High Surf in Hawaii, April 9 to 10, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
854 PM HST Mon Apr 9 2018

.SYNOPSIS...
High pressure passing by to the north of the islands will keep a
moderate to breezy trade wind flow in place through Wednesday. The
trades will ease for the end of the work week as a front passes by
to the north of the state, with windy trades then returning over
the weekend into early next week as strong high pressure builds
north of the island chain. Clouds and showers will favor windward
and mauka areas, with a few showers spilling over into leeward
areas from time to time due to the strength of the trades.

&&

.DISCUSSION...
Currently at the surface, a 1028 mb high is centered around 1150
miles northwest of Kauai and is driving moderate to locally breezy
trade winds across the island chain this evening. Aloft, an upper
level trough has shifted just south of the Big Island, with upper
ridging now building over and to the north of the western islands.
There is quite a disparity in deep moisture across the islands,
with precipitable water values ranging from around 0.8 inches over
Kauai to around 1.8 inches over the Big Island. Infrared satellite
imagery shows mostly cloudy skies over the Big Island as well as
windward sections of the smaller islands, with partly cloudy
conditions over leeward areas of the smaller isles. Radar imagery
and rain gauge observations indicate that scattered to numerous
showers continue to affect windward sections of Maui and the Big
Island, with no more than isolated windward showers over the
remaining islands. Main short term concern revolves around rain
chances.

Tonight through Wednesday night,
High pressure northwest of the state will track steadily eastward
along 32/33N over the next couple of days while strengthening
slightly to around 1032 mb. This will keep a moderate to breezy
trade wind flow in place across the island chain through the
period. The deepest moisture will remain over Maui and the Big
Island with precipitable water values generally in excess of 1.5
inches. This should keep a fairly wet trade wind pattern in place
over the eastern end of the state through the period. Over Kauai
and Oahu drier air will hold in place tonight, with precipitable
water value recovering back up into the 1.2 to 1.4 inch range on
Tuesday and holding at these levels through Wednesday night. As a
result, expect fairly dry conditions tonight and Tuesday, with
light showers limited to windward and mauka areas. The moistening
up of the airmass Tuesday night through Wednesday night should
result in a more typical trade wind shower pattern over the
western islands, with a few showers spilling over into leeward
locales from time to time due to the strength of the trades.

Thursday through next Monday,
A cold front will pass by well to the north of the islands during
the end of the work week, and this should result in a slight
easing of the trade winds into the moderate range. The front will
stall out well north of the islands Friday night, with a strong
high reaching around 1040 mb building north of the state over the
weekend. This should bring breezy trades back to the islands
Friday night, with windy conditions expected across the entire
state over the weekend through early next week.

As for sensible weather details, persistent trade winds along
with precipitable water values in the 1.2 to 1.5 inch range
should keep a slightly wetter than normal trade wind pattern in
place through the period. Bands of enhanced clouds and showers
are expected to push through the islands from time to time, but
trying to time these features through the area more than a couple
days out is very difficult. As a result, the forecast will
feature a slightly wetter than normal trade wind shower regime
through the period. Clouds and showers will favor windward and
mauka areas, with a few showers spilling over into leeward areas
from time to time.

&&

.AVIATION...
A strong surface high located far northwest of the islands will
be moving east through Tuesday. This motion will help maintain
strong and gusty trade winds over the area. Therefore, AIRMET
Tango will continue until further notice. Tango is for mechanical
turbulence below 8k feet, south thru west of mountains all
islands.

An abundance of low level moisture coming in with the trades, will
continue to harass the windward sections of Maui and NE facing
side of the Big Island overnight and likely into Tuesday. AIRMET
Sierra is currently in effect for widespread ceiling below 3k
feet and scattered light showers. PHOG has improved slightly in
the past hour, with the ceiling lifting to 4k feet, but the
significant weather remains east of the airport, towards Hana. The
air mass was somewhat unstable over the Big Island today, due to
an upper level trough that is currently moving south. This
resulted in a flare-up of spotty heavy showers and even a
thunderstorm this afternoon. The air mass should be a bit more
stable Tuesday with the upper trough situated south of the Big
Island. However, the low level trade wind moisture will linger on.

Judging from pilot reports off windward Big Island low cloud tops
are around 65 hundred feet. This matches up well with weather
radar estimated tops, and this goes for other areas as well. High
level layered cirrus clouds, based at 25k feet, cover Maui county
and the Big Island this hour. The cirrus will be thinning out in
the next 24 hours as the upper trough moves south.

During the overnight hours, we expect the trade clouds and showers
to pick up slightly on the other islands, with these elements
affecting the windward and mountain areas. Otherwise, VFR is
prevailing flying condition.

&&

.MARINE...
The High Surf Advisory for exposed north and west facing shores
of the smaller islands has been upgraded to a warning this
evening, which will continue through the overnight hours. The
nearshore PacIOOS buoy at Hanalei is showing a much larger swell
than was predicted this evening, which should quickly begin to
show at the Waimea and Pauwela buoys through the late evening
hours. If heights hold at these levels into the late night hours,
the warning could be extended through the morning hours Tuesday
or through noon. Heights should hold at and above advisory levels
through the second half of the day Tuesday before lowering
further through midweek.

In addition to the large northwest swell, trades have returned
and are holding in the fresh to strong category, which should
persist through much of the week as strong high pressure (1030+
mb) builds north of the state.

A smaller long-period swell is possible out of the northwest
by Thursday, which will become reinforced by a larger source out
of the same direction Friday into the weekend. Surf should remain
below advisory levels Thursday with the initial swell, then
potentially near warning levels Friday night into Saturday as the
reinforcement fills in. Surf should trend down below advisory
levels along north and west facing shores through the second half
of the weekend into early next week.

Surf along east facing shores will steadily rise through midweek
as fresh to strong trades persist locally and upstream of the
state.

Small background swell will continue to generate small surf along
south facing shores through the week. A slightly larger south swell
from recent activity over the southern Pacific can't be ruled out
late Sunday through early next week.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 AM HST Tuesday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Windward Haleakala.

Small Craft Advisory until 6 PM HST Tuesday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Windward Waters.

&&

DISCUSSION...Jelsema
AVIATION...Lau
MARINE...Gibbs