High Surf Warning 5, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
831 PM HST Mon Feb 14 2022

.SYNOPSIS...
Moderate to breezy trade winds are expected through the upcoming
holiday weekend. A trend toward wetter weather is anticipated the
next couple of days as an upper-level low moves over the islands.
Showers, some briefly heavy, will favor windward areas, but a few
showers can be expected over leeward areas as well.

&&

.DISCUSSION...
To summarize, while it will be difficult to nail down the
particulars, it appears that our recent run of mostly dry weather
will trend wetter over the next couple of days as the island
atmosphere becomes quite unstable. Some showers could be briefly
heavy, with most favoring windward areas. Trade winds of varying
speeds and directions are expected into the weekend as a N-S
oriented surface trough lingers near the islands.

Latest satellite and radar imagery confirms that a low-level trough
is near Oahu this evening. Radar shows scattered moderate showers
moving S and SW over Kauai and Oahu, with clouds and showers near
Maui County and the Big Island moving toward the W and NW, with some
embedded heavy rain cores noted. Water vapor imagery depicts a
cutoff low aloft to the E of the Big Island supporting widespread
thunderstorms well E of the islands, between 140W and 150W.
Additionally, a shortwave trough aloft is approaching the islands
from the NW. With the low aloft nearby, the PHTO 00Z sounding
indicated an unstable but relatively dry (PWAT of 0.9") profile over
the Big Island, with the instability allowing clouds to extend as
high as 20-25 thousand feet. This has allowed lightning to develop
within a couple cells about 50 miles E of the Big Island this
evening. The overnight forecast anticipates mostly windward showers,
with a few briefly heavy, but coverage may remain limited due to the
relatively low PWAT.

A surface trough will linger near and N of the islands for most of
the week, with subtle modulations in its position and strength,
while strong high pressure persists far to the NE. Moderate to
locally breezy trade winds will prevail. While not all that
impactful with regard to the public forecast, this will lead to
changes in the direction and strength of the trade winds, with a
tendency for NE winds over Kauai and Oahu, and E winds over Maui and
the Big Island. This means that incoming moisture will primarily
focus over windward areas.

Over the next 24-36 hours, the low aloft to the E of the islands
will weaken and move E, as the trough moving in from the NW helps to
form a new cutoff low over the islands. 500 mb temperatures over the
islands will drop from around -11C to -16C, leading to significant
destabilization of the island atmosphere. While relatively low PWAT
(near 1.1") may limit shower coverage, some heavier shower cores are
expected. Thunderstorms are already included in the forecast for the
Big Island, but they may need to be added to other areas. Any
upslope precipitation will likely be a wintry mix over the summits
and upper slopes of the Big Island, with the potential for dangerous
freezing rain over the Haleakala Summit. The low aloft will move NE
and away from the islands Wednesday night and Thursday, leading to
an increase in stability, and a reduction in trade showers. A
passing shortwave trough may bring a brief period of instability
Friday night and Saturday, with the potential for an increase in
windward showers.

&&

.AVIATION...
Light to locally moderate trades will focus showers over windward
and mauka zones through the period. The resident weakly unstable
environment will support isolated heavier showers, particularly over
and in the vicinity of Windward Big Island through tonight. However,
limited shower coverage is expected to limit the duration of any
MVFR. Trades will then slowly strengthen, becoming locally breezy by
Tuesday evening. Pockets of heavier showers will once again be
possible over upslope portions of Leeward Big Island.

&&

.MARINE...
Light to moderate trade winds will hold in place tonight and early
Tuesday, with the trades strengthening to moderate and locally
strong levels late Tuesday afternoon and Tuesday night. A Small
Craft Advisory (SCA) may be needed for the typically windy waters
around Maui And the Big Island for a roughly 12 hour period Tuesday
night. In addition, a large northwest swell may build seas to the 10
foot SCA threshold around Kauai Tuesday and Tuesday night. The
trades are expected to ease slightly Wednesday and Wednesday night,
but should remain at moderate speeds. A surface trough will then
sharpen up to the east of the islands Thursday into the weekend,
which should shift the trades to more of a northeasterly direction
and increase speeds back up into the moderate to strong range. SCA's
may once again be needed for portions of the marine area during this
time.

An incoming west-northwest swell has arrived at the NOAA buoys
northwest of the state, and will continue to move into the islands
tonight and Tuesday. The swell appears to be running pretty close to
predicted levels at the moment and is expected to bring advisory
levels surf to exposed north and west facing shores beginning
tonight across the smaller islands and early Tuesday over west
facing shores of the Big Island. The swell may approach warning
levels along the leeward Big Island coast Tuesday afternoon and
Tuesday night, but we expect it to fall just short of reaching the
warning threshold of 12 feet. Advisory level surf may continue
through Wednesday for exposed north and west facing shores, then
lower Wednesday night through Friday. A new northwest swell is
expected to arrive Friday night, peak below advisory levels
Saturday, then lower Saturday night through early next week.

An out of season south swell will arrive tonight, with south shore
surf getting a boost up close to the summertime average Tuesday
through Thursday. This swell will lower Thursday night and Friday
with surf returning to mainly background levels over the weekend
into early next week. East shore surf will remain smaller than
normal through Wednesday, then rise close to the seasonal average
Thursday through the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Wednesday for North and West
facing shores of Niihau Kauai Oahu Molokai and for North facing
shores of Maui.

High Surf Advisory from 6 AM Tuesday to 6 AM HST Wednesday for
West facing shores of the Big Island.

Small Craft Advisory from 6 AM Tuesday to 6 AM HST Wednesday for
Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward
Waters.

&&

DISCUSSION...Birchard
AVIATION...JVC
MARINE...Jelsema