High Surf in Hawaii, April 11 to 15, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
337 AM HST Thu Apr 11 2019

.SYNOPSIS...
Wet and unstable conditions will continue across the eastern end
of the state into the upcoming weekend as an broad upper low
remains in the area. Showers will become heavy at times,
potentially leading to localized flooding for Maui and the Big
Island. In addition to the threat for heavy showers, a few
thunderstorms will remain possible over and around Maui County
and the Big Island. Elsewhere, clouds and showers will favor
windward areas with occasional showers making it into leeward
sections. Improving conditions will be possible through the
weekend into early next week. Trades will remain strong through
the weekend, then trend down early next week.

&&

.DISCUSSION...
Recently animated water vapor loop showed a broad upper low
drifting westward toward the eastern end of the state this
morning. Plenty of moisture associated with this feature combined
with cold air aloft and lowering upper heights are triggering
heavy showers and thunderstorms over a large area near and just
east of the Big Island (including the windward Big Island waters).
Rainfall accumulations continue to steadily climb over sections of
the windward Big Island as frequent showers quickly move through.
Peak 24-hr rainfall accumulations just inland of Hilo are up to
1.8 inches (Glenwood), as of 2 AM HST. Mostly quiet trade wind
conditions are occurring elsewhere, with showers favoring windward
locations.

The latest guidance remains persistent and supports the wet
pattern continuing over the eastern end of the state into the
weekend as the aforementioned upper low remains in the area.
Showers will become heavy at times, potentially leading to
flooding. Although most of the activity will focus over windward
areas with the strong trades in place, heavy showers and
thunderstorms will also remain possible across leeward areas of
Maui and the Big Island. For the higher elevations on the Big
Island, bursts of convection due to the eroded inversion and
plenty of moisture and cold air aloft will likely translate to
periods of snow over the Summits through Friday night. Trades will
remain strong through the weekend due to 1042 mb high pressure
northeast of the state.

Although the better instability and deep source of moisture will
remain focused east, expect the wet trade wind pattern to fill in
over the western end as well today through Friday. Provided the
strength of the trades, some shower activity will manage to spill
over into leeward locations periodically.

For the long range, guidance is now in decent agreement and shows
the upper low beginning to lift northeast and away from the state
over the weekend into early next week, which should result in
improving conditions. Trade winds should trend down for a brief
period early next week as the ridge weakens due to a front passing
to the north.

&&

.AVIATION...
A strong surface high northeast of the state will produce breezy
to locally windy trade winds across the Hawaiian Islands today and
tonight. An upper level low 300 NM east of the Big Island will
continue to destabilize the atmosphere and enhance showers moving
in with the trade winds.

Expect periods of MVFR ceilings and showers mainly along windward
and mountain slopes of each island with more frequent shower
coverage and isolated thunderstorms over Maui and the Big Island.

AIRMET Sierra in effect from Kauai to the Big Island for tempo
mountain obscuration along windward slopes due to low ceilings and
showers.

AIRMET Tango remains in effect for low level moderate turbulence
over mountains and south through west sections of each island.

&&

.MARINE...
High pressure north of the state will keep strong to near Gale
force conditions in place across the marine area through Sunday,
with a brief period of Gale conditions not out of the question for
the Alenuihaha Channel later today and tonight. The Small Craft
Advisory (SCA) has been extended for all marine zones for winds,
seas, or a combination of the two through Sunday, and this may
need to be extended into early next week for some areas.

A large fetch of strong to near gale force winds aimed at the
state will result in a building easterly swell through the
remainder of the work week. Based on buoy observations, surf has
risen to advisory levels along east facing shores early this
morning and will continue to rise over the next couple of days,
with the swell peaking out Friday night and Saturday, before
gradually lowering Sunday through the middle of next week. The
High Surf Advisory (HSA) has been extended for east facing shores
of Kauai, Oahu, Molokai, Maui and the Big Island through Sunday,
and this may need to be extended through Monday.

Small northwest swells will continue to roll through the islands
through the middle of next week, with surf remaining well below
advisory levels along north and west facing shores. Surf along
south facing shores will remain small and well below seasonal
levels through the middle of next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory through 6 PM HST Saturday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West- Windward
Haleakala-South Big Island-Big Island North and East.

Flash Flood Watch through Friday afternoon for Maui-Kona-South
Big Island-Big Island North and East-Kohala-Big Island Interior.

Wind Advisory until 6 AM HST Friday for Lanai-Kahoolawe-Maui
Leeward West-Maui Central Valley-Kohala.

Winter Weather Advisory until 6 AM HST Saturday for Big Island
Summits.

Small Craft Advisory until 6 PM HST Saturday for all Hawaiian
waters-

&&

DISCUSSION...Gibbs
AVIATION...Bohlin
MARINE...Jelsema