Heavy Rain in Hawaii, May 13, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
1005 AM HST FRI MAY 13 2016

.SYNOPSIS...
Light winds will prevail through the weekend, with breezy trade
winds returning early next week. The light winds will allow clouds
and showers to develop over interior and leeward areas of the
islands this afternoon, and again on Sunday. However, from late
tonight through Saturday, moisture from an old front currently near
the Big Island will move slowly north, at the same time a weak upper
level disturbance passes by. This combination will bring more
widespread clouds and showers, with a few locally heavy showers
possible on Saturday.

&&

.DISCUSSION...
In summary, light winds will prevail through the weekend, with trade
winds returning early next week, likely becoming breezy by Tuesday
as high pressure strengthens far NE of the area.  The light winds
will allow sea breezes to drive cloud and shower development over
the islands during the afternoon today and Sunday, but a pair of
synoptic scale features are expected to interrupt this pattern from
late tonight through Saturday. An E-W oriented band of enhanced
moisture currently near the Big Island, marking the remnants of a
stalled front, will move slowly N through Sunday, with a weak trough
developing along the boundary as it does so. As this occurs, a mid-
level shortwave trough is forecast to move from W to E across the
area, potentially triggering a few locally heavy showers, mainly
Saturday afternoon and evening. The returning trade winds may be
showery early in the week as a disturbance aloft NE of the area
destabilizes low clouds arriving on the trade wind flow, mainly
affecting windward slopes and coasts.

Currently, total precipitable water (TPW) imagery highlights a sharp
difference in the moisture content of the island atmosphere, with
values close to 2 inches near the Big Island, and 1 inch near Kauai.
This is primarily due to moisture associated with a stalled and
dissipated front near the Big Island. Surface winds are light trade
winds, with overnight land breezes in the process of giving way to
sea breezes in many island locations. The pressure gradient is
relatively weak as latest surface analysis features several low
pressure areas N of the islands, and high pressure centered far to
the ENE. High clouds streaming over the islands from the W are
associated with an eastward-moving closed low aloft that is centered
about 950 miles W of the islands. This morning's mostly sunny skies
are already in the process of giving way to increasing low clouds as
sea breezes converge over the islands due to heating of the terrain.

The weekend forecast remains a little uncertain due to the expected
interaction between the enhanced moisture, an approaching mid- and
upper-level trough, and the diurnal cycle of land/sea breezes. The
passage of the trough aloft is expected to lower 500 mb temps by 2-3
degrees C, somewhat destabilizing the island atmosphere on Saturday.
The timing of the northward-moving moisture axis complicates the
picture, as early morning clouds and showers on Saturday could
interrupt the land/sea breeze cycle that typically leads to sunny
morning skies. Either way, expect a somewhat cloudy and showery
weather pattern to prevail in most areas of the state on Saturday.
The exception is expected to be the Big Island, as decreasing
moisture is expected to allow for lots of morning sun, with clouds
and a few showers developing over the slopes in the light wind flow.

By Sunday, the trough aloft will move NE of the area, but winds will
remain light enough to allow land/sea breezes to drive island
weather, with morning sun giving way to afternoon and evening clouds
and showers statewide. Lingering moisture may allow a few of these
showers to become briefly heavy as well.

High pressure will strengthen far NE of the state early next week as
the surface trough near the islands dissipates. Trade winds will
strengthen in response, and an upper level disturbance NE of the
islands may cause the trade wind flow to be on the wetter side early
in the week.

&&

.AVIATION...
Clouds are greatest in coverage over and adjacent to the Big Island.
Bases are generally VFR...but with occasional MVFR ceilings and
visibilities in showers. Over land the showers (and accompanying
MVFR conditions) are more common across windward and southeast
sections...where an AIRMET for mountain obscuration remains in
effect. Moisture and clouds are more limited across the smaller
islands...and are currently focused across windward sections.
Background winds are light enough to allow for at least localized
sea breezes...with additional clouds expected across leeward
sections during the afternoon (and isolated MVFR conditions as well).

Scattered to broken high clouds above FL200 will continue to stream
over the state today as a shortwave and upper jet approach from the
west-southwest. The upper level system is not sharply defined and
models show limited wind shear and deformation...so moderate or
greater turbulence is not anticipated.

&&

.MARINE...
A small craft advisory (SCA) will probably be needed for the
typically windy waters around Maui County and the Big Island by late
Sunday and for most of next week as trade winds strengthen. Until
then, winds and seas will stay below SCA levels.

A small, long-period NW swell will build today and peak early
Saturday before diminishing Sunday and Monday, with peak surf
heights well below advisory levels. S facing shores will continue to
see small long period swells through most of the next week, with the
potential for a small increase from the SSW around mid-week. Worth
noting that the source of this swell is a low in the Tasman Sea
between Australia and New Zealand, which typically reduces
confidence in the swell's arrival in Hawaii due to the reduction in
swell energy as it passes through numerous island groups. Otherwise,
short-period wind waves will increase as trade winds strengthen
early next week, bringing an increase in wave heights along E facing
shores.

&&

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

&&

Discussion/Marine...Birchard
Aviation...Bravender