Heavy Rain in Hawaii, April 20 to 21, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
833 PM HST Thu Apr 20 2017

.SYNOPSIS...
Moisture associated with a weak front will stall near Oahu and Maui
county overnight as the front gradually dissipates. Meanwhile a
surface ridge near the Big Island will keep a dry and stable weather
pattern in place there through tonight. High pressure building north
of the islands will bring strengthening trade winds through Friday,
with moderate trade winds persisting into next week. The trade winds
will push lingering moisture over the smaller islands for the next
several days, and a wet trade wind flow is expected, with clouds and
showers especially active over windward slopes and coasts. Humid
conditions will continue through the weekend while drier trade
wind weather is expected early next week.

&&

.DISCUSSION...
A stalled front continues to bring clouds and showers over Oahu
and Maui County. Rainfall totals across the central islands show
moderate rain rates of 1/4 to 1/2 inch per hour with some embedded
heavier showers. The front will continue to sit over the central
islands tonight and linger into the weekend as the front weakens.
High pressure will build north of the state Friday and trade
winds will take over as the front dissipates over the state.
Clouds and showers left behind by the front will continue to
plague the central islands and Kauai into Sunday as the trades
push the frontal moistures back over the islands from the east.
So, wet trade winds are expected for the smaller island this
weekend with the majority of rainfall over windward areas with
passing showers over leeward areas. The Big Island looks to remain
outside of the main band of clouds and only isolated showers are
expected through friday until trade winds bring limited scattered
showers to windward areas Friday night. Humid conditions will
remain across the state through the weekend before dew points
start to drop next week.

High pressure north of the islands will build trades back over the
state Friday, and maintain moderate trade winds through next week.
Drier trade weather will be on tap as the frontal moisture pushed
west of the state and lower dew points filter in from the east.
Limited showers will fall mainly over the mountains and windward
slopes.

&&

.AVIATION...
A weak front over the isles will continue to bring continuous
light to moderate rain, especially from Oahu to Maui. N to NE
winds have already become established over Kauai, Oahu, Molokai,
and Maui. This flow direction should help focus the majority of
the clouds and showers along the north through east facing slopes
tonight, with leeward areas seeing some improvement later.
Scattered areas of MVFR CIGS, and possibly VIS, can still be
expected for Oahu, Molokai, Lanai, windward Kauai, and portions of
Maui. Isolated IFR conditions cannot be ruled out either.

Big Island will remain relatively quiet tonight with prevailing
VFR conditions. Daytime sea breezes will give way to land breezes
tonight, and help to clear out most land areas.

&&

.MARINE...
A large, late-season northwest (approximately 330 degrees)
swell continues to spread across the island chain early this
evening. The most recent readings of swell height at buoy 51001
have dropped below the maximum of just over 17 feet observed late
this afternoon. As a result, surf heights are expected to reach
the low-end warning criteria along most exposed north and west
facing shores of the smaller Hawaiian Islands starting later
tonight or early Friday morning, and a High Surf Warning has been
issued. Note that the west facing shores of Oahu remain under a
High Surf Advisory. We will monitor buoy observations in the
vicinity of the islands overnight in case a High Surf Advisory may
need to be issued for the north facing shores of the Big Island
on Friday. This northwest swell is expected to gradually decline
from Friday afternoon through this weekend.

The large northwest swell will likely cause seas to reach the
Small Craft Advisory (SCA) criteria over coastal waters exposed
to this swell from later tonight through Friday. Strengthening
trade winds later this weekend may reach the SCA threshold over
the typically windy waters adjacent to the islands of Maui County
and the Big Island, although these conditions are not currently in
the forecast.

As the northwest swell continues to subside this weekend, a small
north swell is expected to arrive Saturday. In addition, the
first of a series of small west-northwest swells arriving Sunday
night is expected to continue through first half of next week.
Elsewhere, a south swell will gradually fill in later tonight, and
linger into this weekend. Model guidance continues to indicate a
series of small south swells will spread across the area from this
weekend into the middle of next week. Finally, strengthening
trade winds will cause an increase in short-period choppy surf
along east facing shores through early next week.

&&

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

High Surf Advisory until 6 PM HST Friday for Waianae Coast.

Small Craft Advisory until 6 PM 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.

&&

Morrison...Discussion
Kinel...Aviation
Houston...Marine