Drought in Hawaii, December 1 to 5, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
858 PM HST Thu Nov 30 2017

.SYNOPSIS...
High pressure north of the state will keep a moderate to breezy
trade wind flow in place through the weekend. Unsettled weather
will continue through at least Friday due to an upper trough to
our west and moist air over the island chain. Conditions are
expected to gradually improve this weekend. The trade winds will
ease on Monday as a cold front approaches from the northwest. The
cold front along with a band of light showers is expected to move
southward through the island chain Monday night and Tuesday
followed by strong north to northeast winds. Cooler and drier
conditions are expected by midweek.

&&

.DISCUSSION...
Currently at the surface, a 1033 mb high is centered around 1150
miles north of Kauai, driving moderate trade winds across the
island chain this evening. Aloft, a southwest to northeast
oriented upper level trough is situated just to the north and west
of the state. Infrared satellite imagery shows mostly cloudy skies
across the island chain, with radar imagery showing a large area
of showers and thunderstorms just to the south of Kauai and Oahu,
with another batch of showers moving into windward sections of
Kauai. Some light showers are also moving into windward sections
of the smaller islands, with very little leeward shower activity
at this time. Main short term concern revolves around rain chances
over the next few days.

Tonight through Friday night,
High pressure north of the state will shift slowly eastward
through the period, resulting in a gradual strengthening of the
trade winds. Moderate trades will prevail tonight, then
strengthen to breezy levels Friday and Friday night. At the same
time, the trade winds will shift around from an east and east-
southeasterly direction this evening to a more typical east-
northeasterly direction by Friday and Friday night. Aloft, the
upper level troughing to the west and northwest of the state is
forecast to edge closer to Kauai through the period, and this will
keep conditions somewhat unstable across the island chain.

Model solutions have been persistent in showing shower activity
expanding across the state, particularly the western islands and
windward sections of the eastern islands tonight. With the
convergence associated with the strengthening and shifting
trades, the upper troughing encroaching on Kauai, and deep
tropical moisture with precipitable water values between 1.7 and
2.0 inches, increasing shower activity seems reasonable overnight
statewide. The areas that are expected to see the most notable
increase in shower activity are expected to be Kauai, as well as
windward sections of Oahu, Molokai and Maui. The Big Island
continues to see showers roll into windward areas and this should
continue through the night as well.

Deep tropical moisture will remain over the island chain on
Friday, with the deepest moisture then beginning to shift
southwestward and away from the islands Friday night, with PW
values dropping to around 1.5 inches or less by daybreak Saturday.
With the upper level troughing continuing to slowly encroach on
the western islands, conditions will remain somewhat unstable, so
we expect a wet trade wind pattern to continue. Showers will
continue to focus primarily over windward and mauka areas, with
some of the shower activity reaching leeward areas as well.

A Flash Flood Watch remains in effect for the Big Island, Maui and
Kauai through 6 PM Friday. A Winter Weather Advisory remains in
effect for the Big Island Summits through 6 AM Friday.

Saturday through next Thursday,
High pressure north of the area will keep moderate to breezy
trade winds in place through the weekend, with the trades easing
on Sunday night and Monday as a cold front approaches from the
northwest. The cold front will then shift southward through the
island chain Monday night and Tuesday, with strong north-
northeast winds building in across the state following the frontal
passage. The strong winds are expected to be brief however, as
high pressure quickly settles southward and closer to the
islands, with winds diminishing to light levels by the middle and
latter part of next week.

As for sensible weather details, weather conditions should be
improving by Saturday as PW values decrease closer to normal
levels across the state. That said, with the upper level troughing
swinging southward across the state Saturday and Saturday night,
a bit wetter than normal trade wind weather will likely continue,
particularly over windward and mauka areas. Much nicer weather is
then expected for Sunday and Monday as PW values drop into the 0.8
to 1.0 inch range. Showers both Sunday and Monday, should be
confined primarily to windward and mauka areas.

