Strong Wind and Heavy Rain in Hawaii, February 5 to 6, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
405 AM HST Sun Feb 5 2017

.SYNOPSIS...
High pressure will shift eastward and further away from the state
today as a cold front approaches from the west. Kona winds will
increase ahead of the front, with winds becoming breezy to windy
this afternoon through Monday. A band of heavy showers will
accompany the front as it pushes from west to east across the
island chain tonight through Tuesday. High pressure and drier
conditions will then overspread the state Tuesday night through
Thursday night, with another cold front expected to push down the
island chain late next week.

&&

.DISCUSSION...
Currently at the surface, a 1019 mb high is centered around 2150
miles east-northeast of Hilo, with the ridge axis extending
westward across the Big Island and south of the rest of the island
chain. Meanwhile, the cold front which will bring big changes to
the local weather, is located around 300 miles northwest of
Kauai. Partly cloudy skies are in place across most areas early
this morning, with some enhanced cloud cover and a few light
showers over the waters to the south of the islands and moving
into leeward areas. Main short term concern revolves around the
timing and impacts associated with the approaching cold front.

Today,
The high pressure ridge axis will settle slowly south and east of
the state as the cold front makes steady progress eastward toward
the island chain. The boundary layer will continue to moisten up
as a result of the increasing southerly flow, but mid-level
ridging will keep the low level inversion intact, limiting the
coverage of shower activity in the warm air advection regime. The
convergent low level southerly flow will allow for isolated to
widely scattered light showers to develop and push over the
islands through the day, but rainfall amounts are expected to
remain light. The more noticeable change in the local weather will
be the increasing Kona winds, with breezy to locally windy
conditions expected by this afternoon across much of the state.
High resolution model guidance continues to indicate that gusty
downslope winds of 40 to 45 knots (45 to 50 mph) are likely over
and downwind of the higher terrain on Oahu and Kauai, and this
lines up well with the current Wind Advisory. Although breezy
conditions are expected across the other islands, winds are
expected to remain below advisory thresholds through the day.

Tonight through Tuesday,
The cold front will continue to approach the state from the west
tonight, then steadily push eastward through the island chain
Monday through Tuesday. The front is expected to move through
Kauai Monday morning, Oahu late Monday afternoon into early Monday
evening, Maui County Monday night, and the Big Island during the
day on Tuesday. A band of heavy showers with gusty winds is
expected to accompany the front as it moves through the islands,
with thunderstorms not entirely out of the question. The
probability for thunder continues to appear low, but with surface
based CAPE values around 500 J/KG and 0-6 KM bulk shear values
between 35 and 45 knots (supportive of organized convection), we
will need to keep a slight chance of thunderstorms in the
forecast. The showers are expected to be moving fast enough to
limit the potential for any significant flooding, although some
ponding of water and some minor flooding of poor drainage areas
may be possible in areas which see some good downpours.

Breezy to locally windy conditions are expected along and ahead
of the front, with winds decreasing and shifting around to the
west and northwest following the frontal passage. The current Wind
Advisory is in effect through 6 PM Monday for Oahu and Kauai, and
this Advisory may eventually need to be expanded to include
portions of Maui County and the Big Island for the daytime hours
on Monday.

Additionally, it appears that the moisture will be deep enough
and temperatures cold enough to allow for some snow on the Big
Island summits late late Monday night through early Tuesday
evening as the front moves through. A few inches of snow is
certainly not out of the question, and a Winter Weather Advisory
may eventually be needed.

Tuesday night through Thursday night,
The cold front will exit to the east of the Big Island Tuesday
night, with high pressure then racing by to the north of the
islands Wednesday and Wednesday night. The area of high pressure
will shift quickly east of the state Wednesday night through
Thursday night, with the ridge axis on the western extent of the
high settling southward and over the island chain. This is
expected to result in a period of drier weather, with the best
chances for rain expected across windward sections of the Big
Island where some lingering moisture from the old front will
remain.

Friday through next Saturday,
The GFS and ECMWF both show another cold front moving through the
islands late in the work week and into the upcoming weekend,
although there are some timing differences between the two models.
For now, will show an increase in rain chances across the state,
and await better agreement before trying to pin down the details.

&&

.AVIATION...
An east-northeast to west-southwest oriented surface ridge near
the Big Island continues to shift slowly southward, as a front
approaches the state from the northwest. This will result in
strengthening south and southwest low-level flow across the state
through tonight.

The south to southwest low-level flow will likely carry low
clouds and scattered showers into the western islands. This may
produce periods of MVFR conditions over the higher terrain and
south through west sections of Kauai starting this morning, and
on Oahu starting later this afternoon. Low clouds and isolated
showers will also affect the mountains and south through west
sections of Molokai, Lanai and Maui, where brief periods of MVFR
conditions are possible today. Otherwise, VFR conditions are
expected to prevail across the remainder of the state today.

The increasing southwest winds and a low-level inversion (for now
well ahead of the front) will likely lead to TEMPO moderate low-
level turbulence north through east of the higher terrain of Kauai
and Oahu. As a result, AIRMET Tango will likely be required for
the western islands after sunrise this morning. Expect this AIRMET
to be expanded to include the islands of Maui County later today
or tonight. Also, as the southwest winds continue to strengthen,
pockets of isolated severe turbulence may eventually develop this
afternoon or tonight.

&&

.MARINE...
An active pattern across the northern Pacific is expected to persist
through the week, which will lead to significant impacts across the
Hawaiian waters. The latest surface analysis showed a cold front
approaching from the northwest that was located a few hundred
nautical miles northwest of Kauai and a broad 973 mb low
encompassing much of the northwest Pacific. A large swath of strong
to gale-force west-northwest winds was shown in the last several
ASCAT passes across this region extending west-northwest from the
aforementioned frontal boundary northwest of the local area to the
Kuril islands.

The latest model guidance has initialized well with this large-scale
pattern and depicts strong southwest winds developing across the
local waters today through Tuesday as the front approaches and moves
down the island chain. These winds combined with short-period,
locally generated, wind waves up to eight to 11 ft and a large
northwest swell will translate to dangerous boating conditions
through this time, even across the marine zones that are typically
protected during strong trade winds or from large northwest swells,
such as Maalaea Bay and the waters along the leeward coast of the
Big Island. In addition to the rough marine conditions, surf along
exposed south facing shores will rise to near advisory levels (8 ft)
tonight through Monday. Winds will shift out of the west-
northwest in the wake of the front Monday through Tuesday and
remain fresh. As a result, a Small Craft Advisory due to winds and
seas has been issued to cover this period.

A large west-northwest swell associated with the previously
discussed low over the northwest Pacific will begin to fill in and
build across the local waters tonight. This source will result in a
long-duration event with surf reaching advisory levels for exposed
north and west facing shores late Monday, warning-levels Tuesday
through Wednesday, before slowly easing Thursday through Friday.
Wave run-up issues will become a possibility through this period,
which could translate to overwash onto the typical vulnerable
coastal roadways Tuesday through Wednesday, mainly at and around the
times of high tide.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from noon today to 6 PM HST Monday for Kauai-Oahu.

Small Craft Advisory until 4 AM HST Monday 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.

&&

DISCUSSION...Jelsema
AVIATION...Houston
MARINE...Gibbs