Flash Flooding and Heavy Rain in Hawaii, September 12 to 16, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
338 PM HST MON SEP 12 2016

.SYNOPSIS...
Deep tropical moisture over the region this week will bring wetter
than normal conditions to the islands. A weak surface trough near
Maui will continue to move over the smaller islands overnight,
while an upper level trough will move down the island through
Tuesday night. By Wednesday night, an upper level low within the
trough will add additional instability, leading to the possibility
of locally heavy rainfall. High pressure far northeast of the
islands could bring increase trade winds across the area late in
the week.

&&

.DISCUSSION...
A surface trough near Maui this afternoon continues to move to the
west across the islands. Meanwhile, a surface high remains nearly
stationary far northeast of the islands. The high is maintaining a
trade wind flow over and to the northeast of the islands, while
the trough is helping to introduce an unstable airmass and
increased low level moisture. The trade wind flow is helping to
usher that moisture over the islands. There have been showers
across the islands today, particularly focused over Maui, Molokai,
and Oahu. Most of the showers have been over the windward areas,
but with no discernible inversion, the showers have been deeper
than typical trade wind showers and have been carried over to
leeward areas.

The afternoon soundings from Lihue and Hilo had precipitable
water (PW) values around 1.63 inches. This is very little change
from the overnight sounding, and remains on the high end of normal
for September. Satellite based total PW continues to show values
at or above what the sounding shows to the northeast of the
islands.

A northeast to southwest oriented upper level trough is found
near Kauai this afternoon and is forecast to move further down the
island chain through Tuesday night. Tuesday night, the ECMWF and
GFS develop an upper level low within the trough, northeast of the
islands. Wednesday through Thursday, that low drops to the
southwest and moves over the islands. This will further
destabilize the atmosphere over the islands. In addition, at the
surface, the high pressure system to the far northeast will move
to the east, and the winds could become a bit more east
southeasterly. Combined with the abundant low level moisture,
showers will be plentiful, and some could be heavy. Have locally
heavy rainfall possible in the forecast across the island chain
Wednesday through Thursday.

The main question that remains during this time is whether or not
the atmosphere will hold the potential for thunderstorms. Have
continued to hold off on the mention of thunder in the forecast
quite yet, preferring to monitor the next couple runs of the
models to see what pans out.

Beyond Thursday, a new high builds behind the current one,
allowing trades to fill back in for the weekend. Low level
moisture though remains high, so anticipate at least wet trades to
persist into the weekend.

&&

.AVIATION...
A weak surface trough over the state will bring clouds and
showers across the area, particularly for the smaller islands.
Gentle east to northeast winds will focus most of the showers over
windward areas, however many of the showers will make it over to
the leeward areas too.

AIRMET SIERRA is in effect for tempo mountain obscurations across
Oahu, Molokai and Maui due to the extensive clouds and showers
this afternoon. These showers will continue into tonight and
likely Tuesday. It is possible the AIRMET may need to be expanded
to other portions of the state tonight depending upon the movement
of the showers. No other AIRMETs are in effect.

&&

.MARINE...
A northwest swell building tonight is expected to peak tomorrow
below advisory levels, before gradually tapering. Another
northwest swell is expect to build late Friday, and peak Friday
night through Saturday below advisory levels before subsiding. A
series of small south to southwest swells will persist through
Monday.

Winds and seas are both expected to remain below small craft advisory
conditions through Thursday. As the high pressure system begins to
build north of the islands toward the end of the week, winds may
increase to bring small craft advisory conditions to the typically
windier locations near Maui and the Big Island.

&&

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

&&

M Ballard