Debris Flow and Flooding in Hawaii, December 20 to 21, 2023

Area Forecast Discussion
National Weather Service Honolulu HI
352 AM HST Wed Dec 20 2023

.SYNOPSIS...
A wet trade wind pattern will persist for the next few days, with
rainfall favoring windward and mauka areas. Showers may spread to
leeward areas as well from time to time. Expect clearer skies and
drier weather by the holiday weekend as upper ridging builds over
the islands.

&&

.DISCUSSION...
A high far north of the state continues to drive breezy trade
winds across local waters this morning. Embedded bands of showers
favor windward and mauka areas of the islands. High clouds
marking the subtropical jetstream continue to block views of the
sky as an associated upper trough deepens several hundred miles
west of Kauai. Expect the wet pattern to persist today as a band
of moisture focused on Kauai and Oahu sags southward.

Cloudy, wet, and breezy trade wind weather will prevail into
Friday. Models show the high to our north will merge with another
high moving in from the northwest Pacific on Thursday, keeping
trade winds at breezy levels across local waters. Winds will
weaken slightly Friday as the high moves eastward. Upper troughing
will drift closer to the islands through Thursday, leading to
decreased stability and denser high cloud cover. The low cloud
band focused on Kauai and Oahu will sag southward across all
islands today, spreading shower coverage. Decreased stability may
cause showers to become heavier. However, breezy trades will keep
showers from focusing on any given area, reducing the potential
for flooding. There may be periods of snow on the highest
elevations of Mauna Kea and Mauna Loa. Models show that moisture
should decrease on Thursday.

Expect drier trade wind weather heading into the holiday weekend.
Upper ridging will build in from the west, bringing a return to
more stable conditions. Trade winds will gradually decline as high
pressure to our north is pushed closer to the state.

&&

.AVIATION...
An area of surface high pressure, centered well north of the
state, will continue to generate breezy trades through the
forecast period. Low level moisture caught up in the flow will
bring scattered to numerous showers along the windward slopes and
allow for periods of MVFR ceilings and visibility. Isolated
showers may push over the higher terrain and affect leeward
locations.

AIRMET SIERRA remains in effect above 2000 feet for north through
east sections of all islands for tempo mountain obscuration due
to clouds and showers.

AIRMET TANGO remains in effect below 8000 feet over and
immediately south through west of mountains due to tempo moderate
turbulence. AIRMET TANGO also remains in effect between FL240 and
FL350 for tempo moderate turbulence.

&&

.MARINE...
Surface high pressure far north the islands continues to drive
fresh to locally strong trade winds across the majority of the
Hawaiian coastal waters. High winds, a stubborn northeast swell
and an incoming northwest swell are all reasons to justify the
continuance of the all-coastal water Small Craft Advisory and
east-facing coast High Surf Advisory (HSA) for yet another day,
or at least through this afternoon. As the high drifts farther
east, trade magnitudes will generally remain fresh across more
exposed waters...locally strong within the typically windier
channels and bays through Thursday before falling off to more
moderate to fresh speeds Friday.

Moderate size, short period northeast swell has remained fairly
steady with little to no decline. Thus, with little reprieve from
stronger trade flow, the swell remains elevated enough to warrant
extending the High Surf Advisory (HSA) for east-facing shores
through the day. East shore surf is expected to eventually fall
below advisory heights this evening or early Thursday. A small,
medium period northwest swell will boost north northwest-facing
surf a bit today but remain well below advisory levels through
Thursday. A larger northwest swell is scheduled to arrive late
Thursday and this swell will likely lift north and west-facing
shore surf to HSA levels as it peaks out early Friday. While
confidence remains low, the next northwest swell that will come
in just after Christmas may be high enough to produce XL warning
level surf along north shores. A small, medium to long period
south to southwest swell passing through late this week will keep
south shore surf from going completely flat.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for Olomana-Kauai
East-Koolau Windward-Molokai Windward-Molokai Southeast-Windward
Haleakala-Kipahulu-Big Island East-Big Island North.

Small Craft Advisory until 6 PM HST this evening for all
Hawaiian waters-

&&

DISCUSSION...Powell
AVIATION...JT
MARINE...Blood