Heavy Rain in Hawaii, December 22, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Tue Dec 21 2021

.SYNOPSIS...
Breezy and showery trade wind weather will persist overnight, with
most showers favoring windward and mauka areas, periodically moving
over leeward communities. Wet conditions and a chance of
thunderstorms will continue across the Big Island into Wednesday,
while some drying occurs elsewhere. Slightly weaker trade winds,
with a typical pattern of mainly windward and mauka showers, is
expected Thursday through Christmas. Southeasterly winds and
unsettled conditions may develop near Kauai Sunday, potentially
spreading to all islands early next week.

&&

.DISCUSSION...
Breezy and rather wet trade wind weather continues, with unsettled
conditions near the Big Island. A highly amplified pattern over the
Pacific is maintaining a strong 1039 mb surface high far north of
the state. The high is driving breezy and gusty trade winds and has
pushed a narrow band of diffuse low level moisture toward Maui and
the Big Island. Meanwhile, the deep ridge aloft supporting the
surface high is being undercut by an upper level trough NE of the
state, leading to instability that is enhancing local rainfall. A
smaller, negatively tilted trough is passing S of the islands, and
associated strong difluence aloft is sending a shield of mid- and
high-level clouds over the islands from the S, while also fueling an
area of thunderstorms S and SE of the islands. This strong difluence
aloft will move E away from the islands over the next 24 hours, but
a broad upper trough N of the islands will linger. As a result,
windward areas on all islands will continue to experience active
showers, with showers periodically blowing across leeward sections.
Leeward convection on the Big Island has produced a few
thunderstorms along with some snow on the high summits.

Expect showery trade wind weather to persist tonight, with the focus
for unsettled weather on the Big Island, where a Flash Flood Watch
and a summit Winter Weather Advisory are in place. The trough aloft
passing just S of the state may still trigger heavy showers and
thunderstorms over and around the Big Island, but current trends
indicate that this threat may be easing somewhat. If showers re-
develop, the greatest potential for heavy rain/flooding will be on
windward slopes.

Breezy trades will hold on Wednesday, but a decrease in shower
activity is expected across Kauai, Oahu, and Molokai as the low
level band of moisture pushes farther south. Wet and unsettled
conditions will linger on the Big Island, especially windward, as
the upper level disturbance drifts off to the E.

Moderate to locally breezy trade winds and a somewhat typical
pattern of windward and mauka showers is expected for Thursday
through Christmas holiday weekend. The GFS and ECMWF models depict
the band of low level moisture moving just S of the islands. High
pressure will hold far N of the state and will maintain moderate to
locally breezy trade winds. Drier mid-level air will push down from
the N, but the atmosphere will remain slightly unstable as upper
level troughing lingers overhead. Expect rainfall to be focused over
windward slopes of all islands. As showers develop over leeward and
interior Big Island each afternoon, we cannot rule out a
thunderstorm.

Late Sunday and early next week, another kona low is expected to
develop several hundred miles NW of the islands, and then move S
into the middle of the week before moving W. Forecast models have
lacked run-to-run consistency, with the latest deterministic GFS run
similar to the 12Z ECMWF run. This guidance is trending toward a
wetter solution for the islands, with a convergence band moving over
Kauai from the W on Sunday, spreading over the other islands Monday
and Tuesday. With strong high pressure remaining in place far N of
the islands, locally strong SE winds are also possible, with
potential impacts highly dependent on low-confidence forecast
details.

&&

.AVIATION...
A strong high pressure system far north of the state will keep
breezy to locally strong trade winds in forecast through Wednesday
night. An upper level disturbance over the region will keep wet and
unstable weather conditions in the forecast through Wednesday night.
MVFR conditions with periods of IFR conditions are forecast mainly
over north and east slopes of all islands. Isolated thunderstorms
are also possible near the Big Island. Drying weather trends are
forecast on Thursday for most areas.

AIRMET Sierra for IFR is in effect for Windward Big Island and for
tempo mountain obscuration/MVFR for north through east sections of
the remaining islands.

AIRMET Tango for moderate turbulence remains in effect below 025
over mountains and south through west of all islands.

AIRMET Zulu for moderate icing was expanded to include all Hawaiian
Islands and adjacent waters due to expansion of mid level cloud
ceilings.

&&

.MARINE...
A strong 1037 mb high centered far north of the state will continue
to feed moderate to strong trade winds across the entire area for a
couple of more days. Over the weekend, the high moves further north,
so expect the trade winds to trend slightly weaker. With a surface
low and front stalling west of Kauai next week Monday, the trade
winds will shift to the southeast across the western islands.
Instability from this system will also threaten the western offshore
and coastal waters with heavy showers and thunderstorms Sunday
through at least Monday.

A Small Craft Advisory (SCA) remains posted for all Hawaiian Coastal
Waters due to these strong winds and associated seas. An unstable
airmass in the vicinity of the islands will maintain a threat of
locally heavy showers and thunderstorms over the leeward coastal
waters and western offshore waters through Wednesday.

A High Surf Advisory (HSA) is in effect through Wednesday for east
facing shores from an elevated north-northeast swell. North facing
shores will also experience surf heights just below HSA levels due
to its higher criteria. This swell will lower very slowly through
Friday then hold over the weekend while the swell shifts from a
northeast direction. Expect surf heights to likely remain at or just
above HSA levels for east facing shores.

Swells from the west and northwest directions will be well below
seasonal levels for this time of year with just small pulses
expected through the remainder of the week and on into early next
week. South facing shores will remain small throughout the forecast
period with just background swell energy expected.

&&

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

Flood Watch until 6 AM HST Wednesday for the Big Island.

High Surf Advisory until 6 PM HST Wednesday for East facing shores
of Kauai Oahu Molokai Maui and the Big Island.

Small Craft Advisory until 6 PM HST Thursday for all Hawaiian
waters.

&&

DISCUSSION...Birchard/Wroe
AVIATION...Bohlin
MARINE...Lau