A band of showers is then expected to accompany the strong cold
front as it shifts southward through the area Monday night and
Tuesday, with the showers mainly affecting north and northeast
facing slopes. PW values are then expected to drop into the 0.6 to
1.0 inch range Tuesday night through Thursday, with very little
if any shower activity expected statewide. The weather looks
nearly perfect during the middle and latter part of next week,
with plenty of sunshine and cool and dry conditions expected
across the entire island chain.

&&

.AVIATION...
AIRMET SIERRA continues for mountain obscuration along the east
sides of most islands due to clouds and showers. Expect these
conditions to continue throughout the night.

Locally strong trade winds continue to produce turbulence to the
lee of the mountains on all islands. As such AIRMET TANGO
continues for mechanical turbulence south through west of
mountains statewide below 8000 feet. This is also expected to
continue overnight.

While VFR will dominate at the terminals, MVFR ceilings and
visibilities are possible in clouds and showers, though those
conditions will be focused over the windward sides of the islands.

There is a slight chance of thunderstorms overnight, mostly over
Kauai, windward areas of Maui and the Big Island, and to the
waters to the southwest of the islands. IFR conditions are
possible in and around any thunderstorms that form.

&&

.MARINE...
Rough boating conditions will continue into the weekend due to a
combination of fresh to strong trades and a large north-northwest
swell. The current small craft advisory in place through Friday
afternoon for all waters will be extended through Saturday in the
early Friday morning package. Model guidance suggests the local
winds will temporarily relax and shift out of a more northerly
direction Sunday night through Monday as a front approaches and
moves into the area from the northwest. Strong northerly winds will
fill in across the waters in the wake of this boundary Monday night
into Tuesday, then relax through midweek as a ridge builds over the
islands.

The current north-northwest (330 deg) swell has peaked at the buoys,
but will likely hold through Friday before slowly trending down over
the weekend. The high surf advisory for exposed north and west
facing shores will remain in place through Friday, before dropping
below advisory levels through the weekend.

The latest surface analysis showed a storm-force low (962 mb)
entering the far western portion of the Bering Sea. Satellite data
showed gale- to storm-force west-northwest winds to its south over
the far northwest Pacific extending east from the Kuril Islands.
Altimetry data near the head of this fetch region showed 25 to 30 ft
seas. Wave model guidance supports long-period forerunners
associated with this source filling in across the local waters
through the day Sunday and Sunday night out of the northwest (320
deg), then building into the upcoming week. This source will become
reinforced Tuesday through midweek as a gale- to storm-force low
develops and lifts northeast across the central Pacific near the
Date Line over the weekend and strong northerly winds setup north
of the islands behind the aforementioned front expected.

Surf along exposed north and west facing shores associated with this
active pattern could near advisory levels again Monday through
Monday night, then potentially reach warning levels Tuesday night
through midweek as the reinforcing north-northwest energy fills
in.

Surf along east facing shores will remain rough into the weekend due
to the strong trades expected. Additionally, a reinforcing northeast
(050 deg) swell associated with low pressure developing several
hundred nautical miles northeast of the state over the weekend
will likely keep the surf elevated along east facing shores early
next week as the winds shift around out of the north.

Surf along south facing shores will likely come up slightly over the
weekend due to low pressure passing south-southeast of New Zealand
last weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Friday afternoon for Kauai-Maui
Windward West-Maui Leeward West-Maui Central Valley-Windward
Haleakala-Leeward Haleakala-Kona-South Big Island-Big Island
North and East-Kohala-Big Island Interior.

High Surf Advisory until 6 PM HST Friday for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Olomana-
Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala-South Big Island-Big Island North and East.

High Surf Advisory until 6 AM HST Friday for Waianae Coast.

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

Small Craft Advisory until 6 PM HST Friday for all Hawaiian
waters-

&&

DISCUSSION...Jelsema
AVIATION...Morrison
MARINE...Gibbs