Gale Warning 6, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
915 PM HST Sat Dec 17 2022

.SYNOPSIS...
Showers and thunderstorms will steadily increase in coverage
tonight through Sunday as southwest winds strengthen in advance
of a strong cold front. Strong and potentially damaging kona, or
southwest, winds and strong thunderstorms will move through the
area Sunday night through Monday evening in association with the
front. Breezy westerlies will then focus showers over leeward
slopes through Tuesday. Drier conditions and lighter southwesterly
winds are then expected Wednesday through late next week.

&&

.DISCUSSION...

The initial wave of ascent associated with the approaching low is
underway as evidenced by blossoming shower coverage and emerging
kona winds over the western end of the state this evening.
Instability is already supporting thunderstorms in the Kauai channel
and along the land breeze near Windward Maui. Although the intensity
and location of rapidly developing convection is in line with
expectations, development is occuring much earlier in the night than
anticipated and significant faster than even the most aggressive hi-
res modeling depicted. With confidence, this supports a much more
aggressive approach to the near term portion of the forecast and a
Flash Flood Watch has been issued beginning Sunday morning and
continuing through the event. Although flash flooding is not
considered to be the primary threat, the latest trends suggest the
threat has increased to warrant headline issuance. The flood threat
will then continue into Monday night as waves of heavy showers and
thunderstorms precede the cold front, eventually punctuated by
potentially the most convection along the cold front itself on
Monday. Otherwise mainly cosmetic changes to the forecast this
evening, such as increasing pops and forecast rainfall amounts, with
no significant changes to expected impacts going forward. The
previous discussion follows.

Previous Discussion.
The strong cold front will make steady
progress toward the state tonight and Sunday, then move from
northwest to southeast down the island chain late Sunday night
through Monday evening. Winds will remain light tonight, with sea
breezes transitioning over to land breezes this evening. South to
southwesterly winds may begin to develop late tonight, but will
pick up more noticeably Sunday morning, then become breezy to
locally windy Sunday afternoon. Depending on the degree of
stability Sunday afternoon, a Wind Advisory may be needed for
windward areas due to gusty southwesterly downslope winds. The
winds associated with this storm system are expected to peak
Sunday night and Monday, potentially at warning levels. At a
minimum, a Wind Advisory will likely be required for the entire
state during this time, with the strongest winds expected in the
areas typically referred to as windward locations. A High Wind
Watch has been issued from 6 PM Sunday through 6 PM Monday for all
islands due to the potential for very strong and damaging winds.

Behind the cold front, breezy to locally windy conditions are
expected to continue through Tuesday, before easing off Tuesday
night as a weak front approaches from the northwest. Light to at
times moderate southwesterly winds will then prevail Wednesday
through Friday as a progressive pattern across the Pacific keeps
the surface ridge axis suppressed south of the state. The ridge
axis could lift north of the islands next weekend, bringing a
return of light to moderate trades.

As for the remaining weather details, quite an active next
several days can be expected across the island chain as the strong
storm system approaches and moves through the region.

Tonight and Sunday,
Mid-level moisture and forcing for ascent will increase across the
state as the night progresses. Daytime heating driven showers and
thunderstorms should end fairly quickly this evening, with most
showers then confined to the coastal waters by mid to late evening.
Deep layer southwesterly flow increasing after midnight should
lead the development of scattered showers and thunderstorms which
will begin to move onshore. Moisture and instability will hold
over the islands through the day Sunday, and with wind fields
increasing the potential for organized convection, a few
thunderstorms could become strong with the capability of
producing gusty winds and even hail given the cold temperatures
aloft. The potential for brief downpours appears high, however the
flow should remain progressive enough to keep the chances for
widespread flooding low, and a Flood Watch is not anticipated at
this time.

Sunday night through Monday night,
The potential for organized thunderstorms will peak Sunday night
through Monday evening as deep layer 0-6 km shear values
increase to between 40 and 60 knots. This combined with 1500 to
2000 J/KG of surface based CAPE and cold air aloft with -20C
heights lowering to between 18 and 20 kft, and H7 to H5 lapse
rates dropping to -7 to -8 C/KM, all point to the potential for
strong thunderstorms capable of producing damaging winds and large
hail. The greatest potential for Severe weather will exist along
and immediately ahead of the cold front. Some locally heavy
rainfall can't be ruled out either, and given the potential for
training of deep convection, a Flood Watch may eventually be
required for the Sunday night through Monday evening time frame.
Lingering deep moisture and instability could keep some strong
thunderstorms over the Big Island Monday evening, with conditions
quickly stabilizing after midnight as the front exits to the east.

