High Surf in Hawaii, February 19 to 20, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
414 AM HST Sun Feb 19 2017

.SYNOPSIS...
Locally strong north to northeast winds are expected today through
Monday as high pressure builds in from the northwest, and a weak
cold front passes south of the Big Island. Clouds and showers will
focus over windward areas and northern slopes. Breezy trade winds
will continue into the middle of next week, with clouds and brief
showers favoring windward areas, especially during nights and
mornings.

&&

.DISCUSSION...
A weak, shallow front has reached the Big Island this morning
with strong and gusty north winds following the front down the
island chain. Clouds and showers will mainly affect north slopes
and windward areas with only light to moderate rainfall totals
expected. A few showers will reach leeward locations as the strong
winds push moisture over the terrain but these will be brief in
nature.

The front will continue moving south and exit south of the Big
Island later this evening. Gusty north winds are filling in
behind the front as high pressure builds in from the northwest.
Winds will accelerate over and through the higher terrain and a
wind advisory has been issued for sections of Maui County and the
Big Island, including the higher terrain of the Lanai and over the
Kohala Mountains on the Big Island.

North winds behind the front will push dry air over the islands
with associated precipitable water values in the 0.6 - 0.8 inch
range. This dry air will limit most clouds and showers to the
windward slopes through Monday. Gusty trade winds will remain
through Wednesday as the high travels north of the islands, with
limited showers focused over windward and mountain areas. While
another high is expected to build northwest of the islands by the
middle/end of next week, a developing low east of the high may
act to limit any local increase in wind speeds. Latest model guidance
indicates that light to moderate winds will back to the northeast,
keeping a cool and relatively dry weather pattern in place.

&&

.AVIATION...
A weakening cold front will continue to move southeastward,
pushing through the Big Island later today. A band of lower
clouds and showers will accompany the front as it moves through
the Big Island, with reduced ceilings and visibilities expected
across windward and mauka areas through much of the day. Across
windward sections of Maui County, conditions will be improving
through the morning hours, with predominantly VFR conditions
expected by late this morning. Across Kauai and Oahu, VFR
conditions are generally expected, although brief MVFR ceilings
may drift into windward areas from time to time. Strong north to
northeast winds are expected across the entire island chain
through tonight.

AIRMET Sierra is in effect for mountain obscuration across
windward sections of Molokai and Maui. The AIRMET will likely
need to be expanded to include windward sections of Big Island
prior to daybreak. Conditions should improve by 19/1600z in
Molokai, and by 19/2100z in Maui. Mountain obscuration will likely
continue across windward sections of the Big Island through much
of the day.

AIRMET Tango is in effect for moderate low level turbulence over
and downwind of the mountains from Kauai eastward to Maui. This
AIRMET will likely need to be expanded to include the Big Island
later this morning.

&&

.MARINE...
Strong to gale-force winds associated with strong high pressure
building north of the state in the wake of a cold front will
continue through Monday. Gales are expected across the Pailolo and
Alenuihaha Channels and Maalaea Bay this afternoon through Monday
before gradually trending down Tuesday through Wednesday. Local
winds will veer from the north this morning to the northeast this
afternoon and toward the east Monday. The overnight ASCAT pass
clipped the Kauai waters and showed winds up to 25 kt out of the
north. These winds will generate very rough/short-period seas across
the local waters.

In addition to the winds and seas, a large north-northwest (330-350
deg) swell will fill in across the waters today, peak this
evening/overnight, then gradually ease Monday through Tuesday. This
swell is associated with a storm-force low that was analyzed just
under 1000 nautical miles north of the state. Provided the proximity
of this storm to the local area, wave periods will come in within
the 11-13 second energy band, which are shorter periods than
typically observed from our large northwest sources. Buoy 51101
observations (northwest of Kauai) have remained in line with the
latest wave model guidance through the early morning hours and
continue to climb as of this discussion time.

Surf along exposed north and west facing shores will quickly respond
and reach advisory levels today as conditions evolve and the north-
northwest swell fills in. These conditions will continue through
Monday before trending down. Later packages may need to include the
northern shores of the Big Island as the storm passes well to the
north later tonight through Monday.

Surf along east facing shores will gradually rise through the week
and near advisory levels Tuesday through Wednesday as the winds veer
out of the east.

On Wednesday, a moderate long-period west-northwest (310 deg) swell
associated with a developing gale east of Japan is expected to fill
in, peak late Wednesday through Wednesday night, then gradually ease
Thursday through Friday. If this source comes in slightly higher
than predicted, advisory-level surf along north and west facing
shores will become a possibility Wednesday and Wednesday night.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Monday for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Molokai-Maui
Windward West-Maui Central Valley-Windward Haleakala.

Wind Advisory until 6 PM HST Monday for Lanai-Kahoolawe-Big
Island North and East-Kohala.

Small Craft Advisory until 6 PM HST Monday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Big Island
Windward Waters-Big Island Leeward Waters-Big Island Southeast
Waters.

Small Craft Advisory until noon HST today for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel.

Gale Warning from noon today to 6 PM HST Monday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel.

&&

Morrison...Discussion
Jelsema...Aviation
Gibbs...Marine