High Surf in Hawaii, April 28 to 29, 2017

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
446 AM HST Fri Apr 28 2017

.SYNOPSIS...
A cold front and a couple of upper level disturbances will bring an
extended period of unsettled weather to the main Hawaiian Islands
that will likely last through Monday of next week. There is the
potential for heavy rain and and a slight chance for thunderstorms
each day beginning today.

&&

.DISCUSSION...
A good bit of the evening showers left the land masses for the
adjacent waters as an offshore land breeze formed. The exceptions
were the Big Island where some heavy showers occurred along the
leeward Kohala coast and near Waikoloa Village between midnight
and 3 am, prompting a flood advisory. On the windward side of the
Big Island, scattered showers were streaming into the Puna district
and skirting the Kau coastline. The wind flow over the Big Island
is a bit different from elsewhere with a steady easterly flow.
The wind pattern there, however, will be changing to the SE this
afternoon. The other island is Kauai where the showers would time
to time ping its coastline. Otherwise, the evening has been
quiet and dry.

cor...The forecast looks on track. Shower activity will be
picking up especially this afternoon, not only because of the
front reaching Kauai but also due to daytime heating and the light
winds. On the latest satellite imagery, the front is not as
distinctive as earlier in the evening as light northerly winds
have snuck ahead of the front. As of 3 am HST, the front is
roughly 90 miles W of Kauai. An ASCAT pass shows some 25 kt or
small craft advisory (SCA) level winds behind the front. Because
of this data, a SCA is in effect for the waters primarily around
Kauai. SCA level winds are expected through tonight only. On
water vapor imagery, the upper trough is noted 165 miles W of
Kauai. It is on track in reaching Kauai shortly after day break
this morning.

The front is expected to travel as far as northern Big Island by
Saturday afternoon. A cool dry air mass with dew point to as low
as the upper 50's will accompany the brisk northerly winds that
reach only as far as Oahu. East of Oahu, the weather will be of
scattered showers, a few locally heavy, and a slight chance of
thunderstorms.

The nice weather over Kauai and Oahu will only be brief. The
weather after Saturday evening, mainly after midnight, will start
to turn sour across Kauai and Oahu as the next upper level
disturbance, a low with a pool of very cold air of minus 19
degrees C at 500 mb, comes into play. The models are in sync with
this feature settling over Kauai and Oahu by Sunday daybreak.

This is an messy picture synoptically speaking. The surface front
remains stalled over Maui County on Sunday which may act as a
enhancer to the development of the heavier showers. In short, the
active weather favors Maui County and the Big Island over the
weekend. A Flash Flood Watch for at least Maui is still not out of
the question at this point. We will see how persistent the models
are with this unsettled scenario.

The front weakens to a trough over the central islands Sunday night
with a moist southerly wind flow east of the trough. The upper low
stalls out some 200 miles SSW of Kauai, and lingers there through
Monday. The ECMWF and GFS solutions differ thereafter in the
placement of the upper low. The GFS has it just to the south of Oahu
Monday night while the EC still has it 200 miles SSW of Kauai.
Either scenarios points to unsettled weather extending into Monday
night and perhaps Tuesday.

&&

.AVIATION...
A slow moving front will move through Kauai later today and
through Oahu tonight. Light winds ahead of the front will allow
for daytime sea breezes in most areas from Oahu eastward, while
the winds will pick up out of the north on Kauai later this
morning and Oahu tonight behind the slow moving front. Moisture
ahead of the front along with an upper level disturbance moving
overhead will keep conditions unsettled, with scattered showers
and temporary MVFR cigs/vsbys possible across the entire island
chain through the next 24 hours. The best chances for reduced
cigs/vsbys will be this afternoon and this evening as showers
develop as a result of daytime heating. Additionally, MVFR
cigs/vsbys are expected to become more widespread across Kauai
later today and Oahu tonight along and behind the slow moving
front, with the most unsettled conditions expected over windward
and mauka areas.

AIRMET Sierra remains in effect for mountain obscuration across
northwest thru northeast sections of Kauai. These conditions will
likely remain in place through much of the day. The AIRMET will
likely need to be expanded to include the interior of some of the
other islands this afternoon.

&&

.MARINE...
A complex weather pattern is expected for the next several days,
introducing a slight chance for thunderstorms across most of the
coastal waters today through Monday. A front and upper level
trough just west of Kauai will continue to move southeast before
stalling over the central islands Saturday. Small Craft Advisory
winds and seas will fill in over the waters surrounding Kauai
today as the front passes. A small break in thunderstorm chances
should occur behind the front, before spreading across all waters
late Saturday as an upper low and developing surface low slide
over the island chain. The forecast confidence remains low on the
exact placement of thunderstorm activity, but mariners should stay
tuned to the updated forecast today and this weekend.

Surf along north facing shores will see a small bump tonight into
Saturday from a short period choppy swell associated with the
stronger winds behind the front. Surf will stay below advisory
conditions. A series of small, longer period northwest swells are
then expected for next week.

A series of long period south swells will grace the south facing
shores into next week. The first set of swells will bring advisory
level surf (8 ft) to exposed south facing shores today into early
Saturday morning. A High Surf Advisory has been posted. Please
refer to the Coastal Hazard Message, CFWHFO, for more information.
Surf this weekend, along with a reinforcing south swell Monday
and Tuesday remain borderline for advisory conditions.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Saturday for south facing shores
of Niihau, Kauai, Oahu, Molokai. Lanai, Maui, and leeward Big
island.

Small Craft Advisory until 6 AM HST Saturday for waters around Kauai
and windward waters of Oahu.

&&

Lau...Discussion
Eaton...Marine
Jelsema...Aviation