High Surf in Hawaii, May 8 to 9, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
331 AM HST Tue May 8 2018

.SYNOPSIS...
Clouds and showers associated with a dissipated front will linger
over portions of the state into midweek, primarily affecting
windward areas. While Kauai and Oahu may see some clearing by
Wednesday, windward portions of Maui and the Big Island will
continue to see cloudy and damp weather. Breezy trade winds will
persist through Wednesday, while lighter winds are expected Thursday
and Friday. The light winds may allow afternoon showers to develop
over leeward areas. A gradual drying trend is anticipated over the
weekend and into early next week with a return of a more typical
trade wind pattern.

&&

.DISCUSSION...
Water vapor imagery showed plenty of middle- to upper-level moisture
pooling northeastward over the state due to a weak upper trough
moving into the area. This moisture and upper feature are supporting
a shield of high clouds moving over the islands. A combination of
these high clouds passing through and a persistent band of broken to
overcast low clouds associated with a dissipated frontal boundary
stalled near the Big Island continues to support mostly cloudy,
moist, and showery conditions. The latest radar loop around the
state showed the bulk of the showers, mostly light, remaining
focused over windward and mauka locations this morning. The latest
rainfall summary depicted peak six hour (through 2 AM HST this
morning) accumulations just under a tenth of an inch on Kauai,
less than half of an inch on Oahu, 1.58" at West Wailuaiki on
Maui (windward), and up to 1.81" at the Honokaa gage on the Big
Island. Accumulations south of Hilo through this period exceeded
half of an inch with 0.64" at Pahoa and Glenwood through this time
and up to around 2" for the 24 hr period.

Short-term (through Wednesday) guidance remains in good agreement
and generally depicts no significant changes to the current pattern
and trends described above. A general consensus supports the
aforementioned band of low clouds/moisture associated with the
dissipated frontal boundary lingering over/near the Big Island. This
will translate to a continuance of cloudy, moist and showery
conditions with occasional moderate to heavy showers passing
through, mostly favoring windward areas of the Big Island and Maui.
Windward sections of the smaller northwest islands could experience
a downward trend in shower activity beginning today as the back
edge of the moisture axis inches southward (PWs dropping toward an
inch; drier air). Moderate to breezy trades will hold in place as
high pressure, currently north-northwest of the state, slowly
builds eastward. Low-level trade wind flow will begin to veer from
the northeast (070 deg) to the east (090 deg) late tonight
through Wednesday as high pressure shifts to an area due north of
the islands.

Later in the week (Thursday-Friday), a weakness within the ridge
north of the state is forecast to develop as high pressure continues
eastward and an upper trough and surface front approach the
region. Local winds will shift out of a more east-southeast
direction and weaken (potentially becoming light and variable for
Kauai/Oahu) as a pre-frontal trough develops over the smaller
islands Thursday night into Friday. While the trough over the area
Thursday/Friday will likely increase the instability, potential
effects are still uncertain at this time. Most model guidance
indicates the trough will develop over the central portion of the
island chain, with moisture convergence maximized over waters to
the northeast of the islands. If this verifies, the best chance
for rain over land may occur over leeward areas of the smaller
islands in the afternoon as light winds allow converging sea
breezes to trigger shower development. For Maui and the Big
Island, east-southeast flow will likely continue to focus clouds
and showers over windward locations.

For the upcoming weekend and early next week, conditions are
forecast to begin improving as upper heights rise, drier air, and
trades gradually return. This drying trend will continue into Monday
with PWs possibly dipping below an inch as high pressure builds
north of the state.

&&

.AVIATION...
High pressure north of the state will keep a moderate to breezy
trade wind flow in place through tonight. Low clouds and showers
will favor windward and mauka areas, with a stray shower spilling
leeward from time to time. The most persistent shower activity and
lowest ceilings are expected over windward sections of Maui and
the Big Island.

AIRMET Sierra remains in effect for mountain obscuration across
windward sections of all islands. Conditions should improve across
Kauai, Oahu and Molokai by late this morning, but will likely hold
in place across Maui and the Big Island through the day.

AIRMET Tango remains in effect for moderate low level turbulence
over and downwind of the terrain. This AIRMET is expected to
continue through tonight.

&&

.MARINE...
A moderately strong surface high with a central pressure of around
1030 mb is currently located 580 nm NNW of Kauai. It will be
moving gradually eastward, passing north of the islands this
evening. The result of this, is a boost in the trades to moderate
to locally strong, and this has led to the issuance of a Small
Craft Advisory (SCA) for waters around Maui and south of the Big
Island. Above normal seas due to a large north swell brought on
the expansion of the SCA involving most of the windward and channel
waters around the main Hawaiian Islands.

The reinforcing north swell currently influencing our north shore
will peak later this morning with a swell height of around 9 feet
and a period of 13 to 14 seconds. A gradual decline follows that
will continue through Wednesday. A High Surf Advisory is in effect
for the north facing shores for most islands through 6 am HST
Wednesday. Since the swell was generated from a deep low pressure
NE of the islands, islands closer to the low will have slightly
larger surf, such as Molokai, Maui, and the Big Island.

Both swell and wind should diminish below SCA criteria over all
Hawaiian waters by 6 pm HST Wednesday, and where the SCA ends.

The rest of the new work week will have a series of smaller, but
long- period swells, up to 16 seconds, from the northwest to
north- northwest to keep surf elevated, but below advisory level
for north and west facing shores.

Surf along east facing shores will be a couple of feet higher
from yesterday, brought on by the strengthening trade winds. This
short period locally generated swell will drop off on Wednesday
night as the trades soften.

The south facing shores will continue to have small surf through
the forecast period.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Wednesday for North facing shores
of Niihau Kauai Oahu Molokai Maui and the Big Island.

Small Craft Advisory until 6 PM HST Wednesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory until 6 PM HST Tuesday for Kauai Windward
Waters-Kauai Channel-Oahu Windward Waters-Kaiwi Channel-Maui County
Windward Waters-Big Island Windward Waters.

&&

Synopsis...Birchard
Discussion...Gibbs
Aviation...Jelsema
Marine...Lau