Strong Wind in Hawaii, February 20, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
303 AM HST Thu Feb 20 2020

.SYNOPSIS...
Gusty trade winds will continue today, then gradually decrease
tonight through Saturday as a front approaches from the
northwest. Although rainfall will focus over windward and mauka
locations, strong winds will push showers into leeward areas at
times as well. Winds will become light and variable by Sunday as
the front nears the islands, with sea and land breezes expected
statewide. Showers will favor interior and mountain areas during
the day and locations near the coast at night. Trade winds are
expected to return early next week, with rather wet conditions
possible over the western islands as the front stalls out in this
vicinity.

&&

.DISCUSSION...
Currently at the surface, a 1033 mb high centered around 900
miles north-northeast of Honolulu is driving strong and gusty
trade winds across the island chain early this morning. Infrared
satellite imagery shows the back edge of a swath of low clouds
approaching the islands from the east, with mostly cloudy skies
present across much of the state at the moment. Radar imagery
shows numerous showers pushing into windward areas from Maui to
Kauai, with coverage beginning to diminish over windward Big
Island. Considerable leeward spillover continues into leeward
sections of the smaller islands, while leeward Big Island is
mainly dry. Main short term concerns revolve around trade wind
trends and rain chances through the remainder of the work week
and into the weekend.

Strong high pressure will slide eastward and weaken slightly over
the next few days as a cold front approaches from the northwest.
Strong and gusty trades will continue across the island chain
today, and as a result, the Wind Advisory is now in effect for
most areas of the state through 6 PM this evening. The trades are
expected to trend lower tonight and Friday, then shift around to
the east-southeast on Saturday as the front moves closer. Light
and variable winds will likely develop by late Saturday across the
western islands while moderate trades hold on across the eastern
end of the state. The gradient will slacken further Saturday
night, allowing a land breeze pattern to develop statewide, with
this sea/land breeze pattern holding in place through at least
Sunday night. Model solutions are in fairly good agreement showing
the front or its remnant band of moisture pushing into the
western islands Monday, then stalling out in this area through the
first half of next week before dissipating. As the front stalls,
a new high will slide by well to the north of the island chain.
Trade winds look to return Monday, with moderate to breezy trades
taking hold Monday night and continuing through the middle of
next week.

As for weather details, the back edge of the showery low clouds is
expected to push westward through the islands over the next few
hours, with a much drier airmass overspreading the island chain
around or shortly after daybreak. This should bring a more
typical trade wind shower pattern to the local area through
Friday night, with showers favoring windward and mauka areas. A
few showers will continue to spread leeward, although there will
likely be a reduction in leeward shower coverage given
precipitable water values dropping to around 0.75 inches.

Over the weekend, a transition over to a sea breeze/land breeze
pattern will keep showers favoring areas near the coast in an
east-southeast to southeast boundary layer flow at night, and
interior and mauka areas during the afternoon/early evening. A
transition over to a trade wind pattern will bring a return of
more windward/mauka showers and the occasional leeward spillover
early next week. Rather wet conditions could develop over the
western islands during the first half of next week however, as the
front or its remnants stall out in this vicinity.

&&

.AVIATION...
Gusty easterly trades and showery conditions due to a band of
moisture/clouds moving through will continue through the early
morning hours today. Although most of the shower coverage has
been focusing over windward sections of the islands, the strong
winds are managing to carry some showers over into leeward areas
as well. Satellite imagery did show much drier air trailing this
band of moisture, which has already begun to push into windward
areas of the Big Island. This should result in a gradual drying
trend through the morning hours from east to west. Until then,
MVFR ceiling and visibilities will continue for windward areas,
especially in and around passing showers. The strong trades should
begin to trend down later this evening and into the overnight
hours tonight.

AIRMET Sierra for tempo mountain obscuration is in effect for
windward sides of Kauai, Oahu, Molokai, Maui and the Big Island.
This may be cancelled by mid morning as conditions gradually
improve and the aforementioned drier air fills in.

AIRMET Tango remains in effect for tempo moderate low level
turbulence over and downwind of the terrain.

&&

.MARINE...
A strong surface high pressure system is moving steadily east far
north of the state. The very tight pressure gradient south of
this feature is producing strong to near-gale force easterly
trade winds over Hawaiian waters, with winds reaching gale force
in areas where the wind accelerates around the islands. A Gale
Warning remains in effect through this afternoon for the typically
windier waters adjacent to the islands of Maui County and the Big
Island. A Small Craft Advisory (SCA) also remains in effect
through early Friday morning for all other Hawaiian Waters. The
SCA will likely be scaled back to include fewer areas as winds
and seas slowly diminish from Friday through Friday night.

By this weekend, the surface high will move farther away to the
east, as a front approaches the state from the northwest. This
will allow the trade winds to weaken, and start to shift out of
the east-southeast over most areas by Saturday. The front is
expected to move into the far northwest Hawaiian offshore waters
Sunday, with increasing northeasterly trade winds returning
behind the front.

The strong trade winds will maintain rough, elevated surf along
east facing shores through Friday. As a result, a High Surf
Advisory remains in effect through Friday afternoon for east
facing shores of the islands. This surf will subside over the
weekend as the trade winds weaken.

The current small northwest swell will continue through this
evening, then gradually lower from later tonight through Friday.
A slightly larger, long-period northwest swell arriving late
Friday will likely peak on Saturday, then gradually lower from
Sunday through Monday. A new large northwest swell is possible
early next week. Elsewhere, a series of small south and south-
southwest swells will maintain small background surf along south
facing shores through this weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM HST this evening for Kauai-Oahu-Molokai-
Lanai-Kahoolawe-Maui Windward West-Maui Leeward West-Maui
Central Valley-Windward Haleakala-Leeward Haleakala-South Big
Island-Big Island North and East-Kohala.

High Surf Advisory until 6 PM HST Friday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Gale Warning until 6 PM HST this evening for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 AM HST Friday 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.

&&

DISCUSSION...Jelsema
AVIATION...Gibbs
MARINE...Houston