Heavy Rain in Hawaii, July 31, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST SUN JUL 31 2016

.SYNOPSIS...
Gentle to moderate trade winds will increase and become locally
breezy by the middle of the upcoming week. Areas of enhanced
moisture embedded in the trades will maintain a rather wet pattern
over windward areas with some showers carrying over into leeward
areas. A nearby low aloft will provide enough instability to produce
a few heavier downpours the next couple of days. Although the low
will move away from the islands by the middle of the week, enhanced
moisture from the remnants of dissipated Hurricane Frank may fuel
additional showers Thursday through Friday.

&&

.DISCUSSION...
A relatively strong and nearly stationary 1033 mb surface high is
centered far NE of the islands this morning. A low aloft centered
about 750 miles NE of Kauai is drifting N and becoming embedded
within an E-W oriented trough located along 30N, while a ridge aloft
is centered well S of the islands along 15N. The prevailing westerly
flow aloft is carrying thin high clouds over the islands, while the
proximity of the low has elevated and weakened the subsidence
inversion, with morning soundings detecting a mixed layer that is
about 10 kft deep.

With an especially showery cluster of low clouds currently
traversing the central portion of the island chain from Maui to
Oahu, radar is detecting a few small cores of heavy showers embedded
in an area of otherwise light to moderate rainfall. Other areas of
showers are affecting windward portions of Kauai and the Big Island,
and the forecast was updated to increase PoP/Sky for many areas
today. At this time, it does not appear that this showery cloud
cluster will diminish significantly with daytime heating, and a
fairly cloudy and wet weather pattern is expected through the day,
especially windward.

The forecast calls for the high to the NE to remain nearly
stationary through the upcoming week, strengthening to near 1038 mb
by mid-week before weakening somewhat by next weekend. Latest model
guidance indicates that the low aloft and associated trough will
move and change minimally over the next couple of days. The presence
of the low and trough will continue to keep winds over the islands
somewhat muted due to a subtle weakening of the low-level pressure
gradient, and keep the island atmosphere somewhat unstable.
Therefore, a somewhat wet trade wind weather pattern is expected
through Tuesday.

The slow-moving low aloft is expected to weaken and move
sufficiently far NW of the islands by Wednesday to allow the low-
level gradient to tighten, and trade wind speeds to increase. While
the departure of the low would typically lead us to expect a trend
toward a drier trade wind weather pattern, especially with a mid-
level ridge building in from the NE, global models are indicating
that moisture associated with the remnants of now-dissipated
Hurricane Frank may arrive on the trade wind flow Thursday and
Friday, leading to increased showers. Another potential moisture
source (a remnant of newly-formed Tropical Depression Nine-E in the
eastern Pacific) could become embedded in the trade wind flow to
fuel more showers next weekend, but confidence is low with respect
to that portion of the forecast. For more information on Nine-E, see
advisories issued by the National Hurricane Center in Miami.

&&

.AVIATION...
AIRMET Sierra for mountain obscuration is in effect for most
windward areas this morning but may be allowed to expire at noon due
to the diurnal lifting of the cloud base. Conditions may redevelop
late tonight or early Monday.

Later today, sea breeze driven convective buildups will develop over
leeward and interior portions of the Big Island. Elsewhere clouds
and showers will continue to favor windward slopes and coasts.
Isolated mvfr ceilings possible in showers; otherwise VFR conditions
will prevail through late evening all areas.

&&

.MARINE...
Winds and seas are expected to remain below small-craft-advisory
(SCA) levels for the next couple of days due to the presence of a
nearby low aloft, and its weak surface reflection, which has
loosened the low-level pressure gradient. As the low moves N around
the middle of the week, the pressure gradient will tighten and trade
wind speeds will increase. Winds may become sufficiently strong at
that time to support the issuance of a SCA for the windier marine
zones around Maui and the Big Island.

There will be a series of small long-period S/SW swells through the
next week, but surf will remain below advisory levels on S facing
shores. Trade winds will drive short-period wind waves toward the
islands from the E, producing moderate and below-advisory surf along
E facing shores through the forecast period.

&&

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

&&

DISCUSSION/MARINE...Birchard
AVIATION...Bedal