Heavy Rain in Hawaii, June 11, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
1003 AM HST Sun Jun 11 2017

.SYNOPSIS...
Light to moderate trade winds will prevail for the next several
days, although winds may become breezy around the middle of the
week. The trade winds will periodically deliver clouds and showers
to the islands, as moisture associated with a dissipated front
streams in. Most of the clouds and showers will favor windward
slopes and coasts, with a few showers spreading to leeward areas.
Fewer showers and lighter winds are expected by next weekend.

&&

.DISCUSSION...
A surface high centered far NNE of the islands will drift slowly E
the next couple of days while weakening slightly. The high will then
remain nearly stationary to the distant NE of the islands through
next weekend. The high is expected to support moderate trade winds
through Thursday, although global models are indicating a slight
uptick in wind speeds around midweek. A trend toward lighter trades
is expected by the end of the week as the associated ridge N of the
islands is weakened by a passing front. As the front stalls and
retreats, the long-range forecast anticipates trade winds
restrengthening to moderate speeds.

A front that dissipated to the NE of the area several days ago has
injected its moisture into the trade wind flow upstream of the
islands, and continues to fuel areas of showers. As the moisture
band appears to be consolidating into distinct patches, island
weather will likely be characterized by periods of showery weather
interspersed with periods of clearing.

Model guidance supports this as well as recent trends - on Oahu for
example, today started fairly dry and sunny, but an area of clouds
and showers is approaching from the E to make for a cloudier and
wetter afternoon/evening. Meanwhile, only isolated small showers are
in the short term forecast for windward Big Island, as limited
moisture is noted immediately upstream. Although very little in the
way of low clouds is noted upstream of Kauai, skies are mostly
cloudy island wide due to orographic forcing. A forecast update will
be issued to account for recent trends. Increased moisture currently
along 150W is expected to keep showers active over the smaller
islands through Monday, while also moving in to windward Big Island
on Monday.

While a break in the moisture may move across the islands Monday
night, global models are indicating another slug of moisture riding
in on the trade wind flow Tuesday and Wednesday. Additionally, a
developing low aloft is expected to be tracking toward the islands
from the NE, increasing the instability of the island air mass, and
likely leading to the development of at least a few locally heavy
downpours. As latest GFS and ECMWF guidance continue on this theme,
forecast confidence is increasing, and the afternoon forecast update
will likely include a mention of heavy showers. The low aloft is
forecast to move slowly W, likely keeping some instability over the
islands through Thursday before a high aloft moves in from the E,
leading to a more stable weather pattern by Friday into next
weekend.

Rainfall will be concentrated over windward areas, but some of the
showers will spread leeward, especially during the night and morning
hours. The developing low aloft will likely produce periods of high
clouds over the islands through the first half of the coming week.
These clouds are not expected to produce any precipitation that
reaches the surface, but will keep the skies gray at times. Leeward
Big Island could receive some locally heavy afternoon and evening
showers this week as well, due to the presence of the low aloft, and
increased low-level moisture.

&&

.AVIATION...
Areas of brief to TEMPO MVFR will exist over windward and mauka
sections across the island chain today as showers ride the trade
winds. Leeward sections will see VFR conditions predominantly.

No AIRMETs are currently in effect.

&&

.MARINE...
Small background swells from the south and southeast are expected
through Friday. A moderate long period south swell is forecast to
build in along south facing shores Friday night into next
weekend.

Trade winds will remain moderate to locally fresh through much of
the week, as high pressure persists to the north-northeast of the
islands. Small, short period swell will affect the east facing
shores as a result.

&&

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

&&

Aviation...Kinel
Marine...Stall
Discussion...Birchard