High Surf in Hawaii, March 7 to 8, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
339 AM HST Sat Mar 7 2020

.SYNOPSIS...
Strong high pressure passing north of the islands will support
windy conditions over the islands today, with trades gradually
easing tonight into early next week. A cooler air mass will bring
drier conditions and fewer trade showers over the weekend, with
periods of high cloudiness also anticipated. Light winds and
warmer temperatures are forecast beginning Tuesday and Wednesday
as a front approaches the state, with the front potentially
affecting island weather late next week.

&&

.DISCUSSION...
Currently at the surface, a 1036 mb high is centered around 1150
miles north-northwest of Honolulu and is driving breezy to
locally windy trades across the island chain early this morning.
Infrared satellite imagery shows a mix of high and low clouds
resulting in mostly cloudy conditions in most areas. Radar
imagery shows scattered to numerous showers moving into windward
areas, with regular spillover into leeward communities due to the
strength of the trades. Main short term concern revolves around
trade wind trends.

Strong high pressure will hold in place to the north-northwest of
state through the weekend, with a weakening/dissipating front
shifting southward toward the island chain late tonight and
Sunday. The trades will peak today, with advisory level winds
expected across the entire state, and warning level winds across
some of the windier areas of Maui County and the Big Island. The
trades will begin trend down slightly tonight through Sunday as
the front approaches from the north, but advisory level winds may
hold in place across portions of the state right through the day
Sunday. The front or its remnants will shift closer Sunday night,
then move through the islands on Monday, while a weakening high
shifts eastward well to the north of the state. The trades should
lower to moderate levels statewide by Monday, then hold in that
range through Tuesday. Winds should weaken and shift around to the
southeast Wednesday as a new front approaches the islands from
the west, with sheltering effects from the Big Island resulting in
a land/sea breeze pattern over the smaller islands. Forecast
details become much more uncertain by late next week, and will
hinge on the strength of the front moving through or into the
island chain.

As for weather details, showery trade wind weather is expected to
continue this morning, with quite a bit of leeward spillover due
to the strength of the trades. Shower coverage will then trend
considerably lower by this afternoon as precipitable water (PW)
values fall to around 0.7-0.8 inches. Rather dry conditions will
then continue statewide Saturday night through Sunday night, with
PWs dropping as low as 0.5-0.6 inches on Sunday. A few
orographically induced showers will likely continue in
windward/mauka areas due to the strong trades, but rainfall
amounts will be light along with limited leeward spillover. A
considerable amount of high cloud cover is expected today and
tonight, with the high clouds thinning out and shifting east of
the state during the day Sunday.

The remnant moisture from an old front is expected to swing
through the islands Monday, and this will bring an increase in
trade wind showers. Fairly typical trade wind weather should then
prevail Monday night through Tuesday night, with showers favoring
windward and mauka areas along with the stray leeward spillover. A
brief convective sea breeze pattern may develop on Wednesday as
the flow tips southeast, with showers favoring interior and mauka
locations. Considerable uncertainty continues during the longer
range portion of the forecast, due to rather poor model
consistency and run to run continuity. Overall, it appears that a
front could enter the islands late next week, bringing an increase
in cloud cover and showers to the region, with some areas getting
rather wet.

&&

.AVIATION...
Strong and gusty trade winds will continue through today as a
broad area of strong high pressure, 1036 mb, lingers far north-
northwest of the islands. Thus, expect AIRMET Tango for low level
turb below 8k feet, south thru west of mountains of all islands,
to persist through tonight.

Satellite imagery shows a drier air mass upwind of the islands
depicted by a field of stable stratocumulus clouds. But before the
drier airmass arrives, we will have endure a couple of more hours
of passing showers. AIRMET Sierra for mountain obscuration is
still posted for Oahu and Kauai, but there is a chance it will end
at 16z. If not 16z, then by 18z. This stabilizing process is
already in motion with the clouds tops as high as 12k feet earlier
in the evening, and now at 13z, the tops are down to 7 to 9k
feet.

Dense high cirrus clouds continue to spread across the island
skies from the west this hour. These clouds will linger into
tonight.

Other than the strong winds, VFR conditions will be the
prevailing flying weather. Any showers that do occur, will be
brief, and favor the windward and mountain areas.

&&

.MARINE...
The tight pressure gradient south of a 1036 mb surface high
pressure system located far north-northwest of the state is
producing very strong trade winds and elevated, rough seas across
the islands early this morning. A scatterometer pass across the
islands Friday evening confirmed east winds of 30 to 35 knots
were across much of the ocean surrounding the islands. Therefore,
a Gale Warning remains in effect through early Sunday morning for
the typically windier waters adjacent to the islands of Maui
County and the Big Island. A Small Craft Advisory (SCA) remains
in effect through Sunday afternoon for all remaining Hawaiian
waters. Expect these hazardous marine conditions to persist
through the remainder of the weekend, so the SCA will likely be
extended at least through Sunday night for most waters. The trade
winds are forecast to trend down early next week as the surface
high gradually weakens, and moves to a position far northeast of
the islands. By Wednesday, a front approaching from the northwest
will likely cause the surface winds to become light and variable
over many areas. Moderate west to northwest winds may start to
fill in near Kauai late Wednesday night.

The strong and gusty trade winds around and upstream of the
islands will continue to maintain rough and elevated surf along
east facing shores this weekend. In addition, a medium period
north-northeast swell arriving tonight into Sunday will likely
keep elevated surf along most east facing shores through Sunday
night. A High Surf Advisory (HSA) is currently in effect through
Sunday afternoon for east facing shores of Kauai, Oahu, Molokai,
Maui, and the Big Island. Note that the Wave Watch III model
guidance shows the north-northeast swell may be slightly larger
over the eastern end of the island chain. Therefore, expect the
HSA to be extended through Sunday night, and possibly into Monday,
for most east facing shores. A new northeast swell may fill in
from late Monday into Tuesday, and surf could remain near the HSA
threshold along some east facing shores into Monday night, and
possibly Tuesday.

In addition to the medium period north-northeast swell arriving
this weekend, which was described above, expect a small, long
period west-northwest swell to arrive later today, and persist
from Sunday into Monday. A slightly larger long period west-
northwest swell arriving Monday night is expected to build
through Tuesday night, then gradually lower late Wednesday. Surf
produced by the west-northwest and north-northeast swells will
likely remain below the HSA criteria along north and west facing
shores through the middle of next week. Elsewhere, expect small
background surf along south facing shores due to small south and
southwest swells.

&&

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

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

High Wind Warning until 6 PM HST this evening for Lanai-
Kahoolawe-Maui Leeward West-Kohala.

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

Small Craft Advisory until 6 PM HST Sunday 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...H Lau
MARINE...Houston