Damaging Winds in Hawaii, December 18 to 19, 2022

Area Forecast Discussion
National Weather Service Honolulu HI
859 PM HST Sun Dec 18 2022

.SYNOPSIS...
Numerous showers, heavy at times, and thunderstorms will continue
through Monday as a kona storm moves through the region. Bands of
thunderstorms are moving through islands this evening ahead of a
vigorous cold front that will sweep through the state from
northwest to southeast on Monday. The stronger thunderstorms may
become severe through Monday afternoon with strong winds and the
potential for hail. Strong southwesterly kona winds will increase
into Monday ahead of the cold front, with the strongest wind
speeds forecast for communities along the down wind north and east
slopes of mountain areas. Periods of heavy rain with this storm
will likely result in flash flooding in some areas. Weather will
improve over the western islands by Monday night and across the
rest of the state by Tuesday. Fair weather and light winds
expected from Wednesday through Friday.

&&

.DISCUSSION...
The short range weather forecast remains on track this evening
with three bands of showers and thunderstorms are passing through
the Hawaii Region. The first band is just east of the Big Island
over the marine coastal waters. The second band is passing
eastward through Oahu and should move into Maui County later
tonight. The third band, the strongest of the three, is forming up
along the cold front just west of Kauai. This front will continue
to sweep eastward through all islands with increasing winds for
all islands ahead of the frontal trough.

Weather conditions this evening are just a start to potentially
severe weather conditions that are forecast to move through ahead
and along the cold front as it passes over each island. Strong
sustained southwest winds of 20 to 30 MPH with stronger gusts are
expected along with periods of heavy rain and thunderstorms.
These strong winds will be magnified by thunderstorms, including
mountain peaks and down wind of higher elevation areas. Mountain
peaks and communities along north and east slopes of mountains
will see much stronger wind speeds develop as the front gets
closer to each island. The timing of this cold frontal passage
will vary as the front sweeps eastward. Expect the front to pass
over Kauai on Monday around 2 AM HST, over Oahu around 7 AM HST,
Molokai and Lanai around noon HST, reaching Maui around 1 PM HST,
and finally over the western Big Island around 6 PM HST.

Finally, a severe thunderstorm threat will exist tonight through
through the time of frontal passage on Monday. A Severe
Thunderstorm Watch may be needed and will be considered for this
kona storm. On satellite, we continue to see the deep trough
quickly approaching the state with a line of thunderstorms
developing ahead of the front.

The potential for flash flooding will continue to increase tonight
and Monday for areas ahead and along the cold front. Soil moisture
is already saturated from the rainfall during the day today and
increasing instability will increase the heavy rainfall rates
through Monday. Due to the potential for intense rainfall rates
of 3 to 4 inches per hour through Monday, it will not take much
additional rainfall to trigger flash flooding over any island.

Tuesday into midweek. Well-mixed, gusty westerlies build in behind
the cold front and focus showers over leeward slopes through
Tuesday. Fair weather with much lighter winds are expected by
Wednesday. Beyond Wednesday, we are looking at a progressive
pattern to continue in the North Pacific, which will keep light
winds in the forecast through rest of the week. Another front
could approach the state by Friday, through most of the moisture
and instability looks to stay well north of the island chain.

&&

.AVIATION...
A strong cold front and an upper-level trough associated with an
impressive low pressure system north of the main Hawaiian Islands
continues to advance rapidly towards the state. Ahead of these
features, unstable atmospheric conditions and enhanced moisture
are producing widespread cloud cover and rainfall along with
thunderstorms. This is a high impact and unusual weather event
for the main Hawaiian Islands, and all aviation interests need to
use extra caution through Monday.

Widespread MVFR conditions, and localized IFR conditions, due to
low ceilings and reduced visibilities are occurring across most of
the state. AIRMET Sierra remains in effect for mountain
obscuration above 2 thousand feet for most islands east of Kauai.
This AIRMET will likely be required for Kauai later tonight, and
may remain in effect for most of the state at least through Monday
morning.

