Heavy Rain in Hawaii, August 20 to 21, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
335 PM HST SAT AUG 20 2016

.SYNOPSIS...
A typical trade wind weather pattern can be expected through the
weekend due to high pressure far northeast of the area. Clouds and
passing showers will favor windward and mauka areas. Early next
week the high will weaken as a trough of low pressure develops to
our northwest. Winds will become lighter as a result. Also an
increase in showers is expected as moisture gets drawn up over the
area from the southeast. High pressure will rebuild northeast of
the area later next week with a trade wind weather pattern
returning.

&&

.DISCUSSION...
High pressure is currently centered far to the northeast of the
state. This places the islands in a moderate trade wind
environment. Moisture embedded in the trade wind flow is causing
some typical windward and mauka showers to fall with the trade
winds sufficiently strong enough to blow a few brief showers over
to some leeward locations. There will likely be a slight downward
trend in shower activity later this morning and this afternoon but
daytime heating will likely cause for clouds and showers to build
up over the leeward Big Island slopes this afternoon.

In the mid levels of the atmosphere a ridge is located just
northwest of the area while a trough lies to the east of The Big
Island. This is noticeable in the 12Z soundings as indicated by
higher inversion heights at Hilo as compared with Lihue.

The weather pattern will go through some changes over the next
couple of days. At the surface, the high to the northeast will
slowly weaken over the few days with a gradual weakening of the
trades. The mid level ridge northwest of the state will gradually
weaken and shift to the southwest while the mid level trough to
our east moves to the northeast away from the area. Showers will
continue to be focused over windward and mauka areas through
Sunday with afternoon clouds and showers possible over the Kona
slopes.

During the Monday through Wednesday time period, a low and mid
level trough is forecast to dig down to a position several hundred
miles to the northwest of the area. This will cause the wind flow
to become light favoring a southeasterly direction. The lighter
wind flow will allow for daytime sea breezes and nighttime land
breezes to develop over some areas. In the meantime an area of
enhanced moisture may get drawn up near or over the area.
Depending on how far north the moisture reaches, this would likely
cause for an increase in shower activity first for The Big Island
and then gradually spreading up the island chain. The airmass is
not expected to become all that unstable so the threat of
thundershowers at this time appears low. However there may be
enough heating and deep moisture to allow for an isolated
thundershowers over The Big Island slopes early next week mainly
during the afternoon hours.

As we head in the second half of next week, the trough to our
west will lift away to the north while a mid level ridge builds
over the area from the northeast. Also the area of enhanced
moisture will be moving away to the west and high pressure at the
surface will be rebuilding in to our northeast. This will allow
for a trade wind weather pattern to return.

&&

.AVIATION...
Surface high far northeast of the state will keep breezy trade
winds across the area. AIRMET TANGO is posted for low level
turbulence to the lee of terrain. The NE winds will carry enough
clouds and showers over windward areas to produce isolated MVFR
conditions.

Sea breeze driven convective buildups over the southern and
leeward coasts of the big island may produce locally heavy
showers this evening but will dissipate after sunset. Partial
clearing expected after sunset for leeward areas.

&&

.MARINE...
A Small Craft Advisory remains posted for Maalaea Bay and the
Pailolo and Alenuihaha Channels as high pressure remains firmly
entrenched far northeast of the area. The high has been weakening
and on this trend we expect the trade winds to drop off later
tonight and Sunday. Winds are then expected to remain below
advisory levels into the middle of next week.

Trade winds will continue to produce short period choppy surf
along east facing shores. A series of small southerly swells can
be expected through the weekend and on into the middle of next
week. No other significant swells are expected.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Sunday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel.

&&

Synopsis/Discussion/Marine...Burke
Aviation...Bedal