Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 111430

Area Forecast Discussion
National Weather Service Honolulu HI
430 AM HST Sun Dec 11 2016

An upper level trough near the western end of the state will bring
partly to mostly cloudy skies and showers mainly to the Big
Island today. The clouds and showers associated with this trough
will gradually shift eastward through Monday, resulting in
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.


A couple of weak surface lows are located north of the islands.
These systems are linked together by a slow moving, or nearly
stationary, front. Elsewhere, a 1024 mb high is near 28N 129W...or
about 1720 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 morning. This
feature, which continues to propagate 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 the Big Island had been producing a band of
overcast layered cloud with embedded numerous showers earlier this
morning. Most of these layered clouds and showers are now pushing
east of the Big Island.

Soundings from early this morning indicated precipitable water
values of 1.47 inches at Lihue and 1.85 inches at Hilo, with the
latter showing a nearly saturated vertical profile. Also,
temperatures on the Big Island summits remain just below freezing
this morning with freezing drizzle and fog occurring. A Winter
Weather Advisory has been issued through this morning for the
Big Island summits. Also, a wind advisory has been issued for
strong southwest winds on the summits. This is currently in effect
through this afternoon, but may need to be extended into tonight
if winds remain stronger than expected.

The upper level trough will continue to propagate toward the
east-northeast, while the surface lows will also lift northeastward.
This will allow a weak ridge to build in from the north and bring
gentle north winds across the western portion of the state later
tonight. 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. The Big Island
will likely continue to have wetter than normal conditions through

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, 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, with locally heavy rainfall,
will likely develop starting Thursday.


AIRMET ZULU for light to moderate icing has been cancelled as
most of the thick layered clouds have moved east and northeast of
the area. An upper level trough will continue to move east today.
AIRMET TANGO for tempo moderate turbulence between FL280/FL400
remains posted, but will likely be dropped by early this
afternoon as the trough exits the area. AIRMET SIERRA for mountain
obscuration remains posted over the eastern islands from Molokai
through Big Island early this morning. This will be trimmed as
conditions continue to improve.

Otherwise, the widespread stratiform rain has eased in the past
couple of hours. A series of surface disturbances north-northwest
of state will keep winds light, allowing daytime sea breezes to
form. Development of interior clouds and showers along sea
breezes this afternoon will be likely, as ample moisture remains
in the boundary layer with little to no inversion.


The latest observations at the nearshore CDIP buoys north of the
islands continue to reflect the primary swell energy coming in out
of the north within the 10-12 second band. This persistent northerly
component was generated from a compact gale located around 800 nm
north of Oahu over the past few days. Although the overnight ASCAT
pass just missed this low to the east, strong to near gale force
east-northeast winds were shown to its northeast, which were
slightly weaker than 24 hours ago. This lines up well with the
latest model guidance that depicts this feature continuing to weaken
as it drifts toward the south through tonight. The GFS, ECMWF and
the ensemble guidance, however, all depict a new gale developing in
this same general area Monday through Tuesday in response to a
shortwave trough lifting northeastward. This will translate to a
reinforcing swell out of the north-northeast (010 deg) Tuesday
through the mid-week period locally. Surf will respond and build
through the day Tuesday and may near advisory levels along the
eastern exposures, which will need to be monitored. This source
should hold into Thursday before easing Thursday night and Friday.

A new long-period west-northwest (310 deg) swell associated with a
gale that had developed across the northwest Pacific is forecast to
fill in Monday, peak Monday night, then fade Tuesday through
Wednesday. This source will result in small to moderate surf along
the exposed north and west facing shores.

The latest surface analysis showed a storm-force low centered near
the Kuril Islands. A recent ASCAT pass clipped this area and showed
a decent sized batch of gale to storm force winds favorable for
Hawaii located to its south. A recent altimeter pass over this area
showed seas ranging from 20 to 25 ft, which lined up well with the
latest WAVEWATCH III guidance in this area. A new long-period west-
northwest swell will likely fill in from this source locally
Thursday, peak through the day Friday before trending down over the
weekend. Surf will likely reach advisory levels along north and west
facing shores through this time Friday.

In addition to moderate to large west-northwest swell peaking
through the day Friday, models remain in decent agreement and depict
a strong low developing nearby or just west of the islands. If this
verifies, moderate to fresh southerly winds will become a
possibility through this period across the local waters. A
combination of local winds and this new long-period swell will
likely translate to Small Craft Advisory conditions beginning as
early as Friday, especially for the northwest waters.


Winter Weather Advisory until noon HST today for the Big Island

Wind Advisory until 6 PM HST this evening for the Big Island



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