High Surf in Hawaii, April 14 to 17, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
842 PM HST Sat Apr 14 2018

.UPDATE...
No significant changes in the forecast are anticipated this
evening, as the previous forecast remains on track. The flash
flood watch remains in effect through 6 AM HST Sunday for all
islands and will likely be extended through Sunday in the 4 AM
HST package due to an upper level low, surface trough and plenty
of deep layer moisture in place.

&&

.SYNOPSIS...
Breezy, wet, and unstable conditions will continue through at
least tonight and possibly through the weekend. An upper level
low to the west of the state will tap deep moisture in the breezy
trade wind flow, maintaining the threat of heavy rain and
thunderstorms. While the primary flood threat will be across
windward slopes, all areas could experience heavy rainfall. As the
upper level low slowly moves away on Monday and Tuesday, the
breezy trade wind flow will gradually stabilize, causing typical
rainfall to focus along windward slopes. Breezy trade winds and
extensive high clouds will hold through much of the week, with
unsettled weather possible late in the work week.

&&

.DISCUSSION...
Wet and unstable conditions remain in place. An upper level low
parked several hundred miles west of Kauai is generating the
instability along with a shield of high clouds blanketing much of
the state. The afternoon soundings continued to show no
inversion, and recent satellite data revealed precipitable water
(PW) values holding in the 1.5 to 1.8 inch range, which is well
above the roughly 1.25 inch April normal. A strong surface high
passing far north of the state is driving the breezy and gusty
trades, while a subtle mid level trough appears to stretch from
northeast to southwest just west of Kauai. It is this subtle
trough that has been focusing heavy showers and thunderstorms over
the Garden Isle, where in excess of 3 inches of rain has been
measured at several north shore gauges in the past 6 hours. From
Oahu to Maui, moderate to briefly heavy showers have been largely
confined to windward slopes today. On the Big Island, more
persistent heavy showers and thunderstorms have been pulsing along
windward slopes, and leeward shower activity has been somewhat
subdued, probably due to high cloud cover.

The threat of heavy, flooding showers and thunderstorms in the
breezy trade wind flow will persist through at least tonight. The
subtle mid level trough near Kauai will drift westward, taking a
focusing mechanism for moisture away from the islands. However,
deep moisture will linger over the islands, and the GFS model is
showing a weak shortwave aloft passing over the entire state
tonight. This points toward a threat of continued heavy, flooding
rainfall and a chance of thunderstorms for mainly windward slopes.
That said, grounds are saturated in many locations, including
some leeward areas, and a Flash Flood Watch remains in effect for
all locations on all islands.

The upper level low will begin to drift westward on Sunday, but
unstable conditions will likely remain in place. The mid level
trough will creep west and northwest of Kauai, leaving a broad
area of convergence just north of the islands. High pressure to
the north will maintain breezy trade winds, which will focus
potentially heavy showers across windward slopes. That said,
heavy showers could form over the leeward Big Island and Maui
slopes during the afternoon. As a result, the Flash Flood Watch
may need to be extended, though confidence is not high enough to
do so at this time.

The atmosphere will gradually stabilize on Monday and Tuesday
locally breezy trade winds hold. The upper level low will slowly
fill and drift westward, bringing an increase in stability and
pulling the deepest moisture west of the islands. This will likely
result in a drying trend over leeward areas of most islands,
though moderate rainfall will persist over windward slopes. High
clouds will still hang around over most areas.

Some instability could return to the trade wind flow later in the
week. The GFS and ECMWF show the weakened upper trough to the
west being pushed back over the state on Thursday. If this occurs,
heavy showers and thunderstorms could again be possible, with
rainfall continuing to be focused along windward slopes.

&&

.AVIATION...
A complex weather pattern will continue tonight and tomorrow.
Locally breezy E to ENE trade winds will continue at the surface
through around 10-12 kft before veering out of the S. AIRMET TANGO
for low level turbulence is posted across the state. An upper
level low, providing instability, and ample moisture will
maintain wet conditions with scattered thunderstorms for the
state. The wettest conditions will remain along the windward and
mountain areas, where AIRMET SIERRA for mountain obscuration is
posted across windward areas on all islands and for all of Kauai.
MVFR to isol IFR CIGS/VIS can be expected.

AIRMET ZULU for tempo light ice in any layered clouds above the
freezing level is also posted.

&&

.MARINE...
A Small Craft Advisory (SCA) is in effect for all waters through
most of the weekend for seas and winds. Isolated thunderstorms
remain likely over the coastal waters through tonight, with
likelihood and coverage diminishing somewhat for Sunday, but still
a possibility.

A northwest swell peaked Saturday morning, and the nearshore
buoys continue to show diminishing wave energy. A High Surf
Advisory (HSA) remains through Sunday morning for north and west
facing shores. A small north swell is expected mid-week with a
moderate northwest swell building Thursday night. As trade winds
strengthen Sunday, surf along east facing shores will increase to
advisory levels with elevated east shore surf persisting into the
first half of next week. Otherwise, surf will remain small along
south facing shores, but new swells from the southern hemisphere
will produce a modest increase starting as early as Sunday.

See the Collaborative Surf Discussion for additional details on
surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 6 AM Sunday for all Hawaii islands.

High Surf Advisory until 6 AM HST Sunday for north and west facing
shores of Niihau, Kauai, Oahu, and Molokai, and for north facing
shores of Maui and the Big Island.

High Surf Advisory until 6 PM HST Sunday for east facing shores
of Kauai, Oahu, Molokai, Maui, and the Big Island.

Small Craft Advisory for all waters.

&&

UPDATE...Gibbs
MARINE...Morrison
AVIATION...Eaton
REST OF DISCUSSION...Wroe

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE HONOLULU HI
526 PM HST SAT APR 14 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0959 AM     NON-TSTM WND GST KALIHIWAI               22.22N 159.42W
04/14/2018  U0.00 MPH        KAUAI              HI   DEPT OF HIGHWAYS

            KALIHIWAI BRIDGE WAS CLOSED DUE TO FALLEN BRANCHES, HAS
            REOPENED.

0959 AM     FLASH FLOOD      KAPAA                   22.10N 159.35W
04/14/2018                   KAUAI              HI   LAW ENFORCEMENT

            KUHIO HWY NEAR HAENA BEACH PARK AND DRY CAVE IMPASSIBLE
            DUE TO WATER FLOWING OVER ROAD.

1017 AM     FLASH FLOOD      AINA HAINA              21.29N 157.75W
04/14/2018                   HONOLULU           HI   COUNTY OFFICIAL

            KALANIANAOLE HWY AT HALEMAUMAU DR ONLY ONE LANE OPEN IN

            EACH DIRECTION DUE TO DEBRIS ON ROAD FROM FLASH FLOODING.

&&

MDYE

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE HONOLULU HI
552 PM HST SAT APR 14 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0200 PM     FLASH FLOOD      WAINIHA                 22.21N 159.54W
04/14/2018                   KAUAI              HI   EMERGENCY MNGR

            KUHIO HWY CLOSED IN BOTH DIRECTIONS IN WAINIHA AND
            WAIKOKO THROUGH SATURDAY NIGHT DUE TO THREE LANDSLIDES.

&&

MDYE