Drought in Hawaii, September 1 to 30, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
835 PM HST Thu Aug 31 2017

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

&&

.DISCUSSION...
Latest (00Z) run of the GFS continues to support the ongoing
forecast philosophy that anticipates a light wind pattern developing
over the islands on Friday, and remaining so through about the
middle of next week. This occurs as the trade wind supporting high to
the NE weakens dramatically and a front approaches from the NW.
Overall, the next several days are expected to be drier than normal.

The developing light winds on the large scale will allow island-
scale land and sea breezes to dominate the wind flow over land,
while initially favoring 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. As a weak high
develops ENE of the islands late this weekend, and the stalled front
persists to the N, winds shifting to the S will likely lead to vog
moving over all islands by Sunday. 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. Some weakening of the ridge
aloft and modest mid-level cooling is expected after Sunday, but
with PWAT remaining relatively low, significant rainfall is not
expected.

The front will initially stall just N of the Hawaiian Offshore
Waters through early next week before potentially inching closer to
the islands Tuesday and Wednesday, but 00Z GFS and 12Z ECMWF
guidance continue to indicate that the bulk of the moisture
associated with the front will remain N of the area. High pressure
building far NE around the middle to end of next week should 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 in response, allowing a daytime onshore Sea
Breeze with interior cloud ceilings developing over all islands.
Mountain obscurations and MVFR ceilings will be possible in these
areas, and may require AIRMET Sierra to go into effect. Overnight
land breezes should clear out most of the cloud ceilings after
sunset Friday night.

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