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871 
FXHW60 PHFO 181421
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
421 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.

&&

.DISCUSSION...

Satellite imagery shows an impressive mid-latitude cyclone deepening 
over the Central Pacific Basin as digs southward toward the islands. 
The latest data indicate the surface low has deepened to 988mb south 
of 40N with further deepening expected as it makes its closest 
approach on late Monday night.

Today. During the last several hours, the local gradient has begun 
responding to the approach of this feature as kona, or southwest 
winds, have emerged over the western end of the state. This is 
ushering an existing batch of moisture northeastward across island 
chain and touched off numerous showers and thunderstorms during the 
overnight period. Radar indicated rain rates have ranged from 1 to 3 
inches per along with isolated signatures supportive of small hail. 
As noted in previous days, the supply of instability during the next 
24 to 36 hours will be continuous and impressive courtesy of cold 
WNW flow aloft and strengthening southwesterlies in the lowest 
levels. As a result, the islands will likely remain convectively 
active through today with the greatest coverage and organization 
focusing along the western and northern portions of the moisture 
gradient, the latter of which will lift north of the area during the 
next several hours. Thus, after a potential lull in activity during 
the morning, the western end of the moisture gradient (presently 
draped over Kauai) will slowly work eastward supporting an increased 
coverage of showers and thunderstorms over Oahu and Maui during the 
daytime hours today. Throughout this time, background isentropic 
ascent will steadily increase ensuring frequent regeneration of 
convective cells as strengthening winds lead to more progressive 
storm motions. With multiple flood advisories already issued and 
numerous thunderstorms ongoing in the area, the Flood Watch 
beginning this morning at 6am remains in good shape.

This evening through Monday. Pressure falls spread across the island 
chain in response to impressive mid-level forcing and the 
southwesterly wind field responds accordingly. Sustained southwest 
surface winds of 20 to 30 mph will be common with locally stronger 
winds likely in areas favorable for local accelerations. Of greater 
concern is winds right off the deck, in the 925mb to 950mb layer, 
strengthening to 45 kts or so during the overnight. Windward zones 
and areas downwind of terrain will be highly susceptible to 
downsloping which will force damaging winds to the surface. Windward 
Oahu will be particularly susceptible. 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 into the mid-
week period. 
&&

.AVIATION...

A strong cold front located far northwest of Kauai will move 
rapidly toward the western islands through tonight. An upper- 
level trough is also digging down toward the state. As a result 
of these features, the atmosphere has already become unstable. In
addition, the background low-level south to southwest flow is 
transporting enhanced moisture into the islands. This is causing 
the coverage of clouds and showers to increase this morning. 
Isolated thunderstorms are also in the vicinity of the islands. 
The clouds and precipitation along with the deep convection are 
expected to continue increasing today. This will likely produce 
widespread areas with MVFR ceilings and visibilities. In addition,
as the front moves closer to the area, the southwest winds are 
also expected to strengthen from later this morning through this 
evening.

AIRMET Sierra for mountain obscuration has recently been issued 
for the higher terrain above 2500 ft on Kauai, Oahu, Molokai, and 
Maui. This AIRMET will likely be needed for most of these islands
through this morning, and possibly this afternoon. 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.

&&

.MARINE...

A potent cold front will move SE across the islands tonight 
through Monday night, bringing a wide array of marine hazards.
These include the potential for very 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) for all Hawaiian
waters today will be replaced by a Gale Warning tonight.
Additionally, a High Surf Warning (HSW) for exposed N and W 
facing shores will go into effect at 6 pm HST tonight. Other less-
exposed shorelines may need a High Surf Advisory as the swell 
builds. 

Even after the front moves E of the islands, W winds are expected
to remain strong to near-gale on Tuesday as low pressure passes a
couple hundred miles N of the islands. Winds will decline Tuesday
night into Wednesday as the low departs to the NE, and a ridge of
high pressure builds over the area.

As discussed above, 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. 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 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 Niihau-Kauai Leeward-
Kauai Mountains-Waianae Coast-Oahu North Shore-Olomana-Central 
Oahu-Waianae Mountains-Lanai Mauka-Kahoolawe-Maui Windward West-
Maui Leeward West-Haleakala Summit-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. 

High Wind Warning from 6 PM this evening to 6 PM HST Monday 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. 

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...JVC
AVIATION...Houston
MARINE...Birchard