Flash Flood and Heavy Rain in Hawaii, December 30 to 31, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
953 AM HST Fri Dec 30 2016

.SYNOPSIS...
A slow-moving front will drift southward and stall over Maui
County tonight, then dissipate Saturday night. Numerous showers
will continue along the front, mainly across windward areas, but
breezy trade winds will carry some showers over into leeward areas
of the smaller islands. An upper level trough will approach from
the west today, then pass over the islands tonight and Saturday,
and could produce some locally heavier showers. Weak high pressure
will build north of the islands behind the front, with trade winds
gradually weakening during the weekend and into early next week.

&&

.DISCUSSION...
A weak, slow moving cold front southwest is draped over Molokai
this morning. The front is characterized as about a 200 mile wide
band of overcast low clouds with numerous showers carried east to
west by the overrunning trade winds. Radar over Hawaii shows
numerous showers stretching from windward sections of Oahu to
Maui, with the axis of highest showers over Molokai. Trade winds
are focusing more of the showers onto windward and mauka areas,
with many of the showers reaching the leeward sides of the
islands. Rainfall amounts are generally light to moderate in
intensity. Meanwhile, Kauai and the Big Island are relatively dry
this morning.

Infrared satellite shows abundant high level cloudiness streaming
across the state, due to an upper level trough located several
hundred miles west of the state, and an associated upper level jet
stream passing just south of the islands. The 12Z soundings
reflected the rather shallow moisture along and ahead of the
front, with very dry conditions persisting in the mid levels in
the 12K-25K foot layer, between the low-level frontal moisture and
the upper-level moisture streaming overhead. PW values also remain
unimpressive with only 1.3 inches at Lihue and 1.22 inches at Hilo.

The front will drift another 15 or 20 miles to the southeast and
stall out over Maui County tonight, and slowly dissipate through
Saturday night.

The upper trough to our west will shift eastward and push the
cold pool aloft over the state. This may start to enhance showers
along the front this afternoon and tonight with a few locally
taller and heavier showers becoming possible. However, there are
some limiting factors for heavy rainfall including relatively low
PW values indicated from the soundings, the persistent mid level
dryness, and continued fast motion of the showers due to the trade
winds. An isolated thunderstorm cannot be totally ruled out, but
the chances still appear to be quite low. Thunder is not included
in the forecast at this time. Snow accumulations may be possible
on the Big Island summits by tonight as the cirrus clouds deepen
down the summit levels as the upper trough approaches.
Temperatures at the summits are in the upper 20s and will be cold
enough to support snow as the layered moisture fills in. We will
likely be issuing a Winter Weather Advisory today. Models show the
layered moisture departing east of the Big Island Saturday.

By Saturday afternoon and evening, the cooler/drier airmass will
spread across central parts of the state, as the upper trough
moves east of the islands. Shower activity should decrease during
this time as the upper support exits and moisture along the old
frontal boundary becomes increasingly shallow. Trade winds will
also be weakening during this time with only a weak high to our
north, sandwiched in between a front far to the northwest and a
meandering surface low far to the northeast.

Lingering frontal moisture should be pushed south of Maui County
on New Years Day, with scattered light showers possible over
windward areas of the Big Island, and mainly dry conditions
elsewhere. Gentle trade winds are forecast to prevail through
the first half of next week as a weak surface ridge lingers just
north of the state. Another weak front may drift southward into
the islands toward the end of next week, but models have not been
consistent with the timing or southward movement of this feature,
so it is too soon to determine if it will affect us directly.

&&

.AVIATION...
Moisture associated with a stalling front is E-W oriented over
the area from Oahu through Maui County, with radar detecting
abundant showers, with cloud tops up to 15 kft. The front is
bringing areas of MVFR VIS/CIG in SHRA, and ISOL IFR VIS/CIG in
+SHRA, primarily to windward waters, coasts and slopes, although a
few showers are briefly producing ISOL MVFR conditions as they
spread leeward. AIRMET SIERRA for mountain obscuration is in place
for the windward slopes from Oahu to Maui.

Breezy NE winds lie along and N of the front, and these are
currently affecting Kauai, and to a lesser degree Oahu, where
AIRMET SIERRA remains in place for low-level turbulence to the
lee of the terrain. Moderate high-level turbulence continues over
all islands due to an approaching trough aloft W of the area. The
trough is prompting overcast high clouds to stream over the
islands in the WSW flow aloft, but these are limited to the upper
levels of the island atmosphere, as latest sounding data indicate
that a significant dry layer exists between about 12 kft and 25
kft. The presence of this dry layer should prevent the development
of thunderstorms when the trough moves over the island chain later
today and tonight, but some of the clouds/showers in the stalling
frontal band could grow taller/more intense, especially E of the
islands.

&&

.MARINE...
North and east shore surf will diminish through the day. The
Hanalei/Waimea buoys showed seas peaking overnight around 14/15
feet, and have lowered to 12 feet so far this morning. The
Mokapu/Pauwela/Hilo buoys are near peak around 11/12 feet. Seas
remain higher than the latest WaveWatch III guidance near the
islands, but the overall model trend is steadily downward through
tomorrow. With the lower heights already observed near Kauai/Oahu,
we should be able to downgrade the east shore high surf warning
and discontinue the north shore advisory, but will wait for the
morning surf reports to confirm. When these are dropped, we may
need to extend the east shore advisory into tonight as well.

The overnight ASCAT pass clipped the waters west of Kauai, and
showed a solid area of 25-30 knots behind the front. The front
continues to weaken as it moves southward. High resolution model
guidance shows winds dropping off as well, but remaining above
small craft levels for many areas into tonight. In addition, the
combination of wind waves and the north-northeast swell will keep
seas above 10 feet for some zones into early Saturday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for north and east
facing shores of Maui, and north facing shores of Niihau, Kauai,
Oahu, Molokai and the Big Island.

High Surf Warning until 6 PM HST this evening for east facing
shores of Kauai and Oahu.

Small Craft Advisory until 6 PM HST this evening 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-
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Windward Waters.

&&

DISCUSSION...Foster
AVIATION...Morrison
MARINE...Bravender