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Hawaiian Islands Synoptic Discussion and Guidance

858
FXHW60 PHFO 082001
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1001 AM HST Thu Dec 8 2016

.SYNOPSIS...
Cloudy, wet, and unsettled weather will prevail through the
evening as an upper level disturbance passes over the state. A
decrease in shower activity is expected on Friday, followed by
a potential increase again on Saturday. As a weak front
approaches, a land and sea breeze pattern is expected on Sunday,
producing interior clouds and showers mainly in the afternoon.

&&

.SHORT TERM...TODAY AND TONIGHT
Cloudy, wet, and unsettled weather will continue into tonight. A
shortwave upper level trough approaching from the southwest is
generating a large shield overcast layered clouds and showers with
isolated thunderstorms from Maui to Kauai this morning. Satellite
derived precipitable water estimates of around 2 inches are
observed within this plume of moisture, which will affect the
smaller islands throughout the day, and a Flash Flood Watch is in
effect from Maui to Kauai. While the Big island is expected to
remain along the eastern fringe of the deep moisture, a heavy
shower or thunderstorm cannot be ruled out today. A forecast
update has been sent to increase rainfall chances today.

Unsettled weather is expected into at least the evening. The GFS
and ECMWF show the upper level shortwave weakening and lifting
north of the islands, though the state will remain under a
somewhat weak jet stream aloft that will maintain thick high
clouds and chances for spotty heavy showers. For now, the flash
Flood Watch ends at 6 PM, though we will have to assess the need
to keep it going into tonight. Again, the deepest moisture and
potential threat for flash flooding would be across the smaller
islands. A Winter Weather Advisory is also in effect for the high
summits on the Big Island through tonight. Winter weather chances
appear to rather low today, since the deep tropical moisture is
passing to the west and temperatures are relatively warm. Higher
chances for ice and snow are due tonight.

.LONG TERM...FRIDAY THROUGH WEDNESDAY
The upper level disturbance is expected to lift northeast of the
state by Friday, 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...
Clouds and showers are spreading across the islands from the south
as an upper level disturbance moves over the islands from the
west. Thunderstorm activity within the area have decreased since
daybreak, but there remain isolated thunderstorms in the area. The
clouds, showers, and any thunderstorms will produce MVFR and IFR
conditions primarily over the smaller islands today. Conditions
are 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. AIRMET ZULU is in effect for moderate
icing between 140 and FL220 from Kauai to Maui including the
adjacent waters. AIRMET SIERRA is posted for mountain obscuration
for all of the smaller islands, and for the west side of the Big
Island. These AIRMETS are expected to continue through the day
with some improvement expected overnight.

&&

.MARINE...
The current northwest swell will decline today and no other
significant swell is expected through the weekend. A moderate
north swell is possible early next week. Surf is expected to
remain below advisory conditions into early next week.

Winds will remain on the lighter side and generally southeastward.
Small Craft Advisory conditions are not expected for the coastal
waters into early next week. Isolated thunderstorms will produce
lightning and gusty winds within close proximity of storm cells
through this evening, and possibly again Friday night into
Saturday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 6 PM HST this evening for Niihau-Kauai-
Oahu-Molokai-Lanai-Kahoolawe-Maui.

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


&&

$$

Short Term...Wroe
Long Term...Hui
Aviation...M Ballard
Marine...Foster

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office