High Surf in Hawaii, April 28 to 30, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
408 AM HST Sat Apr 28 2018

.SYNOPSIS...
Showers are developing across Maui County this morning as a
shallow cold front continues to move southeast down the island
chain. The frontal band will reach the Big Island by this
afternoon and stall just to the east of the island on Sunday.
Cool and dry air will move in behind the cold front with a drying
trend for most islands through Monday. Lingering unstable moisture
will continue to drift over the windward Big Island through the
upcoming week. A building high pressure ridge east of the state
and a developing low pressure system to the north of the islands
will slowly draw up the unstable cloud band back northward across
the islands from Tuesday through Friday. Expect scattered to
numerous showers affecting mainly Kauai and Oahu from Wednesday
through Friday as the unsettled cloud band stalls across the
northwestern side of the state.

&&

.DISCUSSION...
In the big picture, a cut off low continues to spin roughly 800
miles north of Oahu. The low remains cut off from the midlatitude
steering flow and will maintain its position for the next several
days. GOES Satellite imagery this morning also shows a large band
of high level cirrus clouds produced by the Sub-tropical Jet
stream. These upper level cirrus clouds are obscuring a shallow
low level cold front with light to moderate showers shown on
doppler radar moving through Maui County. The remnants of this
front will continue to move into the Big Island later this
afternoon enhancing showers across the island. Drier and cooler
air with northerly winds will move in behind the front, decreasing
shower activity across most of the state with dew point
temperatures already falling to near 60 degrees Fahrenheit on
Kauai and Oahu this morning.

A wind advisory remains in effect for the Big Island summits
through Sunday morning. Summit wind speeds will likely increase
from Sunday night through Monday and a High Wind Warning for the
Big Island summits may be needed during this period.

A drier and cooler air mass will linger over Kauai, Oahu and Maui
County on Sunday and Monday with decreasing northerly wind
speeds. Large scale wind speeds will be light enough on Monday
for an onshore sea breeze setting up over Kauai and Oahu with
afternoon interior cloud build ups and a few showers possible.

On Monday and Tuesday, a building high pressure ridge to the east
and a low pressure trough to the north will combine forces and
draw the unstable cloud band near the Big Island slowly northward
across the state. The Big Island and Maui will see increasing
clouds and enhanced showers on Monday and Tuesday as the unstable
band drifts northward.

Wednesday through Friday...The unsettled band of clouds will reach
Oahu and Kauai on Wednesday and the forward motion of the unstable cloud
band will stall out just as an upper level short wave trough
begins to sweep across the northern half of the state. Divergence
aloft ahead of this upper trough will cause deeper convection and
shower activity along the cloud band, producing scattered and
numerous showers over Oahu and Kauai with possible thunderstorms
and periods of locally heavy showers for the end of the week. The
timing of this rainfall remains a combination of model solutions
and we are using this approach to smooth out forecast model run to
run inconsistencies. Additional details on rainfall impacts and
potential thunderstorms will become clearer as the time period
gets closer. Stay tuned for updates to the forecast as next weeks
weather pattern evolves.

&&

.AVIATION...
An ill-defined front is located over Molokai/Lanai, moving east
at near 10 kt. The front should be over Maui in an hour or two,
accompanied by widespread IFR to LIFR weather. These conditions
will continue eastward, spreading over to at least the northern
half of the Big Island by noon, and the rest of the island's
windward side by nightfall. The lower part of leeward Big Island
will be least affected. Meanwhile, conditions continues to improve
across Oahu. With a somewhat drier and cooler air mass sweeping
across the smaller islands, we probably can lift AIRMET Sierra for
Oahu by 16z, Molokai and Lanai by 22z. We can say Kauai has
experienced this off hand. After a few brief showers a couple of
hours ago, the air mass around Kauai appears to have dried off
with dew points lowering down to 60 deg F at both LIH and Barking
Sands. Models continue to hint at a VFR status for this island
today.

The front stalls just off the Big Island's (BI) eastern shores
this evening, and pretty much stays there through the rest of the
weekend. Windward BI will continue to have limited effects from
the close proximity of the stalled front.

The base of the dense layered clouds over the BI got to as low as
12k feet about an hour ago. It has since lifted back to 17k feet,
and there are indications it has already met its lowest point.
We probably will drop AIRMET Zulu.

Models point to a burst of moderately strong northerly winds
today. It will be strong enough to warrant AIRMET Tango for low
level turb for today and this evening only.

&&

.MARINE...
Fresh to strong northerly winds in the wake of a cold front that
is forecast to shift eastward over the Big Island waters through
the weekend combined with a rising north-northwest swell (12-13
sec periods) will lead to dangerous boating conditions today.
Small craft advisory seas are expected over the exposed northwest
waters through the weekend. Winds are forecast to quickly drop
off into the light to moderate range out of the north tonight
through early next week as the aforementioned front stalls in the
vicinity of the Big Island.

A reinforcing north-northwest (330 deg) swell associated with a
storm-force low around 800 nm north of the state is expected to
fill in through the day today, peak Sunday, then fade into the
upcoming week. Swells associated with nearby sources like this
typically come in on time, but larger than projected by a few feet
around the peak. The overnight ASCAT pass showed storm-force northerly
winds focused at the islands associated with this source, which
was 5 to 10 kt stronger than predicted and should correlate to
larger seas/swell by a couple of feet. As a result, warning-level
surf will be likely for Kauai and Niihau, then advisory-level
surf for exposed coasts on Oahu, Molokai and Maui as the swell
peaks tonight through Sunday night. A gradual downward trend is
expected Monday through Tuesday.

A new south-southwest (200 deg) swell from recent activity near New
Zealand is expected to arrive tonight into Sunday, which will
translate to rising surf along south facing shores. A similar
reinforcement is expected by midweek.

Surf along east facing shores will remain small through the first
half of the upcoming week as the winds remain light locally and
upstream.

See the latest Collaborative Nearshore Swell and Wind Forecast for
more details regarding surf on Oahu at:
www.prh.noaa.gov/hnl/pages/SRF.php#Discussion

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from 2 PM this afternoon to 6 AM HST Monday
for Waianae Coast-Oahu North Shore-Oahu Koolau-Molokai-Maui
Windward West-Maui Central Valley-Windward Haleakala.

High Surf Warning from 2 PM this afternoon to 6 AM HST Monday
for Niihau-Kauai Windward-Kauai Leeward.

Wind Advisory until 6 AM HST Sunday for Big Island Summits.

Small Craft Advisory until 6 AM HST Monday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters.

&&

Discussion...Bohlin
Aviation...Lau
Marine...Gibbs