Heavy Rain in Hawaii, December 1 to 2, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
330 PM HST Wed Dec 1 2021

.SYNOPSIS...
Unsettled weather will persist through the forecast period. An
upper low along with a weak low level trough will linger near the
Big Island through tonight. A cold front will move across the
island chain Thursday night and Friday then stall near the Big
Island Saturday. Low pressure will drop down toward the state
later in the weekend and on into early next week with wet weather
expected across portions of the state.

&&

.DISCUSSION...
A rather unsettled weather pattern is setting up for the Aloha
State during the upcoming forecast period. Water vapor imagery
currently shows an upper low centered just west of the Big Island.
Meanwhile a low level trough is located near the Big Island.
Instability caused by the upper low along with abundant low level
moisture has allowed for some locally heavy showers and isolated
thunderstorms to form over and around the Big Island. Meanwhile
moisture embedded in low level easterly has been providing for
some passing showers mainly over windward and mauka areas of the
smaller islands. The upper low will gradually weaken tonight then
lift off to the northeast Thursday and Thursday night. The low
level trough will linger in the island vicinity with scattered
showers continuing mainly for windward and mauka areas.

Next up will be an area of low pressure along with its associated
frontal boundary. The front is forecast to reach Kauai Thursday
night then move across the island chain Friday before stalling
near the Big Island Friday night and Saturday. The front will be
accompanied by a band of showers as it moves down the island
chain. Also there is a slight chance for thunderstorms along the
frontal boundary as well. Cool northerly flow will follow in the
fronts wake with showers becoming focused over north facing
slopes. The post frontal winds may be strong enough over Kauai for
a Wind Advisory needing to be issued as we get closer to the
event. Also with the front stalled near the Big Island, showers
along with a slight chance of thunderstorms will continue to fall
along the boundary.

Over the Big Island summits, very strong winds are expected to
develop by Friday night and continue on into early next week. A
High Wind Watch will likely needed for this event. Also, accumulating
snow is expected as the front reaches the Big Island. A Winter
Storm Watch will likely be needed for this event as well.

The area of low pressure will then drop southward to a position
just north and west of Kauai. Instability along with low level
convergence will likely allow for increased rain chances for a
good portion of the state Sunday through Tuesday. However with
this type of weather scenario, it is very difficult to pin down
the details. Heavy rainfall will be a threat and shower bands may
line up over portions of the state for extended periods of time
increasing the chances for flooding issues. A Flash Flood Watch
will likely be issued as we get closer to the event. Winds will be
blowing from the south and southwest later this weekend and on
into early next week. Some areas over and downwind of terrain may
see locally gusty winds at times.

&&

.AVIATION...
A northeast to southwest oriented surface trough remains near the
Big Island and Maui this afternoon, with a nearby upper level low
evident on water vapor. Most of the focus of clouds and showers
with these systems has been over the Big Island today, with some
clouds and showers forming late this morning onwards over Maui.
The surface trough and upper level feature are expected to weaken
over the next day, bringing some improving conditions. Until then, MVFR
and isolated IFR conditions are possible - more so over the Big
Island, but also possible over Maui. AIRMET Sierra remains in
place for the north and east side of Maui and the Big Island, and
for the southeast side of the Big Island for mountain obscuration.

The remainder of the islands will see lighter winds spread across
the region tonight and tomorrow, but clouds and showers should
remain focused over the mountains and eastern sides of the
islands with an easterly background flow. While not currently
under any AIRMETs, one can't be ruled out as clouds and showers
develop.

&&

.MARINE...
The upper level low is slowly weakening but should continue to
support thunderstorm development near the Big Island and Maui
through tonight. Trade winds will continue to weaken through tonight.

A cold front is forecast to enter the northwest offshore waters
Thursday and move down the main Hawaiian Islands Thursday night
and stall near the Big Island late Friday. Strong and gusty
north-northwest winds and rough seas are expected along and
behind the front. Winds are expected to be strongest over the
western end of the state and a Gale Watch has been issued for the
waters around Kauai on Friday. Over the weekend the stalled front
will then move west as a trough, and eventually become a low
pressure system west of the state. This Kona Low will bring
widespread rainfall and thunderstorms over the marine area Sunday
through early next week.

A small to moderate northwest swell is filling in this afternoon
and will peak Thursday. A very large north-northwest (350 degree)
swell will arrive late Friday and peak Saturday with surf heights
well above warning level thresholds for north and west facing
shores. In addition to high surf, significant harbor surges are
expected along exposed north facing harbors such as Kahului
harbor. With the peak of the swell coinciding with a spring high
tide late Saturday night, this could potentially cause
significant beach erosion with water reaching vulnerable low-lying
coastal roadways.

Surf along east facing shores will decline as the trade winds
subside. Surf should rise this weekend along east facing shores
due to the very large north-northwest swell wrapping into select
areas. Surf along south facing shores may rise slightly over the
weekend due to a series of background of south swells.

The arrival of the strong northerly winds and large north-
northwest swell Thursday night through Friday will boost the seas
to 10 feet or higher covering most of the Hawaiian waters, thus
prompting a Small Craft Advisory.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Gale Watch from Friday morning through Friday afternoon for
Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward
Waters-Kauai Channel.

&&

DISCUSSION...Burke
AVIATION...M Ballard
MARINE...Shigesato/Kino