Heavy Rain in Hawaii, August 22 to 24, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
1012 AM HST MON AUG 22 2016

.SYNOPSIS...
Hot and humid conditions expected as trade winds decrease and
veer from the southeast over the next few days. Showers will
become more frequent, especially during the afternoons and early
evenings, as a moist and somewhat unstable airmass moves across
the area. Locally heavy rainfall and isolated thunderstorms will
be possible on the Big Island through Tuesday. Drier trade wind
weather will return Thursday and Friday.

&&

.DISCUSSION...
A weak front to the north has pushed our normally tradewind-
producing high pressure far into the East Pacific. This in turn,
has weakened the pressure gradient over the islands and weakened
the tradewinds to light to moderate levels. In addition, tropical
moisture is working its way over the state from the southeast,
bringing relative humidity levels up into the uncomfortable zone.
The early morning Hilo sounding shows a very juicy sounding
holding 2.2 inches of precipitable water, and a conditionally
unstable airmass. The instability and enhanced moisture will
produce some locally heavy showers over the Big Island today with
a slight chance of thunderstorms. The best chance for
thunderstorms will over the mid and upper slopes this afternoon
and Tuesday afternoon with the slopes providing an elevated heat
source.

The enhanced moisture will also effect the smaller islands with
increased showers, and high humidity expected through Wednesday.
Winds will gradually shift from the southeast today and Tuesday
effectively blocking the trade winds over the leeward areas of the
smaller islands, and allowing sea breezes to develop. With the
afternoon sea breezes, expect increasing clouds and showers over
interior sections early in the afternoon with showers drifting
over coastal areas later in the day. Clouds and showers will
diminish after sunset as the sea breezes dissipate and land
breezes set up late at night.

Enhanced showers will also form along low- level convergence
bands as the southeast winds flow around the islands and converge
downstream. Historically, southeast flow with a moist atmosphere
have created the potential for heavy rain over the Koolau
Mountains as winds converge downstream of Molokai.

Fortunately, the upper levels of the atmosphere will remain fairly
stable with 500 mb temperatures remaining near 5 degrees C, well
too warm for widespread thunderstorms. So, the forecast for a slight
chance thunderstorms is relegated to the Big Island for now where
low level moisture is the highest and elevated land heating is
present.

Drier trade wind weather should return to the state beginning
Thursday and continuing into the weekend, as high pressure builds
to our north and a drier airmass moves in from the east. This
should suppress seabreeze activity, with passing showers becoming
focused over windward areas.

&&

.AVIATION...
AIRMET Sierra for mountain obscurations is posted as an area of
showers moves across the area causing some MVFR conditions across
windward and mountain areas. Expect these conditions to continue
through the day, possibly spreading over to some interior and
leeward areas this afternoon.

&&

.MARINE...
Winds and seas will remain below Small Craft Advisory levels
through at least Wednesday. Winds could approach advisory levels
from Thursday through the rest of the week across the typically
windy waters around Maui County and the Big Island, as high
pressure rebuilds far northeast of the area.

Small swells from the northeast and south will continue to produce
small surf along exposed shorelines this week, but no significant
swells are expected, and surf will remain well below advisory
levels on all shores.

&&

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

&&

Morrison...Discussion and Marine
Burke...Aviation