Drought in Hawaii, December 1 to 21, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Tue Nov 30 2021

.SYNOPSIS...
Unsettled weather will persist through the forecast period.
Locally heavy rainfall and thunderstorms are possible for Maui and
the Big Island through Wednesday morning. Breezy trades will turn
southeasterly and diminish Wednesday and Wednesday night. Gusty
northerly winds will accompany a cold front that will push through
the islands Thursday night through Friday. Heavy rainfall and
flash flooding are possible from Friday into next week.

&&

.DISCUSSION...
A surface trough southeast of the Big Island will approach the
islands over the next 24 hours, then move up the chain Wednesday
night and Thursday. Winds will weaken and shift to southeasterlies
as it passes. Associated clouds and showers will move in early
Wednesday morning. Localized heavy rainfall may occur at times.
Ahead of the surface trough, trade wind showers may be locally
heavy, but will move fast enough to minimize flooding concerns
tonight. However, as winds taper off Wednesday and Wednesday
night, localized flooding could occur. Satellite loop shows an
upper low just south of the Big Island this evening, providing
enough instability to keep a slight chance of thunderstorms in the
forecast for windward and southeast portions of Maui and the Big
Island through Wednesday morning.

Models show a deep trough will develop over the central Pacific
and drive a cold front through most or all of island chain
Thursday night through Friday. As the deepening trough approaches
from the north, the closed upper low currently over the state will
transition to an open wave and quickly move eastward. However,
lingering moisture will be readily available for the incoming
front. Heavy rainfall and potential flooding are possible as early
as Friday morning over north-facing slopes of Kauai within
post-frontal northerly flow. The flood threat would then quickly
shift to the Big Island, where the front appears likely to stall
Friday night through Saturday.

Sunday into early next week, a band of moist southerly flow will
develop east of the upper trough axis as a deep low closes off
near or west of Kauai. This moisture band will fuel the greatest
potential for heavy rain and flash flooding. Models vary in their
placement of this moisture band as it shifts across the islands,
decreasing forecast confidence. However, it is becoming clear
that active wet weather is in store for the state through the
forecast period, especially from the weekend onward.

&&

.AVIATION...
Moderate to locally breezy easterly winds will push occasional
showers, a few locally heavy, over windward and mauka sections of
the smaller islands through the forecast period. Brief MVFR or IFR
will be possible within heavier showers. Over the Big Island,
instability within the core of an upper low will support a slight
chance of thunderstorms, mainly over the waters south and east of
the islands. However, southeast flow in the mid-levels will
periodically steer heavy showers and isolated thunderstorms into
the southeast-facing slopes where AIRMET Sierra is now in effect.

AIRMET Sierra for mountain obscuration is in effect for the Big
Island from Cape Kumukahi to Apua Point to South Cape.

AIRMET Tango remains in effect tempo moderate upper-level
turbulence between FL250 AND FL350 over Maui and the Big Island.
Conditions are expected to gradually improve through tonight.

&&

.MARINE...
A low pressure system aloft continues to produce isolated
thunderstorms in the vicinity of the Big Island. The low is
slowly weakening but should continue to support thunderstorm
development near the Big Island and Maui through the rest of
tonight. The locally strong trade winds over the nearshore waters
of the Big Island and Maui will be trending down tonight. The
Small Craft Advisory (SCA) covering these waters expires at 6 am
Wednesday. This weakening trend will continue through Wednesday.

A dissipating cold front is forecast to enter the far northwest
offshore waters Thursday and move down the main Hawaiian Islands
Thursday night and Friday. Strong and gusty northerly winds and
rough seas will follow in behind the front. More widespread
unsettled weather is on tap for all the islands this weekend.

A moderate northwest swell is slated to arrive Wednesday and hold
through Thursday. A large north-northwest swell comes in Friday,
peak Saturday. This large swell is from a storm-force low
forecast to develop about 1000 nm north of Kauai Wednesday night
and drift south through Thursday. A High Surf Warning is very
likely for the north and west facing shores of most islands. For
the other shores, the east facing shores will see surf trending
down as the trade winds weaken. Surf along south facing shores
will remain flat through the forecast period.

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

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Wednesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Windward Waters-
Big Island Leeward Waters-Big Island Southeast Waters.

&&

DISCUSSION...Powell
AVIATION...JVC
MARINE...Lau