Flash Flood and Heavy Rain in Hawaii, December 8 to 11, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Thu Dec 8 2016

.SYNOPSIS...
The weather will become a bit more active in the next few days due
to another trough of low pressure aloft approaches the state from
the west. Shower activity is expected to increase today and continue
off and on through the weekend. Mainly light winds will continue
through the rest of the week with widespread daytime sea breezes and
nighttime land breezes. A frontal boundary may reach the state late
this weekend and stall out in the island vicinity.

&&

.DISCUSSION...
Moisture did increase across much of the state overnight and as an
upper level disturbance passes near the islands early this morning,
latest radar data indicated thunderstorms did develop over the Kauai
channel as well as south coastal waters of Maui county early this
morning. Much of the clouds are being drawn up from the south as the
main upper level trough from the west moves closer to the area.
Believe the atmospheric conditions have deteriorated enough to
warrant the issuance of a Flood Watch for the smaller islands today,
since the threat of heavy rain with thunderstorms is quite elevated.
Big Island may still see its share of weather by this afternoon,
though looks like the bulk of the moisture will stay west and
therefore has excluded that island from the Watch at this time. On
the other hand, a Winter Weather Advisory remains posted for the Big
Island summits due to possible wintry weather at times through early
Friday morning. Mainly light southerly winds are expected as well,
with land breezes early this morning becoming sea breezes later in
the day.

The upper level disturbance is expected to lift northeast of the
state tonight, so atmospheric conditions in the island vicinity may
improve slightly. But then, the main upper level trough will still
be west of the area through Saturday. Meanwhile, weak southerly wind
flow at the lower levels will continue and possibly may bring in
more moisture to the state. Therefore the somewhat unsettled weather
is still expected tonight through Saturday, with scattered showers,
possibly locally heavy at times.

Latest model runs still indicate the upper level trough may finally
push across the islands by Sunday. The solutions are now closer,
with both models show some boundary reaching the western islands on
Sunday. The ECMWF has this boundary passing east of the Big Island
by Monday while GFS has the boundary stalling near the eastern
islands. Regardless, both models hint the band of moisture
associated with this boundary will linger in the island vicinity
early next week, with possible more clouds and showers for the area.

&&

.AVIATION...
A weak pressure pattern will remain in place today and tonight,
keeping winds light across the area. An upper level disturbance
approaching from the west early this morning, will move over the
island chain today, bringing some unsettled weather and the
potential for thunderstorms. MVFR conditions are expected to
develop around or shortly after daybreak in many areas, with these
conditions then lingering into the early evening hours.
Conditions are then expected to slowly improve tonight as the
upper level disturbance lifts northeastward and away from the
island chain.

AIRMET Tango is in effect for upper level turbulence due to strong
jet stream winds. These conditions will likely continue through
the day.

AIRMET Zulu is in effect for moderate icing between 140 and FL220
from Kauai to Maui including the adjacent waters. These conditions
will likely continue through the day.

AIRMET Sierra will likely be needed later this morning for
mountain obscuration across much of the island chain.

&&

.MARINE...
The current northwest swell will slowly decline today and no other
significant swell is expected through the weekend. A moderate north
swell is possible early next week.

Winds will remain on the lighter side, and with no major swell
expected, small craft advisory conditions are not expected for the
coastal waters through the next few days. But then, thunderstorms
may bring hazardous conditions with gusty winds and lightning at
times.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this afternoon for Niihau-Kauai-Oahu-
and Maui County.

Winter Weather Advisory until 6 AM HST Friday for Big Island
Summits.

&&

Aviation...Jelsema
Rest...Hui