Flash Flood Warning 12, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST Sun Dec 18 2022

.SYNOPSIS...
Numerous showers and thunderstorms will continue through Monday.
Some storms may be severe tonight and Monday. Any severe storms
will be capable of producing strong winds and hail. Strong and
damaging southwest, or kona, winds will develop tonight and
continue through Monday. Periods of heavy rain throughout this
time may result in areas of flash flooding. Weather will gradually
improve Tuesday with fair weather and much lighter winds expected
by Wednesday.

&&

.DISCUSSION...
Satellite imagery shows an impressive mid-latitude cyclone deepening
over the Central Pacific Basin as it digs southeastward toward
the islands. Ahead of this cyclone, we have increasing southerly
winds this morning which is pulling moisture from the tropics and
producing widespread moderate rainfall with brief periods of heavy
rainfall across the smaller islands. The moisture axis remains
centered over the western half of the state this morning, but as
I write this discussion we are already seeing the moisture axis
shift eastward towards Maui County. With the moisture shifting
east and instability increasing throughout the day, Oahu, Maui
County and the Kona side of the Big Island should see increasing
rainfall today with pockets of heavy rain. A Flood Watch is in
effect for all Hawaiian Islands through Monday night. Winds will
also be increasing from the southwest at the surface and aloft
this afternoon which will increase the storm motion. This should
help decrease the flooding threat slightly, but with the
widespread rainfall from this morning saturating the soils
already, it would only take one intense thunderstorm sitting over
a particular area briefly to cause flooding concerns.

The main weather concern looks to start from this evening and
continue through Monday. As the mid-latitude cyclone continues to
move closer to the state, kona winds will increase with sustained
southwest surface winds of 20 to 30 mph with stronger gusts
expected. In areas of local accelerations, such as the Windward
zones and areas downwind of terrain will be highly susceptible to
downsloping which will force damaging winds to the surface. Wind
gusts of over 60 mph will be possible especially over Windward
Oahu and downwind of Mt.Kaala, north and east Maui, and north and
east Big Island. Convective instability will also be maximized
during this time as 0-1km MLCAPE increases to at least 1500 j/kg.
Any particularly vigorous showers will therefore have the
potential to mix down severe winds up to 60 mph. For this reason,
the High Wind Warning includes the entire area - not only favored
downsloping areas.

Finally, a severe thunderstorm threat will exist tonight through
through the time of frontal passage on Monday. Lapse rates aloft are
modeled to remain fairly impressive, in the neighborhood of 7 to 7.5
km/C in the prefrontal environment. Strong, deep unidirectional wind
profiles in the presence of moderate convective instability will
contribute to an enhanced threat for thunderstorms to evolve into
northeast-moving bowing segments capable of producing damaging
straight line winds. 0-6km bulk shear in excess of 40 kts and cold
temperatures aloft will also support a marginal risk for hail up to
1 inch in diameter. The potential for a Severe Thunderstorm Watch
will be evaluated today and this evening.

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 trade
winds out of the forecast through rest of the week. Another front
could approach the state towards the end of the week, through most
of the moisture and instability looks to stay well north of the
state.

&&

.AVIATION...
A strong cold front and upper level trough associated with an
impressive low pressure system far north of the main Hawaiian
Islands will advance towards Kauai through tonight. Ahead of this
feature, enhanced moisture has overspread the smaller islands from
Kauai to Maui this morning, producing widespread MVFR ceilings and
visibilities and occasional IFR conditions within heavier
downpours. With ample instability in the area, thunderstorms have
also been observed, especially across adjacent waters. Lightning,
heavy rain, and moderate to severe turbulence have all been
reported via PIREPs at various locations. As a result, SIGMET
Victor has been issued for embedded thunderstorms over a large
portion of the state. This SIGMET will likely remain in effect at
least through the rest of the morning, though the bulk of
lightning is now greater than 40nm NNE of the island chain.

Expect clouds and showers with scattered thunderstorms to
continue as the enhanced moisture eventually makes its way
further south to the Big Island later this afternoon.
Widespread MVFR ceilings and visibilities with occasional IFR
conditions will persist for much of the day and into tonight
across a large portion of the state. Additionally, as the front
gets closer to the state, southwest wind speeds are expected to
increase today and become strong through tonight.

AIRMET Sierra for mountain obscuration above 2500 feet remains in
effect for Kauai, Oahu, Molokai, and Maui. This AIRMET will
likely remain in effect through the afternoon and may need to be
expanded to also include the Big Island later today.

In addition, as the gusty southwest winds increase later today,
AIRMET Tango will likely be needed for TEMPO moderate mechanical
turbulence northwest through southeast of the higher terrain on
most islands. Once it is issued, this AIRMET will likely remain in
effect into Monday. There is also potential for sustained surface
wind speeds to reach 30kts or greater overnight for portions of
the state, which would warrant an AIRMET for strong winds as well.

Finally, model data suggests that moderate to severe clear-air
turbulence will be possible behind the main area of showers this
afternoon into early evening. Should this materialize, an AIRMET
for moderate upper level turbulence or possibly a SIGMET for
severe turbulence from roughly Kauai to Oahu would be needed.
&&

.MARINE...
A robust cold front will move southeast across the islands
tonight through Monday night, bringing a wide array of marine
hazards. These include the potential for gusty winds and hail 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 Small Craft Advisory (SCA) remains in effect for
all Hawaiian waters through this afternoon, but will be replaced
by a Gale Warning tonight. Additionally, a High Surf Warning (HSW)
for exposed north and west facing shores will go into effect
tonight. Less-exposed shorelines may need a High Surf Advisory
(HSA) as the swell builds.

Even after the front moves east of the islands, west winds are
expected to remain strong to near-gale on Tuesday as low pressure
passes 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
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, but a large northwest swell is
possible during the second half of next week. Surf along south
and west facing shores will also see an increase in short-period
wind waves Sunday evening 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 from 6 PM this evening to 6 PM HST Monday for
all Hawaiian Islands.

High Surf Warning from 6 PM this evening to 6 PM HST Tuesday for
Niihau-Kauai Leeward-Waianae Coast-Oahu North Shore-Olomana-Maui
Windward West-Kauai North-Molokai Windward-Molokai North-Molokai
West-Windward Haleakala.

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 from 6 PM this evening to 6 AM HST Tuesday
for Big Island Summits.

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

Gale Warning from 6 PM this evening to 6 PM HST Monday for all
Hawaiian waters-

&&

DISCUSSION...Kino/JVC
AVIATION...Vaughan
MARINE...Powell

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

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

1243 PM     NON-TSTM WND GST 4 SE WAIALUA            21.54N 158.08W
12/18/2022  U0.00 MPH        HONOLULU           HI   COUNTY OFFICIAL

            A TREE FELL ACROSS KAUKONAHUA ROAD NEAR POAMOHO STREET.

&&

JNS