High Surf in Hawaii, April 15 to 16, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
949 AM HST Wed Apr 15 2020

.SYNOPSIS...
Trade winds will strengthen over the state from east to west
through tonight. Showers will be focused over windward slopes with
a few afternoon showers leeward, and a low aloft south of the
state will send a thinning shield of high clouds overhead. A
stable and moderate trade wind flow will prevail Thursday through
Saturday, and will briefly weaken and become disrupted Sunday and
Monday.

&&

.DISCUSSION...
The atmosphere is gradually stabilizing as trade winds build. A
compact upper level low sitting nearly 200 miles southwest of the
Big Island is generating heavy showers and thunderstorms just
outside of the coastal waters south of Oahu and Maui County this
morning, but overnight soundings and recent aircraft data show
increasing stability with a low level inversion returning. We
cannot rule out a briefly heavy shower flaring up this afternoon,
though it is looking unlikely as the upper low is weakening. The
upper low is also sending a shield of high clouds over mainly
Oahu and Maui County. These clouds will thin out through tonight.

Moderate easterly trade winds are already well-established on
the Big Island and Maui County, and as high pressure builds north
of the state, trades will strengthen over the entire island chain
through tonight. The increasingly stable trade wind flow is
carrying in a typical field of stable stratocumulus mixed with
shower-bearing cumulus. As a result, expect modest showers to be
confined to windward slopes, with a few showers briefly popping up
over leeward slopes of mainly the Big Island this afternoon.

Stable trade wind weather will prevail Thursday through Saturday.
Trades will be at moderate strength, and as a mid level ridge
builds in, stable conditions will confine modest showers to
windward slopes.

Trades will shift southeasterly on Sunday and will likely be
disrupted on Monday as an approaching front pushes the
subtropical ridge over the islands. A stable and somewhat dry
land and sea breeze pattern to briefly take over.

Easterly trades are expected to return Tuesday and Wednesday. The
GFS and ECMWF show the front stalling and dissipating north of the
state and the subtropical ridge strengthening as it retreats
northward. As a result, east to perhaps southeasterly winds will
fill in under stable conditions.

&&

.AVIATION...
A ridge will will build towards the island chain from the
northeast and allow trade winds to strengthen to moderate levels
by the end of the day. Winds will carry in a few low clouds and
showers that will focus over windward areas tonight. VFR
conditions will prevail with the potential for isolated mountain
obscurations/MVFR ceilings within showers tonight. Cirrus clouds
will decrease over the state today.

See SIGMET Yankee for details on moderate to severe turbulence
over Oahu and Molokai from FL280 to FL400. This area of turbulence
is expected to decrease by the end of the day.

&&

.MARINE...
Surf along north and west facing shores will trend up today through
the upcoming weekend as a series of northwest swells move through.
The first of these swells is filling in now and expected to peak
tonight, far exceeding advisory levels for exposed north and west
facing shores. A High Surf Advisory is now in effect through 6AM
HST Thursday. However, the latest observations from Buoy 51101 NW
of Kauai show a spike in the swell above model guidance. If this
trend continues, local surf could approach warning levels, so will
continue to monitor closely. A Small Craft Advisory is also in
effect for most Hawaiian waters due to either the large northwest
swell or strong winds around Maui County and the Big Island. As
this first swell fades Thursday through Friday, another northwest
swell will build Friday night into the weekend for another round
of advisory level surf as it peaks Saturday night, then slowly
eases Sunday into Monday.

Surf along south facing shores will remain small through the week.
However, a long-period south swell may affect Hawaii by next Monday,
with surf heights likely exceeding advisory level as it peaks early
next week. Surf along east facing shores will trend upward during
the second half of this week as trade winds strengthen.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Thursday for north and west
facing shores of Niihau, Kauai, Oahu, and Molokai, and for north
facing shores of Maui.

Small Craft Advisory until 6 AM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward
Waters-Big Island Southeast Waters.

&&

DISCUSSION...Wroe
AVIATION...Foster
MARINE...TS