Frequent lightning, heavy rainfall, and severe turbulence have
all been reported via PIREPs earlier today at various locations
in some of the thunderstorms. Hail and significant low-level wind
shear are also possible later tonight in some of the
thunderstorms, which may become severe. Convective SIGMETS are
currently in effect for almost the entire state, and will likely
continue through early Monday morning. We have also mentioned the
potential for light icing occurring in clouds across the entire
state in the layer from 12 to 22 thousand feet in AIRMET Zulu. In
addition, model output suggests that severe upper-level clear-air
turbulence is possible over much of the state early this evening.
SIGMET Whiskey has been issued to account for this. We will
monitor these conditions, and determine if this turbulence threat
may decrease later tonight.

As the front continues to move closer to the state, expect
significant strengthening of the low-level southwest winds from
west to east across the island chain. AIRMET Tango will likely be
needed beginning tonight or early Monday for Tempo moderate
mechanical low-level turbulence northwest through southeast of
the higher terrain on most islands. Once it is issued, this
AIRMET will likely remain in effect into Monday. It is also
possible that sustained wind speeds could reach 30 kts or greater
for portions of the state from later tonight through Monday,
which would also require AIRMET Tango for strong surface winds.

&&

.MARINE...
A robust cold front will move southeast across the islands
late tonight through Monday night, bringing a wide array of
marine hazards. These include the potential for gusty winds, hail
and waterspouts in and around strong to severe thunderstorms,
sustained near- gale to gale-force southwest to west winds, as
well as rapidly increasing seas and surf. A Gale warning is now in
effect for all Hawaiian waters. Additionally, a High Surf Warning
(HSW) for exposed north and west facing shores will go into
effect early Monday morning. Less- exposed shorelines may need a
High Surf Advisory (HSA), to be issued later, as the swell builds
down the island chain.

Even after the front moves east of the islands, west winds are
expected to remain strong to near-gale on Tuesday as low pressure
moves east passing a couple hundred miles north of the islands.
Winds will decline Tuesday night into Wednesday as the low departs
to the northeast and a ridge builds over the area.

The strong low pushing the cold front over the islands will
produce a long-lived and extra-large northwest swell that will
rapidly build into the islands and bring surf heights well above
High Surf Warning (HSW) levels. There is high potential for wave
run- up onto vulnerable roadways and coastal areas in the HSW
areas, with the most likely time for these impacts being Monday
night into Tuesday morning during high tide. This swell is
expected to drop below warning thresholds Tuesday night into
Wednesday. The northwest pacific remains active resulting in a
large northwest swell towards the end of the week near High Surf
Advisory (HSA) levels. Surf along south and west facing shores
will also see an increase in short- period wind waves tonight into
Tuesday due to the strong onshore winds.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through late Monday night for all Hawaiian Islands.

High Wind Warning until 6 PM HST Monday for all Hawaiian Islands.

High Wind Warning until 6 AM HST Tuesday for Haleakala Summit.

High Wind Warning until 6 AM HST Wednesday for Big Island
Summits.

Winter Storm Warning until 6 AM HST Tuesday for Big Island
Summits.

Gale Warning until 6 PM HST Monday for all Hawaiian waters.

High Surf Warning from 3 AM Monday to 6 PM HST Tuesday for
north and west facing shores of Niihau, Kauai, Oahu and Molokai,
and north facing shores of Maui.

&&

DISCUSSION...Bohlin
AVIATION...Houston
MARINE...Shigesato

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE HONOLULU HI
821 PM HST SUN DEC 18 2022

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0630 PM     HIGH SUST WINDS  HALEAKALA SUMMIT        20.70N 156.17W
12/18/2022  M55 MPH          MAUI               HI   PARK/FOREST SRVC

            STRONG WINDS REPORTED BY NATIONAL PARK
            SERVICE WITH GUSTS UP TO 70 MPH.

&&

RCB