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Hawaiian Islands Synoptic Discussion and Guidance

246
FXHW60 PHFO 172004
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1004 AM HST Sun Dec 17 2017

.SYNOPSIS...
A broad, weak surface trough near Kauai will keep winds light over
the state through Monday. Clouds and showers will favor the
afternoon and evening hours for most areas. Showers will increase
near the Big Island on Monday as a moist airmass slowly pushes
northwest up the chain. A strong trough and cold front will bring
the possibility for heavy showers or thunderstorms to parts of the
state Monday night through Wednesday. Brisk, cool northerlies will
follow the front with fewer showers. A reinforcing cold front is
expected to push down the chain on Saturday.

&&

.DISCUSSION...
Our winds are light and variable this morning due to a surface low
just to the N of the islands. A band of remnant moisture, lying
along a weak surface trough, extends across the Kauai channel,
with broken to overcast clouds and showers associated with it. The
MIMIC-TPW product from UW-CIMSS shows a mass of much deeper
moisture lurking just to the SE of the Big Island.

The surface low will continue to weaken as the upper level support
pushes off to the NE. The surface trough and associated moisture
will push W toward Kauai, bringing increased clouds and showers
there. The remaining islands will generally see a convective
pattern with considerable afternoon cloudiness, though weak mid-
level ridging building over the state should limit afternoon
showers. Land breezes tonight will allow for partial clearing in
most areas. Our weather will be similar for Monday, but deeper
moisture pushing NW up the chain will lead to additional afternoon
showers over Maui county and the Big Island. With PWs over 1.5",
some of these could briefly be heavy.

A strong, digging trough aloft, with very cold mid-level
temperatures, will push S toward the islands Monday night and
Tuesday. This will reinvigorate the old surface trough over the
islands (expected to be in the vicinity of Oahu at that time).
Numerous showers and thunderstorms will develop along and S of
the surface trough in the deeper moisture, with fewer showers W
of the boundary. The southern main islands will also be in the
right entrance region of an upper level jet streak. Localized
excessive rainfall, wintry and windy weather at the summits, and a
few strong to severe thunderstorms all seem to be possible if the
ingredients line up as some of the models are suggesting.

A cold front will push slowly down the chain on Tuesday and
Wednesday, with considerable support from the mid- and upper-
level trough. This will keep the chance for heavy rain and
thunderstorms going the longest for the Big Island. Behind the
front, breezy northerlies will bring another shot of chilly air
to the state. Steep low- to mid-level lapse rates and cold mid-
level temperatures will allow for scattered showers in the
unstable air behind the front, even though the airmass will be
relatively dry by Hawaii standards.

The trough aloft will shift E of the islands by Thu night, with
the airmass stabilizing. A ridge of high pressure following the
cold front will also drop S over the islands, weakening our winds
back to more gentle speeds and giving us quiet, cool weather. The
models agree on a reinforcing cold front reaching Kauai Friday
night with locally windy northerlies returning. This front will
not have nearly as much moisture, and weak upper support, so
likely only a band of clouds and scattered showers as it passes.
The front should clear the islands on Saturday with trade winds
finally returning.

&&

.AVIATION...
Mostly VFR conditions will prevail across the state with
localized MVFR CIG and VIS in SHRA, mainly tonight and Monday.
Wind patterns at area terminals will be locally influenced by sea
breezes as light southerly flow prevails.

No AIRMETs are currently posted and none forecast.

&&

.MARINE...
Light winds will dominate the area into the upcoming week as a
trough of low pressure continues to impact the local weather.
Moderate to fresh northerly winds are forecast to fill in around
midweek as another cold front moves down the island chain,
maintaining those levels through the second half of the week.

A new, long-period, west-northwest (300-320 deg) swell will fill
in today. Surf heights will steadily rise along exposed north and
west facing shores, likely peaking below advisory levels later
this afternoon into tonight, and holding into Monday.

Guidance depicts gale winds developing around 1000 nm north of
the islands Tuesday through midweek and sending a shorter-period
(11-12 sec) north-northwest (350-010 deg) swell toward the state.
This swell could result in surf reaching advisory levels along
north facing shores.

A long-period northwest (320-330 deg) swell is expected to fill
in Friday, but surf heights should remain well below advisory
levels.

For the extended, guidance remains in decent agreement for a
north-northwest swell that could bring advisory-level surf for
north facing shores next weekend.

See the Collaborative Nearshore Swell and Wind Forecast for
details on expected swell sources and resulting surf.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until noon HST today for Big Island Summits.

&&

$$
R Ballard/Brenchley/TS

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