Heavy Rain in Hawaii, December 19, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
831 PM HST Tue Dec 18 2018

.SYNOPSIS...
A weakening front moving down the island chain is forecast to reach
the Big Island and stall through the day Wednesday. Showers
associated with the front will favor northern and windward sections
of the islands overnight into Wednesday, then potentially linger
over the eastern end of the state into Thursday as the boundary
stalls and diminishes. Gusty northerly winds along with drier and
cooler air will fill in across the state overnight through
Thursday behind the front. A return of a more typical trade wind
pattern will be possible Friday into the upcoming weekend. A
combination of increasing moisture and another front approaching
could translate to increasing rain chances next week.

&&

.DISCUSSION...
Gusty northerly winds along with drier and cooler air behind a cold
front will continue to fill in across the state from west to east
overnight into Wednesday. The front, currently moving past Oahu
this evening, should reach Maui County by/around midnight, then
to the Big Island Wednesday where it is expected to stall and
diminish. A shallow band of moisture accompanying this boundary
combined with northerly winds and the island terrain will continue
to support scattered to numerous showers for the northern and
windward sections of the islands as it moves through. The latest
rainfall summary showed peak totals exceeding 3" as of 7 pm HST at
Kilohana on Kauai over the past six hours and up to a half of an
inch on Oahu. Similar shower coverage will steadily fill in over
Maui County and the Big Island through the night and Wednesday as
the boundary continues down the island chain.

In addition to the shower coverage, northerly winds at Barking Sands
have already exceeded 30 mph with gusts topping out in the 40-50 mph
range this afternoon in the wake of the boundary. Expect the same
for the rest of the islands overnight through Wednesday behind the
front. This has resulted in a wind advisory for Kauai into Wednesday
morning, then for Maui County and portions of the Big Island (Kohala
and southeast Kau/Puna slopes) through the day Wednesday.

Temperatures may struggle to reach the 80 degree mark through the
afternoon hours Wednesday due to the drier and cooler air filling
in. Dewpoints are forecast to dip into the upper 50s through this
time.

A transition period is expected Thursday as the low-level flow
shifts around to a more typical trade wind pattern. Residual
moisture along the stalled boundary will support higher rainfall
chances lingering over the Big Island through the day Thursday.
Otherwise, guidance supports a return of a more typical trade wind
pattern Friday into the weekend with clouds/showers focusing over
windward areas.

Rainfall chances may climb once again early next week due to a
better source of moisture moving into the area and another cold
front approaching from the northwest. More specifics will come in
later packages as confidence rises.

&&

.AVIATION...
A weak front has been moving through the islands and bringing
breezy north winds and intermittent showers to most of the state.
Brief MVFR conditions will occur in those areas exposed to the
north flow, mainly over mauka and north- and east-facing coastal
sections.

An AIRMET for mountain obscuration is in effect for Kauai, and
soon Oahu. The other isles may be added later if warranted. Also,
an AIRMET for low level turbulence is posted covering the area to
the south and southwest of the terrain from Kauai to Molokai.

&&

.MARINE...
The cold front has cleared Oahu this evening, will move through
Maui County later tonight, then hang up in the vicinity of the
Big Island on Wednesday. Strong north-northeast winds will fill in
behind the front as it moves through. A Small Craft Advisory
(SCA) remains in effect for all Hawaiian waters through Wednesday
night due to the strong winds and seas greater than 10 feet. High
pressure will return well to the north of the islands Wednesday
through early next week, shifting the winds around to a more
typical east-northeasterly trade wind direction late Wednesday
and Wednesday night. Moderate to locally strong trades will then
continue Thursday through early next week.

The current large long-period north-northwest swell will peak
tonight and hold through Wednesday morning, then slowly decline
Wednesday afternoon through the end of the week. A High Surf
Warning remains in effect for exposed north and west facing
shores of the smaller island through 6 PM Wednesday. A High Surf
Advisory remains in effect for north facing shores of the Big
Island. The combination of strong north-northeast winds, above
normal tides, and the large north-northwest swell is expected to
result in above average coastal wave run-up tonight. This will
increase the risk for significant beach erosion and some minor
overwash onto coastal roadways around or shortly after midnight
when the peak of the high tide and the peak of the swell closely
align.

Advisory level surf will likely continue for the exposed north
and west shores of the smaller islands through the day on
Thursday. Along north facing shores of the Big Island the advisory
will likely need to be extended through Wednesday night. Surf is
expected to drop below the winter time average along north and
west facing shores Friday through the weekend. A new small to
moderate northwest swell will be possible early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 8 AM HST Wednesday for Niihau-Kauai Leeward-
Kauai Mountains.

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 from 5 AM to 10 PM HST Wednesday for Lanai-
Kahoolawe-Maui Central Valley-South Big Island-Big Island North
and East-Kohala.

High Surf Advisory until 6 PM HST Wednesday for Big Island North
and East.

Small Craft Advisory until 6 AM HST Thursday for all Hawaiian
waters-

&&

DISCUSSION...Gibbs
AVIATION...Kinel
MARINE...Jelsema