High Surf in Hawaii, December 26 to 29, 2023

Area Forecast Discussion
National Weather Service Honolulu HI
937 AM HST Tue Dec 26 2023

.SYNOPSIS...
Stable northeasterly winds will push shallow bands of showers
over windward slopes of Kauai and Maui County today. Winds will
diminish tonight, then pick up out of the north on Wednesday. A
weak front will move down much of the island chain Thursday and
Friday, bringing a modest increase in rainfall over windward and
northern slopes and ushering in moderate trade winds.

&&

.DISCUSSION...
Very stable northeasterly winds prevail this morning, as a pair
of shallow bands of moisture bring clouds and some showers to
Maui County and Kauai. The northeasterly winds are being driven by
a weakening surface high moving eastward and centered roughly 350
miles northwest of Kauai. The high is pushing a narrow band of
moisture, the tail end of a dissipating front, over Kauai, while
a second moisture band, a weak convergence zone, is stalled over
Maui. Meanwhile, dry conditions are in place on Oahu and the Big
Island. Ridging aloft is maintaining a strong inversion, capping
clouds at 6,000 to 7,000 ft and keeping showers over windward and
northern slopes of Maui County and Kauai rather light. In spite of
the stable environment, the slow moving nature of the band on
Maui allowed a few locations to measure over a half an inch of
rainfall during the past 12 hours. Expect Maui County to remain
cloudy and wetter today, and the rainfall on Kauai should taper
off this afternoon, while drier conditions persist on Oahu and the
Big Island.

Winds will ease tonight and become northerly over Kauai and Oahu
on Wednesday as the surface high to the northwest weakens into a
ridge and is pushed over the islands. The shallow convergence zone
will linger over Maui County, keeping shower chances higher
there, and rather dry and stable conditions will prevail
elsewhere.

Later in the week, the zonal jet stream over the North and
Central Pacific will drift southward, pushing a weak front over
the state. The front will bring showers to mainly northern and
windward slopes of Kauai Thursday morning then move slowly down
the island chain through Friday. While this feature will be a bit
deeper than the current moisture bands over the islands, firm
mid-level ridging will maintain stable conditions and allow only
moderate shower activity. Surface high pressure following the
front will bring a brief shot of locally breezy northeasterly
trade winds, but winds will quickly drop to moderate strength by
Friday as the high moves off to the east.

Trades will hold at moderate levels Saturday and may drop off
Sunday as another front approaches.

&&

.AVIATION...
Light winds will give way to moderate northeast trade winds later
today as the tail end of a dissipating front approaches and moves
into the area from the northwest. With a relatively dry and
stable airmass in place, shower activity will generally be limited
through the period. However, a band of low level clouds and
showers is draped across Kauai and Maui County this morning.
AIRMET Sierra is in effect for tempo mountain obscuration above
2500 feet across windward Kauai, Molokai, Maui and Lanai. Expect
condition to continue into this afternoon. VFR conditions will
prevail across Oahu and Big Island.

&&

.MARINE...
Surf along north and west facing shores will be the main marine
story into next week. As a series of large to extra-large
northwest and north swells move through the islands.

The first swell (330 deg) is currently building down the island
chain today and looks to align pretty well with guidance. Surf,
already nearing the advisory levels for the north and west facing
shores of the smaller islands, will continue to build peaking at
warning levels (20 to 30 feet north shores, 15 to 25 feet west
shores) this evening and Wednesday. A High Surf Warning has been
posted through tomorrow evening. Seas will also reach the Small
Craft Advisory (SCA) threshold for those waters around Kauai,
Oahu, and windward Maui County.

The second swell (340 deg) should start rolling in later Thursday
through Friday. The current High Surf Warning will likely be
extended to continue to cover this activity. The same applies for
the SCA.

Next, a mix of north (360-020 deg) and northwest (320 deg) swell
energy impacting exposed beaches will likely support advisory-
level surf persisting this weekend. For the extended, guidance
depicts the next warning-level event arriving Tuesday of next
week.

Provided the long duration with advisory to warning level surf
expected over the next week, significant erosion along exposed
north and west facing beaches should be anticipated. This erosion
combined with our early morning high tides and warning-level surf
could translate to overwash onto our vulnerable coastal roadways
and impact properties periodically. The most likely time frames
for these impacts will be around daybreak Wednesday and late
Thursday into Friday as the large overlapping events are near
their peak.

Surf along east facing shores will remain small through the week
due to the light winds locally. East shore beaches exposed to
northerly swells will begin to pick up Friday through the upcoming
weekend as the swell direction shifts out of the north to north-
northeast.

Surf along south facing shores will remain around the winter
average through midweek. A slight bump is possible during the
second half.

The winds over the local waters will fluctuate from light and
variable to moderate north to northeast as the tail-end of
passing fronts clip the area. For timing, expect the light winds
on Wednesday and Friday and the moderate northerly winds today,
Thursday, then late Friday through Saturday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST Wednesday for Niihau-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 PM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters.

&&

DISCUSSION...Wroe
AVIATION...Foster
MARINE...EATON