High Surf in Hawaii, December 16 to 17, 2022

Area Forecast Discussion
National Weather Service Honolulu HI
325 AM HST Fri Dec 16 2022

.SYNOPSIS...
A front moving down the island chain from the northwest will
continue to bring unsettled weather to the state into Saturday.
Showers, along with some thunderstorms, will spread over Maui and
the Big Island today, tapering off early Saturday morning. Another
round of unsettled weather and increasing winds, is expected
Sunday through Tuesday, as a stronger front approaches the state.

&&

.DISCUSSION...
Have made some short term forecast modifications, nudging PoPs
upward in some locations to be in better alignment with the latest
run of the HREF. Have also added some areas of locally heavy
rainfall based on the HREF probabilities. This suggests some
locally heavy rainfall could come into the east side of the Big
Island this morning, and over Kauai this afternoon.

In the evening hours, we saw a several thunderstorms pop up over
the coastal waters, and just outside the coastal waters. As the
evening progressed we had some thunderstorms approach Kauai and
other develop and move across Oahu. Most of the showers and
thunderstorms are moving quickly and not becoming anchored,
keeping flooding concerns at a minimum. Will continue to monitor
stream response and radar in the event additional advisories are
needed.

The front itself remains northwest of Kauai, but is already
becoming a diffuse boundary, difficult to discern on satellite.
The clouds, showers and thunderstorms we have experienced
overnight are associated with the increased moisture being drawn
northward ahead of the front. Water vapor imagery shows the
islands at the base of a short-wave trough that is swinging
through, likely aiding in the development of thunderstorms
overnight. Satellite derived precipitable water (PW) shows 1.4
inches extending northward towards Kauai this morning, with some
higher PW pockets around the islands. This is on the high end of
normal for December, and a little below what earlier model runs
had indicated we would have tonight.

While the front is bringing a more unstable airmass to the
islands, the front itself is expected to wash out over the central
islands later today. Meanwhile, the moisture and colder upper
level temperatures are already in place over the islands, so
expect showers to persist into Saturday. However as the upper
level trough associated with the front swings through the islands,
exiting east of the Big Island tonight, thunderstorm activity
should die out. Thunderstorms remain in the forecast for all the
islands today and many areas tonight, however it looks like the
greatest chance will be over Maui County and the Big Island and
waters to the north of them.

Our attention then turns to the next frontal system to impact the
islands during the Sunday through Monday/Tuesday time frame. The
upper level trough associated with this front is expected to be
colder/deeper than this current system, but precipitable water
values are a little lower. While winds are not a concern with the
current front, they are expected to be more of an issue with this
next front. Kona winds will strengthen Sunday, becoming strongest
Sunday night through Monday with potential impacts downwind of
terrain and across windward areas. Expect the winds to then trend
downward Tuesday. The strong upper level trough will bring very
strong winds to the Big Island summits during this event, and to a
lesser degree on Haleakala. Winds are expected to reach High Wind
Warning levels for these areas with this next event. Some lower
elevations in Maui County and on the Big Island may see advisory
level winds. Similar to the current frontal system, thunderstorms
will again be possible with this next front.

A drier and more stable airmass is expected over the state Tuesday
through Thursday, so look for a decrease in shower activity.
However this is expected to be short-lived, as another weaker
front will approach the islands for the end next week.

Temperatures will be hovering around freezing through the week on
the Big Island summits, and with plenty of opportunities for
moisture to reach the summits, snow will remain in the forecast.

&&

.AVIATION...

A front will move down the island chain today, bringing periods
of light to moderate rainfall and pockets of heavy rain with
isolated thunderstorms. AIRMET Sierra for mountain obscuration is
currently in effect for Kauai and Oahu, though conditions seem to
be improving on Oahu for now. AIRMET Sierra may be needed for
mountain obscuration on Maui later this morning. Some periods of
light icing are also possible within the layered clouds ahead of
the front. Overall conditions will improve later today as the
front washes out over the central islands, though a slight chance
of thunderstorms will remain through Friday night. Winds will
remain mostly light through the forecast period.

&&

.MARINE...
A cold front will slowly dissipate across the western and
central islands today with light to gentle southeast to southwest
winds along with some thunderstorms. Lighter winds will continue
through Saturday night, with increasing southwesterly wind speed
trends on Sunday as another cold front approaches the Hawaiian
Islands from the northwest. This second stronger cold front will
sweep southeastward through the islands chain on Monday, spreading
showers and thunderstorms as it quickly passes through the state.
More stable westerly winds will fill in behind the front with
decreasing shower trends as the front quickly passes through the
region. A Small Craft Advisory (SCA) has been issued through
tonight as seas are expected to exceed the 10 foot SCA threshold
as the northwest swell moves through.

A large northwest (320-330 degree) swell will build down the
island chain today producing extra large and dangerous surf along
most north and west facing shores through tonight with decreasing
trends on Saturday. Swell heights at buoy 51001 have spiked a few
feet above guidance this morning. A High Surf Warning (HSW)
remains in effect for north and west facing shores of Niihau,
Kauai, Oahu, Molokai, and north facing shores of Maui through
tonight. These surf heights will trend down to advisory levels by
Saturday. A Small Craft Advisory (SCA) also remains in effect for
the same time period covering all northern waters and southern
waters around Kauai and Oahu for seas 10 feet or higher. Another
extra large northwest (310-320 degree) swell will build into the
region from Sunday night to Monday morning. This long period swell
energy will produce hazardous surf heights well above warning
levels for both north and west facing shores on Monday, and large
mixed period swell from the nearby low pressure source will likely
hold these surf heights above HSW levels through Tuesday.

A small pulse of south-southwest swell is holding today,
otherwise no significant surf is due for east and south shores
into the first half of next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 AM HST Saturday for Niihau-Kauai
Leeward-Waianae Coast-Oahu North Shore-Maui Windward West-Kauai
North-Molokai Windward-Molokai North-Molokai West-Maui Central
Valley North-Windward Haleakala.

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

&&

DISCUSSION...M Ballard
AVIATION...TS
MARINE...Bohlin