Drought in Hawaii, August 1 to 31, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
848 PM HST Mon Jul 31 2017

.SYNOPSIS...
Trade wind weather will persist through the week as a ridge of high
pressure remains to our north. Trade winds are expected to weaken
through midweek due to a front positioned well north of the state,
but will become locally breezy once again Friday into the upcoming
weekend as the ridge strengthens over the region. Clouds and showers
will favor windward and mauka areas. Leeward sections of the Big
island will see increased clouds and showers each afternoon and
early evening.

&&

.DISCUSSION...
Recently animated water vapor loop showed a broad upper low centered
at the Date Line near Midway and an upper ridge along with plenty of
mid- to upper-dry air in place across the main Hawaiian Islands. The
afternoon Hilo and Lihue soundings reflected this mid- to upper-dry
air above a strong subsidence inversion (6-7 kft) with precipitable
water (PW) values coming in around an inch, which is below average
for this time of year. The latest satellite-derived PW imagery
reflected this dry air over the islands, however, did depict a
pocket of enhanced moisture within the trades approaching Maui
County and the Big Island (PWs up to 1.5") and another surge of
moisture located a few hundred miles east-southeast of the Big
Island. This patch of moisture east of Maui could translate to a few
more overnight windward showers for Maui County and the Big Island.
Otherwise, mostly dry and pleasant conditions are anticipated
overnight.

The latest short-term model guidance has initialized well with the
current pattern and continues to depict mostly dry and stable
conditions persisting over much of the area through Thursday.
This will support below average rainfall chances, with the best
chances favoring windward and mauka areas through the late night
and early morning periods. Although the surge of moisture located
a few hundred miles east-southeast of the Big Island will mostly
pass south of the islands, the northern extent may clip the
southern portion of the Big Island and support a few more showers
late Tuesday through Wednesday. The previously discussed weakness
within the subtropical ridge that has developed north of the state
will be enough to cause the trades to relax into the light to
moderate range over the islands each day.

For the upcoming weekend, the extended guidance remains in decent
agreement and supports moderate to locally breezy trades returning
across the state as the subtropical ridge strengthens north of the
islands. The best rainfall chances will favor windward and mauka
areas through the late night and early morning periods with
occasional showers potentially spilling over to the leeward areas
with the breezy trades returning.

&&

.AVIATION...
A moderate low-level trade wind flow is expected to persist
through Tuesday afternoon. Low clouds and showers will continue
to favor the windward and mountain areas, especially during the
nighttime and early morning hours. An area of low clouds and
showers is spreading slowly westward across portions of the
eastern and central islands early this evening. These low clouds
and showers may affect parts of Oahu and Kauai later tonight.
Elsewhere, another area of low clouds and showers upstream of the
the Big Island may affect parts of the eastern Big Island later
tonight or early Tuesday morning. Aviators should be aware that
some of the trade showers may produce periods of MVFR CIGS/VIS,
especially over higher terrain and along windward facing slopes
and shorelines. Otherwise, VFR conditions are expected.

No AIRMETS are currently in effect, and none are anticipated
through early Tuesday morning.

&&

.MARINE...
The pressure gradient across the islands has slackened, and winds
have thus weakened. No Small Craft Advisory (SCA) conditions are
expected through much of the week. However, trades are forecast
to strengthen by Friday as the pressure gradient once again
tightens. An SCA may then be necessary for the windier zones
around Maui County and the Big Island.

Surf will remain below high-surf advisory criteria on all shores
through the week. However, swells from distant tropical systems
in the eastern and western Pacific may boost surf marginally on
east- and west-facing beaches through the week. Swells from the
southern hemisphere may also bring small- to medium-level surf to
south shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION...Gibbs
AVIATION...Houston
MARINE...Kinel