High Surf in Hawaii, September 1 to 3, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
325 AM HST Fri Sep 1 2017

.SYNOPSIS...
Light winds and mostly dry conditions are expected the next couple
of days, with a few night and morning showers over windward areas,
and a slight chance of afternoon showers over interior and upslope
areas. With the light winds turning toward the south late in the
weekend and early next week, showers will tend to favor leeward
areas. Also, vog may spread over the smaller islands, with
conditions becoming increasingly warm and muggy.

&&

.DISCUSSION...
Quiet weather pattern currently in place features light trade winds
(at least at cloud level) delivering a few windward showers, while
many land-based stations are reporting downsloping land breezes.
Overnight soundings show a strong subsidence inversion based between
5500' and 6500' and PWAT just over an inch.

A period of light winds is expected as trade winds continue to
diminish today, continuing into the first half of next week. This
occurs as the trade wind supporting high to the NE weakens
dramatically and a slow-moving front (now about 600 miles NW of
Kauai) moves closer. Overall, the next several days are expected to
be drier than normal, although some showers may affect normally dry
leeward areas.

The developing light winds on the large-scale will allow island-
scale land and sea breezes to dominate the wind flow over land,
while winds initially favor a NE to E direction over water. This is
expected to prevent volcanic haze (vog) from spreading to the
smaller islands from the Big Island through Saturday, while also
managing to deliver a few nighttime windward showers.

As a weak high develops ENE of the islands late this weekend, the
associated ridge will extend over the islands from the E. In
response, winds will shift to the S as the front inches closer,
likely allowing vog to move over all islands on Sunday. The ridge is
expected hold with little change into Wednesday, maintaining a
somewhat voggy light wind regime.

A strong subsidence inversion will remain through Saturday due to a
mid-level ridge directly overhead, and thus afternoon clouds that
develop in response to converging sea breezes will likely produce
only limited rainfall - mainly over interior and upslope areas. A
weak trough aloft extending over the area from the N will lead to
modest mid-level cooling after Sunday, but with PWAT remaining
relatively low, significant rainfall is not expected.

Although moisture associated with the approaching front may
potentially affect Kauai around the middle of next week, forecast
guidance continues to indicate that the bulk of the moisture will
remain just N of the islands. High pressure building far to the NE
around the middle to end of next week is expected to  shunt the
moisture westward away from the islands as trade winds return.

&&

.AVIATION...
The high pressure ridge north of the islands will continue to weaken
in response to an approaching cold front. Our typical northeasterly
trade winds will weaken, allowing a daytime onshore Sea Breezes with
interior cloud ceilings developing over all islands by afternoon.
Mountain obscurations and MVFR ceilings will be possible in these
interior sections, possibly requiring the issuance of AIRMET Sierra.
Downward motions under a mid level ridge will reduce shower
activity. Overnight land breezes will clear out most of the cloud
ceilings after sunset tonight.

No AIRMET's currently in effect.

&&

.MARINE...
Winds are expected to remain light to moderate, and swells will
remain below 10 feet. Thus, no Small Craft Advisory conditions are
likely through early next week.

A south swell, generated by a strong low in the southern hemisphere,
will continue to fill in over the local waters. A High Surf Advisory
is in effect for all south facing shores through Saturday. Surf will
slowly trend down thereafter.

For the extended time frame, the latest model output is presenting a
more active pattern evolving across the northern Pacific for the
upcoming holiday weekend, in response to Tropical Cyclone Sanvu near
Japan and a gale-force low developing near the eastern Aleutians. If
these features develop as forecast, a small west- northwest (300
deg) swell and a larger north-northwest (330-350 deg) swell remain
possible for next week.

For more details regarding the surf, see the latest Collaborative
Nearshore Swell and Wind Forecast for Oahu at:
www.prh.noaa.gov/hnl/pages/SRF.php.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Saturday for South facing
shores of all islands.

&&

DISCUSSION...Birchard
AVIATION...Bohlin
MARINE...Kinel