, leading
toward a weak surface low somewhere in our vicinity on Wed. The
models are struggling with the details of this feature, but in
general they all show an increasing threat for heavy downpours and
thunderstorms at various spots over the state. Initially the
threat will be rather localized, and for today might be limited to
a few spots of active afternoon convection. However, a more
widespread threat may loom Tue into Tuesday night as moisture and
instability maximizes and the upper trough pushes through. We are
concerned about the possibility of a slow-moving or training
N-to-S band of heavy showers and thunderstorms setting up on the
eastern flank of the low, in addition to orographic anchoring.
This has led us to issue a Flash Flood Watch statewide starting at
noon and lasting through daytime Wed. In addition, any deep
convection near the Big Island will have cold enough temperatures
aloft to produce heavy snowfall above about 11000 feet or so, so a
Winter Storm Watch remains in effect for the Big Island summits
starting this evening.

By Wed night the trough will start to pivot to the NE of the
islands. The airmass will stabilize somewhat, but deeper moisture
may remain in place over eastern parts of the chain. We should see
activity settle down, but probably still some lingering convection
near Maui and the Big Island. There is a possibility we may
eventually have to extend the Watch for those areas, but this is
far out in time, and we need to see how the event evolves in the
meantime.

The surface low will shift N of the islands Thu and beyond,
leaving the islands in a relatively moist southerly kona flow
which will probably keep us somewhat humid, voggy, cloudier, and
wetter than usual. A weak surface ridge building over the eastern
part of the chain by Friday should help to keep showers somewhat
less active than the midweek activity.

Another front approaches the islands this weekend with another
increased chance of showers. It does not look like this front
will have nearly as much upper level support, however.

&&

.AVIATION...
An approaching mid to upper level trough centered near 174W will
steadily destabilize the atmosphere today. This feature is already
triggering isolated thunderstorms to the northwest, west and
south of the islands this morning, and given the expected
weakening of the minimal cap seen in the overnight Lihue sounding
around 10000 ft, isolated thunderstorms have been added to all
areas for the afternoon.

Deeper moisture is currently in place near the Big Island and
Maui. AIRMET Sierra for mountain obscuration is in effect for east
Maui and could be expanded to the Big Island by early afternoon.
As easterly trade winds spread the moisture westward, further
expansion of the AIRMET is likely later today or tonight. These
conditions could affect PHTO and PHNY today, while the remaining
TAF sites are expected to remain under VFR conditions.

As the mid to upper level trough edges closer to the islands
tonight, thickening high clouds will likely lead to icing
concerns aloft.

&&

.MARINE...
The current west-northwest (300-320 deg) swell is a bit larger
than expected this morning and the surf forecast for north and
west facing shores has been updated. This swell will ease into
the early portion of the week with surf remaining below high surf
advisory levels. A small reinforcement out of the west- northwest
(300 deg) associated with a recent gale over the far northwest
Pacific is forecast to fill in Tuesday through mid week.

A moderate northeast swell (030 deg) associated with a recent gale
that was centered around 1000 nautical miles northeast of the
islands will continue to fill in across the local area today and
remain in place through Tuesday night before easing. This swell
will peak through today and generate near advisory- level surf
along east facing shores.

Moderate to strong breezes out of the east-southeast direction will
continue through tonight, resulting in a small craft advisory for
the Pailolo and Alenuihaha channels and the waters south of the
Big Island. Some uncertainty remains in the forecast Tuesday
through mid week, with regard to the local winds, as the models
continue to depict a developing area of low pressure or trough
over/near the state lifting northward. Plenty of moisture and
instability associated with this feature, and an upper trough,
will keep locally heavy showers and a chance of thunderstorms in
the forecast through Wednesday.

For the extended, the latest deterministic and ensemble guidance all
depict a gale to storm-force low developing a few hundred miles west-
northwest of Midway Wednesday through Thursday, then tracking toward
the north-northeast across the Date Line Thursday. Despite some
model differences shown with this evolving feature, a large west-
northwest swell will be possible across the local waters this weekend
that may be large enough to generate advisory- level surf.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Wednesday afternoon for all Hawaii
islands.

Winter Storm Watch from 6 PM HST this evening through late
Tuesday night for Big Island Summits.

Small Craft Advisory until 6 AM HST Tuesday for Pailolo Channel-
Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward
Waters-Big Island Southeast Waters.

&&

Discussion...R Ballard
Aviation...Wroe
Marine...Morrison