Heavy Rain in Hawaii, August 13 to 14, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
1008 AM HST SAT AUG 13 2016

.SYNOPSIS...
Fresh trade winds will prevail through Sunday, weakening to a
moderate to locally fresh breezes throughout much of next week.
An upper trough to the northwest of the islands will enhance the
trade wind showers through Monday night, with more typical trade
wind weather then settling in through the middle of next week.
Another area of disturbed weather may affect the island chain late
next week.

&&

.DISCUSSION...
Fresh trade winds are driven by high pressure located far north
northeast of the state. Aloft, an upper-level trough observed in
the water vapor imagery north of Kauai is beginning to close off
into an upper low and dig towards the state. Visible satellite
over the state shows mostly sunny skies across Kauai and Oahu with
a bit more clouds and showers across Maui County and the Big
Island this morning. The trades are focusing most of these showers
on the windward sides of these islands.

The 12z soundings at PHLI and PHTO showed inversion heights have
risen to 8 kft and 11 kft respectively, with precipitable water
(PW) values ranging from 1.6 to 1.8 inches. MIMIC total PW imagery
shows plenty of moisture over the island chain, with PW values
between 1.5 and 1.8 inches, correlating well with the values
observed in the 12z soundings. Upstream of the islands, PW values
drop to between 1.3 and 1.6 inches, still above normal for this
time of year. With moisture levels remaining elevated, along with
the digging upper-level feature aiding in eroding the low level
inversion over the state, we should continue to see the wetter
weather to continue across eastern portion of the state today.

The upper low will stop north of the state tonight, linger through
Monday and begin to lift away to the north Monday night. While the
low is nearby, trade wind showers will remain enhanced across the
islands. Models show the trades gradually pushing the enhanced
moisture westward encompassing most or all of the state by tonight
and continuing into Monday. Again, the trades will focus most of
the rain over windward locations, but leeward sides of the smaller
island could see a boost in clouds and showers too at times.

The trade winds will weaken to moderate to locally fresh breezes
on Sunday night and continue through the middle of next week.
This is in response to the driving high pressure moving a little
bit further from the Hawaiian Islands. Models show the high
strengthening again during the second half of the week and may
boost our trade wind speeds at that time.

Both the 12z GFS and 00z ECMWF continue to show various weak
areas of low pressure developing to the south of the island chain
next week. Given the time of year and warm sea surface
temperatures in the 28 to 29 degree Celsius range, we will
definitely need to keep an eye on the tropics.

&&

.AVIATION...
AIRMET SIERRA remains in effect for mountain obscuration on Maui
and on the east side of the Big Island due an abundance of area
of low level moisture riding in on the trade winds. Low clouds
along with accompanying showers will possibly cause conditions to
persist into the early afternoon hours before diminishing later
this afternoon. But some pockets of low level moisture are again
expected again overnight. The low clouds and showers will spread
to the rest of Maui county today. Otherwise VFR conditions should
prevail in most other areas, especially along the leeward sections
of the islands.

Moderate to locally breezy trade winds are expected to continue
for the next several days. AIRMET TANGO for mechanical turbulence
to the lee of the mountains will likely remain in effect through
Sunday. A tropical cyclone SIGMET Tango has recently been issued
in the west Pacific for Tropical Cyclone Chanthu, currently at
tropical storm strength.

&&

.MARINE...
High pressure located far north northeast of the the state will
drive fresh trade winds through Sunday. The strongest winds are
expected in waters between the Big Island and Molokai through
Sunday. A Small Craft Advisory, mainly due to winds, has been
adjusted slightly is in effect for the areas listed below through
Sunday. See product MWWHFO on our website for more information.
The trades will produce choppy, short period surf along east
facing shores through Sunday but will stay below high surf
conditions. The trade winds will weaken to moderate to fresh
through much of next week.

A very small west-northwest swell from recent Typhoon Omais in
the western Pacific will remain possible through the weekend.

Model guidance suggests a slight increase in surf along the south
shores Tuesday through Wednesday due to a recent gale in southern
Pacific a few days ago.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel.

&&

DISCUSSION/MARINE...Foster
AVIATION...Fujii