Heavy Rain in Hawaii, September 14 to 16, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
358 PM HST Tue Sep 14 2021

.SYNOPSIS...
A surface trough passing just north of the state, will weaken the winds
tonight with light to moderate east to east-southeast winds expected
on Wednesday. Localized sea breezes will develop Wednesday
afternoon and could lead to pockets of heavy rain showers during
the afternoon and evening. Another band of moisture moving in from
the southeast will likely produce more showers primarily for the
windward Big Island Wednesday evening and night. Trade winds will
pick back up Thursday with conditions gradually improving
throughout the day. Drier weather should return from Friday onward
as the trade winds strengthen across the region.

&&

.DISCUSSION...
Latest water vapor satellite imagery shows an upper level low
located roughly 500 miles north of the Big Island. This upper low
is expected to move southwest and should be a few hundred miles
north of the Oahu on Wednesday. At the surface we have a trough
that is associated with the upper level low and as this trough
passes just north of the state on Wednesday, winds will ease
across the state and begin to veer toward the east-southeast.
Localized sea breezes will lead to scattered showers across the
state and with the upper level low nearby, pockets of heavy rain
is expected during the afternoon.

Starting late Wednesday afternoon into Wednesday night, a surge
of moisture is expected to move in from the southeast and will
likely bring some pockets of heavy rain across the east and
southeast facing areas of the Big Island and Maui. The rest of
the island chain will be shadowed from the Big Island, but some
scattered showers are still expected with some brief downpours at
times.

Thursday will be a transition day as the trough begins to move
away from the state and high pressure builds north of the state.
Showery conditions are still expected as the breezy trade winds
fill back in starting from the eastern end of the state. Some
lingering moisture and instability is expected over the western
end of the state, which could lead to some brief downpours over
Kauai on Thursday. By Friday, strong trade winds will return
across the state with drier conditions.

&&

.AVIATION...
Afternoon satellite imagery reveals multiple bands of trade wind
showers passing over the islands. Showers will impact mainly
windward slopes and coasts, especially those of the smaller
islands. The air mass over the state is mostly dry and mostly
stable so any precipitation should remain light and intermittent.

Meanwhile on the Big Island, leeward sea breezes will give way to
weak land breezes after sunset. This will promote temporary
clearing along south and west facing slopes and coasts starting
after midnight. Sea breezes and interior cloudiness will return
to the Big Island Wednesday afternoon.

Brief MVFR CIG or VIS is possible in passing showers, otherwise
VFR conditions will prevail. The are no AIRMETs currently in
effect or anticipated at this time.

&&

.MARINE...
Moderate to fresh trades will briefly ease tonight and Wednesday as
a surface trough moves from east to west north of the state. Breezy
trades will fill in behind the trough due to a building high far
north of the state Wednesday night through Saturday. Small Craft
Advisory (SCA) level trades will likely return Thursday over the
typically windy Maui and Big Island waters. The SCA level trades
may expand to most waters Friday and Saturday, before easing
slightly Sunday into early next week as the high moves east.

A series of long period south and southwest swells will keep surf
along south facing shores near to slightly above the September
average through Thursday, before gradually lower through the
weekend into early next week. Small surf will prevail on north
facing shores through most of the week, with a small boost from a
northwest swell this weekend. East shore surf will decline
slightly tomorrow as trades ease locally and upstream. Rough,
east shore surf will build to near seasonal levels Thursday
through the weekend, as trades return.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION...Kino
AVIATION...Bedal
MARINE...Almanza