Small Craft Advisory 17, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
345 PM HST Thu Mar 30 2017

.SYNOPSIS...
Moderate trades persist over the eastern half of the state with
lighter winds veering out of the east southeast across the
western half of the state. Showers will increase as a band of
clouds moves in from the east tonight and linger through Friday
morning. Trade winds will become breezy beginning Friday night
through the weekend with increasing windward rainfall expected.

&&

.DISCUSSION...
A 1032 mb surface high is centered about 1900 miles north east of
the state this afternoon while a weakening stationary front lies
500 miles to the northwest of Kauai. The high is driving moderate
trade winds across the eastern half of the state, while the front
is forcing winds to veer east southeasterly across the western
half of the state. As Kauai, Oahu, Molokai and Lanai are blocked
by Maui and the Big Island under this wind direction, this has
allowed sea breezes to bring interior clouds and showers this
afternoon. We anticipate much of the leeward and interior clouds
to dissipate around sunset as the land breezes set in.

Upstream to the east, visible satellite shows a showery band of
clouds about 150 miles east of Hilo ushering in with the trade
winds. These clouds and showers will arrive in the island tonight
and focus over the windward and mauka areas through Friday
morning. Leeward areas should stay mostly dry, but a few showers
may move over the mountains at times. Friday afternoon will likely
be a repeat of this afternoon with sea breezes and leeward cloud
build ups over the western islands.

By Friday night, the front to the northwest will lift away to the
north while the high pressure system far to the northeast
strengthens and edges closer to Hawaii. This will result in
strengthening trade winds, which will remain breezy through the
weekend. Clouds and showers will continue tracking in with the
wind flow that will focus over windward and mauka areas. Highest
chance for rain will be during the nighttime and early morning
hours with minimums during the afternoon and early evenings.
The breezy trades will likely carry some of the clouds and showers
onto leeward areas. Ridging aloft over the state will keep the
atmosphere stable and limit vertical height of the clouds. So
we do not anticipate heavy rain or flooding concerns with the
increased shower activity.

The surface high to our northeast will begin weakening Sunday
night in response to a front passing to our north. The trade winds
will maintain through the weekend but begin to gentle to moderate
breezes by Tuesday. A drier airmass will also usher in from the
east on Monday resulting in fewer trade wind showers. Lighter
trade winds and less showers carrying in will continue into
midweek.

&&

.AVIATION...
Moderate trade winds will continue through the period. Expect
predominately VFR conditions, with clouds and showers favoring
windward and mountain areas. Some passing clouds/showers may
introduce brief periods of MVFR CIGS/VIS. There are no AIRMETs in
effect.

&&

.MARINE...
A large west-northwest swell (310 deg) associated with a strong
weather system far northwest of the state should reach the islands
tonight, peak Friday and Friday night, then slowly ease over the
weekend. Warning and/or advisory-level surf for exposed north-
and west-facing beaches is expected during this time. All of the
models remain in close agreement for seas to ramp up quickly
tonight to Small Craft Advisory levels and peak in the 10 to 14
ft range Friday, before slowly easing through the weekend. If the
swell comes in larger or later than predicted, warning- level surf
could continue into Saturday before dropping to advisory levels
Sunday. For now, a High Surf Warning has been issued for north and
west shores of most smaller islands for tonight through Friday
night. The swell will take longer to reach the Big Island, so will
hold off on issuing their high surf product until later tonight.
A Small Craft Advisory has also been issued for hazardous winds
and seas for most waters tonight through Saturday.

Local winds and seas will likely remain below advisory levels
through Friday as the ridge of high pressure north of the state
slightly weakens in response to a cold front that is forecast to
approach the region later in the week. Advisory-level trade winds
will return across portions of the waters over the weekend as high
pressure builds north of the state behind this front.

Surf along east-facing shores will remain small and choppy due to
moderate to fresh onshore winds. Surf will begin to build over the
weekend as trade winds strengthen locally and upstream of the
islands, and may near or reach advisory levels along east-facing
shores by the end of the weekend and into early next week.

A slight increase in surf along south-facing shores is possible
today through Saturday due to a long-period swell (190 deg) from
recent activity across the southern Pacific. In addition to this
south swell, wrap from the previously discussed large west-
northwest swell will add some to the mix by Friday at select spots
along exposed southern facing beaches.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning from midnight tonight to 6 AM HST Saturday for
north and west facing shores of Niihau, Kauai, Oahu, Molokai and
for north facing shores of Maui.

Small Craft Advisory from midnight tonight to 6 PM HST Saturday
for all Hawaiian waters except Maalaea Bay-

&&

DISCUSSION...Foster
AVIATION...Eaton
MARINE...TS