Heavy Rain in Hawaii, December 17 to 18, 2016

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
1018 AM HST Sat Dec 17 2016

.SYNOPSIS...
A low pressure system north of Kauai will gradually move toward
the west southwest over the next few days. Passing showers can be
expected at times and there is a slight chance of thunderstorms as
well. The low will weaken and move away from the area on Monday.
High pressure will gradually build in from the east beginning on
Monday with a trade wind weather pattern setting up for the
remainder of next week.

&&

.DISCUSSION...
Satellite imagery shows an area of low pressure is centered about
200 miles to the north of Kauai. Bands of showers are seen
rotating around the low providing for some passing showers at
times across the area. A broad swath of multi layered clouds is
also seen streaming northward with the western edge just east of
The Big Island. Instability associated with the low will allow for
a slight chance of thunderstorms today through Sunday. Widespread
rainfall is not expected but locally heavy showers are possible
mainly in thunderstorms that may pop up.

Strong winds as well as a wintery mix of weather can be expected
over The Big Island Summits. A High Wind Warning is currently
posted for the summits. These strong winds are expected to
continue through Sunday before diminishing early next week.

Winds across the remainder of the area will be blowing from the
south and southwest today but gradually shift toward the south
and southeast tonight and Sunday.

By Monday, the low will continue to move west away from the area
and high pressure will begin to build in from the northeast. This
will reduce the risk for thunderstorms as well as the threat of
locally heavy rainfall.

High pressure will begin to establish itself northeast of the
area. A trade wind weather pattern will build in from the east
beginning Monday with a trade wind weather pattern returning.
Moderate trade winds will then persist for several day next week.
Clouds and passing showers will favor windward and mauka areas. A
front will begin to approach the area around Friday and will
likely push the high associated ridge axis southward causing a
bit of a drop off in trade wind speeds.

&&

.AVIATION...
A low centered about 200 nm north of Kauai will move slowly SSW
this weekend. This low will make the atmosphere unstable today, so
enhanced cumulus clouds and showers, now over the smaller islands
will move southeast reaching the Big Island later today. There is
a slight chance of thunderstorms developing over portions of the
state today. At this time, we do not have MTN OBSC in the
forecast, but AIRMET SIERRA could be needed later today.

A band of layered clouds with +SHRA and isolated TSTMS will linger
east of the Big Island early this morning. This band could affect
the Big Island later today.

AIRMET TANGO remains in effect for moderate upper level
turbulence over all Hawaiian Islands generated by a trough aloft.
AIRMET TANGO is also in effect for tempo moderate low- to mid-level
turbulence northeast of the Big Island summits.

&&

.MARINE...
A dynamic wind, wave and weather regime will prevail over island
waters the next couple of days, as a Kona low now centered about
200 nm N of Kauai moves steadily SW and weakens. When it was
stronger, the low generated a large and choppy NNW swell that is
now gradually diminishing, but with strong N winds continuing on
the W side of the low, surf remains especially elevated in Kauai
county, where an ongoing High Surf Warning was extended through
the day. Elsewhere, the Warning was downgraded to a High Surf
Advisory.

Surf generated by the Kona low will continue to diminish through
the weekend, but a long-period NW swell is expected to arrive
overnight and Sunday, with current expectations for the swell to
produce surf below advisory levels. However, if the swell were to
arrive just a little larger than expected, surf could remain at advisory
levels along exposed N and W facing shores through Sunday.

A Small Craft Advisory (SCA) is in effect for almost all Hawaiian
waters due to elevated seas, but with seas decreasing in the short
term, some zones will be removed from the SCA as early as tonight,
with additional zones removed on Sunday. Seas should drop below
the advisory level (10 ft) Monday over all waters, but are
expected to build once again Tuesday through the mid-week period
as fresh to strong trade winds return and another NNW swell fills
in - potentially producing advisory-level surf along N facing
shores Tuesday and Wednesday.

Local winds will steadily shift from SW to S today through tonight
as the low moves NW of Kauai...with SE winds Sunday and Monday.
Increasing trade winds Tuesday and Wednesday will lead to SCA-
level winds in the areas around Maui and the Big Island where the
flow is most accelerated. These winds may bring high surf to E
facing shores later in the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for Waianae Coast-
Oahu North Shore-Oahu Koolau-Molokai-Maui Windward West-Maui
Central Valley-Windward Haleakala-Big Island North and East.

High Surf Warning until 6 PM HST this evening for Niihau-Kauai
Windward-Kauai Leeward.

High Wind Warning until 6 PM HST this evening for Big Island
Summits.

Small Craft Advisory until 6 AM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Pailolo
Channel-Alenuihaha Channel-Big Island Windward Waters.

Small Craft Advisory until 6 PM HST this evening for Big Island
Southeast Waters.

&&

Synopsis/Discussion...Burke
Aviation...Houston
Marine...Birchard