Tuesday through next Saturday,
Both the GFS and ECMWF are in good agreement showing lingering
deep moisture over the state keeping conditions showery on
Tuesday. Drier weather should then settle into the region Tuesday
night through next weekend, with limited showers favoring the
typically dry leeward areas due to the southwesterly boundary
layer flow.

&&

.AVIATION...

A strong cold front located far northwest of Kauai will move
rapidly toward the region through Sunday night. The atmosphere is
gradually becoming unstable ahead of this feature. In addition,
background low-level south to southwest flow is beginning to
transport enhanced moisture into the islands, especially the
western end of the state. This will likely bring increasing
cloudiness and showers with a chance of thunderstorms to much of
the island chain and surrounding waters later tonight and Sunday.
This will likely produce periods of MVFR ceilings and
visibilities as these clouds and showers become more widespread.
The winds out of the south and southwest are also forecast to
increase during the day on Sunday as the front continues to move
closer to the area.

There are currently no AIRMETs in effect. As cloud cover and
showers increase Sunday morning, AIRMET Sierra will likely be
required for some of the islands. In addition, as the south to
southwest winds increase, AIRMET Tango may be needed downstream
north through east of the higher terrain on most islands starting
on Sunday.

&&

.MARINE...

A ridge of high pressure near the state will produce light and
variable winds through tonight, but significant changes are
expected to begin on Sunday. The large NW swell that filled in
yesterday is diminishing, but is still producing combined seas of
10 feet in many areas this evening, so a Small Craft Advisory
(SCA) remains posted for most waters, and a High Surf Advisory
remains in effect for exposed N and W facing shores. Seas are
expected to drop below 10 feet early tonight as the NW swell
continues to decline. However, increasing SW winds are expected on
Sunday, and the SCA will likely be expanded in time and area to
include all waters through Sunday.

A seasonably active weather pattern is expected starting tonight as
a potent cold front begins to approach the state. Shower and
thunderstorm coverage will gradually increase overnight, and SW
winds will begin to increase during the day Sunday. As the front
sweeps through the island chain Sunday night and Monday, winds
will increase to near-gale to gale-force SW to W winds. A Gale
Watch is posted for all Hawaiian waters from Sunday night through
Monday. Additionally, severe thunderstorms capable of producing
damaging winds and hail will be possible during this time. Winds
are expected to remain strong and out of the west on Tuesday as
low pressure passes a couple hundred miles N of the islands. Winds
will rapidly decline Tuesday night into Wednesday as the low
departs to the NE, and a ridge of high pressure builds over the
area.

The powerful low sending the cold front over the islands will
produce a long-lived and extra-large NW swell that will bring
surf heights well above High Surf Warning (HSW) levels for most N
and W facing shores Sunday night through Tuesday, and High Surf
Advisory level surf to shorelines that typically do not receive
high surf, i.e., W facing shores of Maui, Lanai and the Big
Island, and N facing shores of the Big Island. 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 NW swell is possible during the
second half of next week. Surf along S and W facing shores will
see an increase in short-period wind waves Sunday evening into
Tuesday due to the strong onshore winds.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Sunday for Niihau-Kauai
Leeward-Waianae Coast-Oahu North Shore-Olomana-Maui Windward
West-Kauai North-Molokai Windward-Molokai North-Molokai West-
Maui Central Valley North-Windward Haleakala-Big Island East-Big
Island North.

High Wind Watch from Sunday evening through Monday afternoon for
Niihau-Kauai Leeward-Kauai Mountains-Waianae Coast-Oahu North
Shore-Olomana-Central Oahu-Waianae Mountains-Lanai Mauka-
Kahoolawe-Maui Windward West-Maui Leeward West-Kona-Kohala-Big
Island Interior-Kauai North-Kauai East-Kauai South-East Honolulu-
Honolulu Metro-Ewa Plain-Koolau Windward-Koolau Leeward-Molokai-
Lanai Windward-Lanai Leeward-Lanai South-Maui Central Valley
North-Maui Central Valley South-Windward Haleakala-Kipahulu- South
Maui/Upcountry-South Haleakala-Big Island South-Big Island
Southeast-Big Island East-Big Island North.

Flood Watch from 6 AM HST Sunday through late Monday night for
all Hawaiian Islands.

Wind Advisory from 6 AM Sunday to 6 AM HST Monday for Haleakala
Summit.

High Wind Warning from 6 AM Sunday to 6 AM HST Wednesday for Big
Island Summits.

Winter Storm Watch from Sunday evening through late Monday night
for Big Island Summits.

Small Craft Advisory until 6 AM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Big Island Windward Waters.

Gale Watch from Sunday evening through Monday afternoon for all
Hawaiian waters-

&&

DISCUSSION...JVC/Jelsema
AVIATION...Houston
MARINE...Birchard