High Surf in Hawaii, February 15 to 17, 2022

Area Forecast Discussion
National Weather Service Honolulu HI
345 AM HST Tue Feb 15 2022

.SYNOPSIS...
Moderate to locally breezy trade winds are expected through the
upcoming holiday weekend. A period of 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. The low
will also bring a slight chance of thunderstorms, mainly tonight and
Wednesday. The chance for heavy showers will diminish Thursday and
Friday.

&&

.DISCUSSION...
In summary, a period of somewhat wetter weather is possible over the
next couple of days as the island atmosphere becomes quite unstable.
Some showers could be briefly heavy, with most favoring windward
areas, with a slight chance of a thunderstorm popping up just about
anywhere. A more stable regime is expected Thursday into Friday.
Trade winds of varying speeds and directions are expected into the
holiday weekend as a N-S oriented surface trough lingers near the
islands.

Latest satellite and radar images indicate a pretty significant
decrease in clouds and showers over and around the islands
overnight, with just a few showers noted over windward waters. VIIRS
imagery showers mostly clear skies over land, except for some
layered clouds aloft moving over Kauai from the W. Water vapor
imagery (when it was available through 12Z) indicated that a cutoff
low aloft to the E of the Big Island was departing to the NE, as a
shortwave trough aloft moved over the islands from the NW. The PHTO
12Z sounding continued to indicate an unstable but relatively dry
(PWAT of 1.1") profile over the Big Island, with the instability
allowing clouds to extend as high as 20 thousand feet. A more stable
profile was depicted by the PHLI 12Z sounding. The short term
forecast anticipates abundant sunshine for most areas, a few mainly
windward showers, with the Big Island slopes producing clouds,
showers and potentially a thunderstorm this afternoon.

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 are expected to prevail, with some
increase in trade wind speeds expected into tonight. While not all
that impactful with regard to the public forecast, there will likely
be somewhat difficult-to-predict changes in the direction and
strength of the trade winds through the week, 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 hours, the low aloft to the E of the islands will
continue to weaken and move away to the E, as the trough moving in
from the NW helps to form a new cutoff low, this time directly over
the islands. 500 mb temperatures over the islands will drop to near
-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. A slight chance of
thunderstorms is in the Big Island forecast for this afternoon, and
for virtually the entire state tonight and Wednesday. Additionally,
lower than normal freezing levels means that 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/icing over the Haleakala Summit. The low aloft will move NE and
away from the islands Wednesday night and Thursday, leading to a
gradual increase in stability, and a reduction in shower intensity.
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...
Mostly clear skies and VFR prevail across the islands this morning.
Although moderate trades continue to carry pockets of heavy showers
toward windward zones, the dry resident airmass is favoring rather
limited coverage. At the same time, land breezes have prevented any
of these showers from moving onshore. As nocturnal stability erodes
during the morning, a more concerted onshore push of isolated to
scattered showers over windward areas will be followed by robust
afternoon shower development for upslope portions of Leeward Big
Island. There, mid-level temperatures will be sufficiently cold to
support a slight chance of thunderstorms during peak heating.
Isolated thunderstorms will be possible areawide by tonight as a
strong upper trough becomes established over the islands. AIRMET
Tango for upper turb may be needed by this afternoon as the
aforementioned trough deepens over the western end of the state.

No AIRMETs in effect.

&&

.MARINE...
Light to moderate trade winds will hold in place this morning, with
the trades strengthening to moderate and locally strong levels this
afternoon and tonight. A Small Craft Advisory (SCA) has been issued
for the typically windy waters around Maui And the Big Island
through 6 AM Wednesday as a result. In addition, a large northwest
swell will build seas to the 10 foot SCA threshold for the coastal
waters not blocked by the islands, so the SCA also includes many of
these marine zones through 6 AM Wednesday.

The trades are expected to ease slightly Wednesday and Wednesday
night, but should remain at moderate speeds. A surface trough will
then sharpen up over or near the eastern islands Thursday into the
weekend, which should shift the trades to a more northeasterly
direction and increase speeds back up into the moderate to strong
range, especially over the western end of the state. SCA's may once
again be needed for portions of the marine area during this time.

An incoming west-northwest swell has arrived in the islands early
this morning and based on observations at the NOAA buoys to the
distant northwest of the state, will continue to build through the
day today. The buoys northwest of the islands indicate that the
swell has risen to several feet above predicted levels, and as a
result some changes were needed to the ongoing hazards. A High Surf
Warning (HSW) is now in effect for north and west facing shores of
Niihau, Kauai, and Oahu as well as west facing shores of the Big
Island. A High Surf Advisory (HSA) remains in effect for north and
west facing shores of Molokai and north facing shores of Maui. The
HSW and HSA are both in effect through 6 PM Wednesday. The swell is
expected to lower Wednesday night through Friday, but could linger
at advisory levels into Wednesday night for exposed north and west
facing shores. A new northwest swell is forecast to arrive Friday
night, peak below advisory levels Saturday, then lower Saturday
night through early next week.

An out of season south swell will will give south shore surf a boost
up close to the summertime average today 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 Warning until 6 PM HST Wednesday for North and West facing
shores of Niihau Kauai and Oahu and for West facing shores of the
Big Island.

High Surf Advisory until 6 PM HST Wednesday for North and West
facing shores of Molokai and for North facing shores of Maui.

Small Craft Advisory until 6 AM HST Wednesday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu
Windward Waters-Kaiwi Channel-Maui County Windward Waters-Maalaea
Bay-Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

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