High Surf in Hawaii, February 19 to 21, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
332 AM HST Wed Feb 19 2020

.SYNOPSIS...
Gusty easterly trade winds will continue through Thursday, then
gradually trend down and shift out of the southeast over the
upcoming weekend as a front approaches the region. Although most
of the showers will focus over windward and mountain locations,
some will make it into leeward areas occasionally with the strong
trades in place. Light and variable winds will become a
possibility by Sunday as the tail-end of the front moves into the
area and diminishes.

&&

.DISCUSSION...
The latest surface analysis showed 1032 mb high pressure centered
north-northwest of the state, placing a strong pressure gradient
over the islands. Local winds are responding and have increased
through overnight period. Observations around the state showed
peak gusts ranging from 40 to 55 mph. Strongest winds will affect
exposed areas such as mountain ridges and points between the
islands, as well as leeward areas downslope of the mountains. In
addition to the winds, a band of shallow moisture is supporting
showery conditions this morning over Maui County and portions of
windward Oahu. These showers have been light with very little
accumulations, mostly focusing over windward and mountain
locations (occasionally making it into leeward areas briefly).

Guidance suggests high pressure will build to around 1035 mb by
tonight, as it shifts to an area due north of the state. A
combination of very dry mid-level air, a strong subsidence
inversion positioned around 6000 to 7500 ft and a strengthening
pressure gradient will support the windy conditions holding into
Thursday. Periods with enhanced shower activity associated with
shallow bands of moisture within the trades moving through and
weak pulses aloft dropping southward through the islands are
expected through Friday. Although most of this activity will focus
over windward and mountain locations, some will spill over into
leeward sections with the strong winds in place. Accumulations
will remain light.

The extended guidance depicts a large-scale pattern shift across
the central Pacific over the weekend as a broad upper trough and
cold front approach from the northwest. Low-level flow will
gradually veer out of the southeast and weaken, potentially enough
for light and variable winds by the end of the weekend into
Monday. As the winds veer, showers will transition to the
southeast slopes of Maui and the Big Island and interior areas
across the western end of the state through the afternoon hours as
sea breezes setup.

The front will dissipate Monday with high pressure building to
the north in its wake. A return of a more typical trade wind
pattern will be possible beginning around Tuesday. A lingering
band of moisture associated with the diminished front, however,
may support decent windward shower coverage continuing over
portions of the state through midweek.

&&

.AVIATION...
High pressure north of the state will keep strong and gusty trade
winds blowing through tonight. Clouds and showers will favor
windward and mauka areas, although showers will also reach leeward
locations more regularly, particularly during the night and early
morning hours. Showers will bring MVFR cigs/vsbys to windward
areas at times, while VFR conditions prevail in leeward locales.

AIRMET Sierra remains in effect for mountain obscuration across
windward sections of Kauai, Oahu, Molokai and Maui. These
conditions will likely improve later this morning.

AIRMET Tango remains in effect for moderate low level turbulence
over and downwind of the terrain, as well as for strong surface
winds across the entire state. These conditions are expected to
continue through tonight.

&&

.MARINE...
A strong high about 700 nm north of Kauai will move east at 15 kt
over the next few days. The pressure gradient south of the high
will produce 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 is in effect
through Thursday evening for the windy zones around the Big Island
and Maui and a Small Craft Advisory (SCA) is in effect for all
other Hawaiian Coastal Waters.

Later in the week as the high moves farther away to the east, the
trade winds will weaken. A front will approach from the northwest
starting Saturday. Winds will shift out of the southeast, then
south ahead of the front. The front is expected to move into the
far northwest offshore waters Sunday night with strong northeast
trade winds returning behind the front.

The strong trade winds will maintain surf above the High Surf
Advisory threshold of 8 feet along east facing shores through
Friday. This surf will subside over the weekend as the trade winds
weaken.

The current northwest swell is subsiding. A new small northwest
swell will peak tonight, then gradually lower through Friday. A
slightly larger northwest swell arriving Friday will peak
Saturday, then lower slowly through the weekend. A new large
northwest swell is possible early next week. A series of small
south swells will continue through the coming weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 AM HST Thursday for Kauai-Oahu.

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.

Wind Advisory until 6 PM HST Thursday for 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.

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

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-Maui County Leeward Waters-Big Island
Windward Waters.

&&

DISCUSSION...Gibbs
AVIATION...Jelsema
MARINE...Donaldson