Heavy Rain in Hawaii, September 23 to 24, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
843 PM HST THU SEP 22 2016

.SYNOPSIS...
The winds are expected to shift to the east-southeast and weaken
by early Friday morning, and then remain light to moderate through
Saturday. Enhanced tropical moisture arriving from the southeast
will bring an increasingly humid and wet weather pattern into
Saturday, especially over the Big Island and Maui. While most of
the low clouds and showers will favor windward areas, the lighter
winds may allow a few leeward showers to develop Friday and
Saturday afternoons. Fewer showers are expected Saturday night and
Sunday. The trade winds will strengthen again from Sunday into
Monday, and remain breezy for most of next week. Trade showers
may increase again Sunday night and Monday, and again on
Wednesday.

&&

.DISCUSSION...
Autumn officially arrived in Hawaii today, with a relatively
benign trade wind weather pattern in place. A nearly stationery
1028 mb surface high is centered near 34N 136W, or about 1400
miles east-northeast of Honolulu. A nearly east to west oriented
surface ridge extends from this high through a point about 600
miles north of Lihue. The pressure gradient southwest and south
of these features is maintaining the trade wind flow across the
region early this evening.

Elsewhere, the axis of a weak surface trough, the remnant of
former Northeast Pacific Hurricane Orlene, is evident just north
of the Kauai Channel this evening. This feature is moving westward
at about 15 mph. Aloft, an upper level trough extends over the
islands from a well-defined closed upper level low centered near
26N 149.5W, or about 550 miles northeast of the Hilo. The presence
of the upper level trough is causing some slight atmospheric
instability. The early afternoon soundings from Hilo and Lihue
indicated a weak low level subsidence inversion was still in
place. Also, precipitable water (PW) values were near 1.3 inches,
which was slightly below normal, at that time. Note that
satellite derived PW indicates values are now 1.7-1.8 inches in
the enhanced tropical moisture already near the windward Big
Island, with even higher values immediately upstream of the state.

The trade winds are transporting varying amounts of low clouds
and showers to windward facing coastlines and slopes across the
state early this evening. Low clouds and showers are already
increasing over the windward Big Island. Wetter weather is also
expected to develop over windward sections of Maui from later
this evening through early Friday morning as PW values increase
to near 2 inches. Somewhat drier than normal trade wind weather is
expected west of Maui through the evening, with showers likely
increasing from some time after midnight HST through Friday
morning.

Despite little change in the strength and position of the surface
high to the east-northeast over the next couple of days, winds
are expected to diminish somewhat later tonight and Friday as the
low level pressure gradient relaxes, and winds shift to the east-
southeast. This will occur as the surface trough, that is the
remnant of former Orlene, moves northwest of the area through
Friday. Additionally, a developing surface low far north of the
area will drag a front toward the region Friday through Saturday,
eroding the western portion of the surface ridge. While the
background flow will diminish only slightly, a wind shift to the
east-southeast typically places the smaller islands in the wind
shadow of the Big Island, and a trend toward lighter winds over
land is likely. At the same time, the arrival of the enhanced
moisture will also increase humidity levels over the islands. The
interaction between the moisture moving in from the southeast,
and the trough aloft is expected to lead to a wet weather pattern
before the upper level trough eventually lifts to the north, and
the moisture finally clears out on Saturday.

Note that the forecast continues to indicate a slight chance of
thunderstorms may develop over parts of the Big Island Friday
afternoon and evening due to the enhanced tropical moisture
combined with the instability due to the upper level trough.

The forecast models indicate the surface front that will erode the
ridge will eventually stall about 400 miles north of Kauai this
weekend, and it is not expected to directly impact island weather.
However, a new high building north of the front will tighten the
local pressure gradient starting Sunday, when winds will back to a
more typical east-northeast direction. Trade winds will strengthen
further Monday as the strong high builds eastward, and a fairly
breezy trade wind pattern is expected for most of next week. The
GFS model continues to indicate another batch of moisture may
arrive in the trade wind flow Sunday night/Monday, and again on
Wednesday. If so, expect a wetter trade wind weather pattern
during those time periods. Otherwise, a rather tranquil weather
pattern is expected next week.

&&

.AVIATION...
A weak trough of low pressure in the vicinity of the island chain
tonight and Friday will bring enhanced clouds and showers to the
islands at times. Widespread MVFR and periodic IFR conditions are
expected across windward sections of the Big Island overnight
through much of the day Friday. Reduced cigs/vsbys will also
impact primarily windward sections of the smaller islands later
tonight and Friday. Predominantly VFR conditions are expected in
leeward areas overnight, with the potential for reduced cigs/vsbys
in these areas during the afternoon on Friday.

AIRMET Sierra is in effect for mountain obscuration over windward
sections of the Big Island. The AIRMET may need to be expanded to
include windward sections of the other islands, particularly the
Maui County islands, later tonight.

&&

.MARINE...
Winds and seas are expected to remain below the Small Craft
Advisory (SCA) criteria statewide into Sunday. East-northeast
trade winds are expected to shift to the east-southeast from later
tonight through Saturday as a surface high to the distant
east-northeast remains nearly stationary, but a weak surface
trough marking the remnants of former Northeast Pacific Hurricane
Orlene passes north and northwest of the islands, and a front
pushing down far north of the region erodes the surface ridge. A
new surface high building far north of the area starting later
this weekend will eventually lead to winds backing to the east-
northeast and gradually strengthening by late Sunday. Trade winds
will likely be strong enough to require the issuance of a SCA for
the typically windier waters adjacent to the islands of Maui
County and the Big Island starting Sunday night or Monday. The SCA
for these waters is expected to be needed for most of next week.

Surf is expected to remain below the high surf advisory threshold
on all shores across the state through next Friday. In the short
term, a south-southwest swell is bringing elevated surf to south
facing shores, but this swell is expected to gradually diminish
through Saturday. While additional swells from the southern
hemisphere are expected through next week, they are expected to be
smaller than the current one. A small and relatively short period
northwest swell is expected to arrive Friday and peak on Saturday,
then diminish Sunday. Another slightly smaller north-northwest
swell may arrive Monday and Tuesday. Elsewhere, surf is expected
to decrease slightly along east facing shores this weekend.
However, the strengthening trade winds will likely cause an
increase in choppy surf along east facing shores next week.

&&

.HYDROLOGY...
While trade wind showers,  even when briefly heavy, don't
normally cause hydrology problems, the Wailuku River (formerly
known as Iao Stream) in windward west Maui has changed course
after last week's severe flooding, and the stream now runs close
to some homes. Therefore, it likely won't take much mauka rainfall
for the river to cause problems again. However, the river gage
was washed away by the flooding, and it is currently uncertain as
to how much rain it would take for the stream to rise to the
level that it would threaten lives and property.

&&

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

&&

DISCUSSION/MARINE...Houston
AVIATION...Jelsema
HYDROLOGY...Birchard