Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 110644
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
844 PM HST Sat Dec 10 2016

.SYNOPSIS...
An upper level trough near the western end of the state will bring
overcast skies and periods of rain mainly from Molokai to the Big
Island through early Sunday morning. This band of clouds and
showers will gradually shift eastward through Monday, resulting in
slowly improving conditions from west to east across the island
chain. Drier weather is then expected from Tuesday through
Wednesday as weak high pressure builds north of the state.
However, this will be short-lived, as an extended period of
unsettled and rainy weather may develop across the islands
starting Thursday.

&&

.DISCUSSION...
A couple of weak surface lows are located northwest and north of
the islands. These systems are linked together by a slow moving,
or nearly stationary, front. Elsewhere, a 1025 mb high is near
28N 129W...or about 1760 miles east-northeast of Hilo. A weak
surface ridge extends from this high to near the Big island. The
close proximity of all of these features to the islands is
maintaining a light synoptic flow across the region. As a result,
gentle south winds continue near the Big Island, while the flow
is mainly light and variable elsewhere.

Loops of water vapor satellite imagery show an upper level trough
near the western end of the island chain early this evening. This
feature, which appears to be propagating slowly toward the east-
northeast, is contributing to the unstable atmospheric conditions
across the state. This instability combined with abundant moisture
in the vicinity of Maui County and the Big Island are producing a
band of overcast layered cloud with embedded numerous showers from
just east of Oahu to the Big Island. The estimated highest tops
of the clouds within the band extend up to over 40 thousand feet.
Radar reflectivity shows mostly stratiform precipitation
concentrated across Maui County, as well as the northern and
northwestern Big Island. Cirrus clouds and scattered to broken low
clouds with a few light showers along the western edge of the
layered cloud band are passing over Oahu. Scattered to broken low
clouds and scattered showers are moving over Kauai and the
adjacent coastal waters.

Soundings from early this afternoon indicated precipitable water
values of 1.41 inches at Lihue and 1.93 inches at Hilo, with the
latter showing a saturated vertical profile. Also, temperatures
on the Big Island summits remain just below freezing this
evening with layered clouds deep enough to support snowfall
tonight. A Winter Storm Warning remains in effect through early
Sunday morning. Snow may be possible from Sunday into Monday
morning for the Big Island summits, but we are anticipating
shallow clouds will eventually move in from the south sometime
after daybreak Sunday morning. Therefore, depending on when the
deep layered clouds begin to depart the area, we will determine if
the Warning needs to be extended beyond 6 am HST Sunday morning,
or downgrade it to a Winter Weather Advisory.

The upper level trough will lift continue to propagate toward the
east-northeast, which will eventually push the layered cloud band
slowly eastward through Sunday night. The surface lows will also
lift northeast, which will allow a weak ridge to build in from the
north and bring gentle north winds across the western portion of
the state on Sunday. Somewhat cooler and drier air will likely
accompany this flow regime. The Big Island, and to a lesser
extent Maui County, will still have some lingering clouds and
showers on Sunday. The Big Island will likely continue to have
wetter than normal conditions through Monday.

Model solutions remain in good agreement showing improving
conditions from Tuesday through Wednesday as weak high pressure
builds north of the island chain, bringing gentle trade winds for
all islands. However, it appears that this improvement will be
short-lived, since the GFS and ECMWF models, as well as many of
the the GFS / ECMWF ensemble members, show a deep stacked low
developing just to the northwest or west of the island chain late
Wednesday. This feature appears to deepen and meander west or
northwest of Kauai from Thursday into next weekend. Although model
solutions differ on the exact details about the low and its impact
on our weather at this time, they do indicate that an extended
period of very unsettled wet weather will likely develop starting
Thursday.

&&

.AVIATION...
Layered clouds continue to spread across eastern Oahu through the
Big Island, producing light to moderate rain across most of this
area. Terminals from PHMK and eastward will likely see periods of
tempo MVFR through the night. AIRMET SIERRA for mountain
obscuration is posted over eastern Oahu and all of Maui County and
the Big Island. AIRMET ZULU for light to moderate icing in layered
clouds is posted from eastern Oahu through Big Island. The rest of
Oahu will see isolated showers passing from the south. Kauai will
see scattered showers, at times heavy, through the night.

An upper jet on the east side of an upper trough over Kauai will
continue to produce tempo moderate upper level turb, AIRMET TANGO
is posted. As the trough shifts east through Sunday, so will this
area of upper turbulence.

&&

.MARINE...
Winds and seas will likely remain below the Small Craft Advisory
criteria through Wednesday. However, a new surface low moving to a
position northwest or west of Kauai on Thursday may cause
increasing south to southwest winds and seas across parts of the
state. Mariners who are planning activities later this week should
monitor future updates to the wind / seas forecasts. In addition,
periods of heavy rain may cause reduced visibilities over portions
of the coastal waters starting Thursday.

A short-period north to north-northeast swell will linger mainly
over the western half of the state through Sunday. A longer-period
northwest swell associated with a former gale near the International
Date Line is expected to fill in from Sunday into Monday, then
peak Monday night, before slowly fading from Tuesday through
Wednesday.

A longer fetch developing between a building surface high far
north of the state and an elongated trough/front extending
northeast from the state will generate a shorter-period north-
northeast swell beginning late Monday/Tuesday. This swell will
gradually veer around to the east by next weekend. Surf is
forecast to remain below High Surf Advisory criteria along east
facing shores. However, there may be a concern with locally
enhanced wave runup due to the larger spring tides and the fact
that water levels at the tide gages around the state have recently
been higher than predicted.

Forecast models remain in good agreement that another gale low
developing northeast of Japan on Sunday will track toward the
Aleutian Islands early next week. A new northwest swell will
likely fill in from this source Thursday, and hold into the
weekend before trending down. Although confidence remains low,
surf may approach the High Surf Advisory criteria along north and
west facing shores of some of the smaller islands on Friday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 6 AM HST Sunday for Big Island
Summits.

&&

$$

DISCUSSION/MARINE...Houston
AVIATION...Eaton



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