Heavy Rain in Hawaii, May 29, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
400 PM HST SUN MAY 29 2016

.SYNOPSIS...
A weak front will cross Kauai tonight, move across Oahu later
tonight and Memorial Day, and Maui county Monday night and Tuesday.
Moisture associated with this gradually dissipating front will then
remain stalled over the island chain for much of the upcoming week.
High pressure building north of the state will bring gradually
strengthening trade winds through Monday, which will focus most of
the clouds and showers over windward slopes and coasts. After
prevailing through the week, trade winds may diminish again next
weekend.

&&

.DISCUSSION...
Light winds allowed clouds and a few showers to develop over the
islands this afternoon, but thus far shower intensity and coverage
have been significantly less than yesterday. This may be related to
a subtle decrease in PWAT, or due to a weakening of the upper-level
trough that continues near the islands. Afternoon soundings do
indicate that the atmosphere has become more stable over the past 24
hours, while water vapor imagery highlights a decrease in moisture
aloft.

The forecast calls for gradually strengthening, mostly moderate
trade winds to spread over Kauai tonight, Oahu on Monday, and Maui
county Monday night and Tuesday as high pressure moves into a
position N of the islands. This high will be pushing a weak frontal
boundary over the islands from the N, with the boundary crossing
Kauai tonight, Oahu later tonight and Monday, and Maui county Monday
night and Tuesday. The front is currently marked by a NE-SW oriented
band of broken to overcast low clouds that is about 75 miles wide at
our longitude, with the leading edge located just N of Kauai, moving
S/SE between 5 and 10 mph.

Guidance continues to differ on the speed of the southward
progression of the frontal moisture, but the general expectation is
that the moisture axis will move very slowly southward down the
island chain, eventually reaching the Big Island. GFS guidance
delays this until Thursday/Friday, while the ECMWF is a little
faster. POP grids from mid-week through the end of the week
represent a blend of these solutions, featuring relatively high
windward POPs statewide.

In the short term, while Kauai and Oahu see a return of trade winds
and windward-based clouds and showers, weather over Maui county and
the Big Island will be dominated by land and sea breezes, with
clouds diminishing overnight tonight. Mostly clear skies Monday
morning will give way to afternoon clouds, but like today, shower
coverage and intensity are expected to be limited.

A mid- and upper level trough currently overhead will be slow to
clear the area the next couple of days. However, guidance indicates
warming in the mid-levels over the next 48 hours, and as this
occurs, the threat of locally heavy rainfall will diminish, despite
an increase in PWAT associated with the front. A low aloft may
develop nearby Thursday/Friday, and enhance the moisture that is
expected to be near the Big Island. This could potentially fuel
heavy rainfall, especially over the Kona slopes. Another front may
approach from the NW late in the week to kill the trade winds for
next weekend, which could bring a weather pattern similar to what
we've experienced the last couple of days, with land and sea breezes
and locally heavy afternoon downpours. The long range wind forecast
has been trended toward lighter winds, while POP/sky/weather grids
now feature a land/sea breeze weather pattern.

&&

.AVIATION...
A weak front currently making its way onto Kauai is moving SE
between 5 and 10 kt. The front will keep high pressure at bay, far
off to the northeast. With the lack of trade winds, light winds will
remain across the state with daytime sea breezes and nighttime land
breezes forming through tonight. Afternoon cloudiness due to daytime
heating has lead to scattered MVFR conditions due to lowered
ceilings and visibilities from clouds and showers with some areas
possibly seeing isolated IFR conditions. Clearing to take place at
tonight with the land breezes kicking in. But the windward sections
of Kauai and possibly Oahu may see some clouds and moisture from the
weak front skirting by tonight.

High pressure building in from the nw behind the front will allow
trade winds to start returning to Kauai on Monday, if not later
tonight, and the rest of the state later into Tuesday.

An AIRMET for mtn obsc is in effect for Kauai and the Big Island for
daytime cloudiness but will likely be discontinued this evening as
the land cools.

&&

.MARINE...
A weak front in the northwest waters adjacent to Kauai will continue
to move southeast, and move through Kauai this evening and Oahu
through Monday. High pressure will fill in behind the front and
moderate to fresh trade winds are expected for most areas by Monday.
Currently, winds are forecast to remain below small craft advisory
level. However, some of the typically windier spotS may near this
level and will need to be monitored.

A small northwest swell should gradually increase tonight and peak
Monday and subside into Tuesday. Buoy 51101 has been reporting the
expected 3-4 ft northwest swell since this morning. With a 12 second
period, the swell should start to arrive this evening in Kauai and
later tonight in Oahu. The current small southwest swell will
continue to decline tonight. Another small southwest swell is
expected to bump surf up slightly again for south shores Wednesday.
The increasing trade winds will gradually re-introduce short period
choppy surf to east facing shores during the coming week.

&&

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

&&

Discussion...Birchard
Aviation...Fujii
Marine...Eaton