Flash Flooding and Heavy Rain in Hawaii, January 1 to 4, 2022

Area Forecast Discussion
National Weather Service Honolulu HI
834 PM HST Fri Dec 31 2021

.SYNOPSIS...
Southerly winds continue to stream unstable tropical moisture
across the islands this evening. Expect periods of rainfall for
all islands, heavy at times, lasting through the holiday weekend.
The highest chances for showers and isolated thunderstorms are
forecast over Oahu and Kauai through Sunday. Unsettled weather
conditions will linger on Monday as we slowly transition into a
drier weather pattern. Light winds are expected by Tuesday with
trade winds returning from Wednesday into Thursday.

&&

.DISCUSSION...
A low pressure system located roughly 300 to 600 miles west to
northwest of Kauai remains cut off from the main polar front jet
stream. This system is producing southerly flow, advecting moist
and unstable tropical air over the Hawaiian Islands this evening.
Radar imagery shows bands of showers moving up from the south with
the heaviest shower activity over Oahu and some additional
showers brewing just south of Kauai County. Snow levels are
hovering around 13,000 feet near the summit elevation level of
Mauna Kea and Mauna Loa on the Big Island. A Winter Weather
Advisory for snow remains in effect through New Year's morning.

Periods of rain showers will continue to affect all islands
through the holiday weekend as this low pressure system slowly
drifts northward. The highest rainfall activity this weekend will
focus over Kauai and Oahu. Expect elevated stream levels and
ponding of water on area roadways at times due to periods of moderate
to heavy rainfall activity. Localized flash flooding is possible
in some areas through Sunday, however the flash flooding threat
level at this point does not appear to be widespread.

On Monday and Tuesday, weather conditions will change as the low
pressure system drifts further northward away from the island
chain. Light winds and scattered showers are forecast on Monday
as rainfall trends begin to decrease. By Tuesday, drier air moves
into the state with light trade winds starting to return to the
region.

By Wednesday and Thursday, light to moderate trade winds will
develop across the state. Passing shower activity will favor the
typical windward and mountain sections of each island with drier
conditions over the leeward slopes.

&&

.AVIATION...
Moist south to southeast flow will bring periods of moderate to
heavy showers to the islands during the next 24 hours, with an
isolated thunderstorm not out of the question as well. MVFR
conditions will impact the smaller island terminals at times, with
brief IFR vsbys possible in heavy showers as they move through.
VFR conditions should prevail for the Big Island terminals.

AIRMET SIERRA remains in effect for mountain obscuration from
Kauai to Maui. Conditions will likely continue through the
remainder of the night and possibly much of New Years Day.

AIRMET ZULU continues to mention light icing from Kauai to the
Big Island. Conditions may improve later tonight.

&&

.MARINE...
Southeast to south winds will remain in place for the next
several days. A surface low centered about 450 nm NW of Kauai is
moving northward near 20 kt, and it's associated slow-moving
surface trough stretches to just north of the islands. These
features are producing moderate to fresh southeast winds that veer
southerly near Kauai and Oahu, along with scattered heavy showers
and a few thunderstorms, mainly over the western half of the
state. The trough will stall to the northeast of the islands this
weekend as the low is absorbed into a broad area of low pressure
sitting northwest of the state. As a result, southeast to south
winds will persist over coastal waters, with a decline expected
late Sunday through Tuesday as the low weakens and is pushed off
to the north by an approaching front. The front will likely fall
apart mid week, allowing building high pressure to usher in
northeasterly trade winds.

Rough seas, mainly in east to northeast wind swell, will gradually
decline this weekend. These seas are being generated from trade
winds produced by surface high pressure far northeast of the state
and are being boosted by strong trade winds north of the trough
sitting just north of the coastal waters. Local seas have been
running about 2 feet above guidance all day, and the Hanalei
PacIOOS buoy has been flirting with the 10 foot Small Craft
Advisory level. This wind swell, and the resulting elevated surf
along east facing shores, will gradually lower below seasonal
normal Sunday and will be small during the first half of next
week.

A series of mainly small northwest swells will continue through
Monday. A larger northwest swell will arrive Tuesday night and
may produce advisory level surf Wednesday and Thursday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 AM HST Saturday for Big Island
Summits.

&&

DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...Wroe