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Hawaiian Islands Synoptic Discussion and Guidance

876
FXHW60 PHFO 101347
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
347 AM HST Sat Dec 10 2016

.SYNOPSIS...
A trough of low pressure west of the state will keep conditions
unsettled today, with considerable cloudiness and periodic
showers, particularly across the eastern end of the island chain.
The trough of low pressure will shift slowly eastward tonight
through Monday night, resulting in slowly improving conditions
from west to east across the island chain. Drier weather is then
expected Tuesday through Wednesday as weak high pressure builds
north of the state. This will be short lived however, as
unsettled and potentially very rainy weather may return for the
latter part of the upcoming work week.

&&

.DISCUSSION...
Currently at the surface, a weak trough of low pressure is located
just to the west of the state, with a 1026 mb high centered around
1800 miles east-northeast of the island chain. This is resulting
in a weak wind regime across the area, with boundary layer flow
generally out of the south and southeast. Aloft, a broad upper
level trough is located just to the west of the state, with
shortwave energy evident southwest of the Big Island and also
just to the west of Kauai. Infrared satellite imagery shows mostly
cloudy to overcast skies across the island chain early this
morning, with deep layered jet stream enhanced cloud cover east of
Kauai. Meanwhile, radar imagery shows numerous to widespread
showers over Maui County and the Big Island with lesser activity
across Niihau...Kauai and Oahu. Main short term concern for the
next several days revolves around rain chances.

Tonight,
The surface trough just west of Kauai and the area of high
pressure well east of the state, will keep a light south-
southeasterly boundary layer flow in place through the night.
This will continue to allow deep layer moisture with precipitable
water values between 1.5 and 2.0 inches as seen in MIMIC total PW
imagery to funnel northward over the island chain. The shortwave
disturbances southwest of the Big Island and west of Kauai will
provide the necessary lift for shower development as they progress
northeastward within the deep southwesterly flow aloft. The most
favorable rain chance overnight appear like they will focus over
Maui county and the Big Island as well as over Niihau and Kauai.
Over Oahu rain chances are expected to remain a bit lower due to
the deepest moisture and best forcing for ascent not aligning as
well.

Today through Monday night,
Unsettled conditions are expected to continue across the state as
the synoptic pattern will be slow to evolve. At the surface, a
weak trough of low pressure will remain west of Kauai today, then
slowly shift eastward across the island chain, and finally clear
the Big Island around daybreak Tuesday. Aloft, broad upper level
troughing will remain centered west of the state through the
weekend. As upper ridging builds to the west of the state Monday
and Monday night, the upper level trough will shift eastward
across the island chain, pushing east of the Big Island by
daybreak Tuesday.

As for sensible weather details, with the lingering surface
trough in the vicinity of the islands along with periodic
shortwave disturbances moving over the state in the deep
southwesterly flow aloft, conditions will remain unsettled. As a
result, we will need to keep rain chances in the forecast through
the period. Rainfall chances will be most prominent today, then
slowly decrease through early next week as the deeper moisture and
better forcing for ascent slowly exits to the north and east.
Model relative humidity fields show that the deep layered cloud
cover will begin to shift eastward today, allowing some sunshine
to be seen across Kauai and perhaps Oahu during the afternoon
hours, while mostly cloudy or overcast conditions continue across
Maui County and the Big Island. The deep layered clouds will
continue to shift eastward Saturday night and Sunday, with some
sunshine expected across the entire island chain by Monday. That
said, with the weak pressure pattern in place, daytime sea breezes
will result in quite a bit of cloud cover over the interior of the
islands each afternoon in areas which see the deep layered clouds
erode, but for the sunshine fans, at least it won't be as cloudy
as the past few days have been.

The Winter Weather Advisory has been upgraded to a Winter Storm
Warning for the Big Island summits above 12 kft, with the warning
in effect through midnight tonight. Reports from the summits
late last evening indicated that 2 to 3 inches of new snow had
fallen since yesterday afternoon. Occasional light to moderate
snow likely has continued through the night and is expected to
continue today and into the evening hours tonight. New snow
accumulations of 6 to 12 inches are expected at the summits
through midnight tonight.

Tuesday through next Friday,
Model solutions are in good agreement showing improving conditions
Tuesday through Wednesday as weak high pressure builds north of
the island chain. As a result, we expect rain chances to decrease
substantially, and more typical Hawaiian sunshine to return. It
appears that this improvement will be short lived however, as the
00Z GFS and 00Z ECMWF, along with support from the GFS/ECMWF
ensemble members, show a deep stacked low developing just to the
west of the island chain Wednesday night and moving little through
the end of the upcoming work week. Although model solutions differ
on the exact details at this time, they both indicate that we may
move back into a very unsettled rainy pattern for the latter half
of next week.


&&

.AVIATION...
A band of showers is beginning to fire up through the Kaiwi
Channel early this morning, with cloud tops near FL250. Scattered
showers will continue to pass over Maui County islands and the Big
Island from the south today. Expect widespread mid to high level
clouds to stay over all the islands. Light winds will help promote
the sea breezes this afternoon with clouds and showers favoring
the central and interior areas of the island, especially on Kauai
and Oahu. The Big Island will see stratiform rain with pockets of
heavy downpours.

AIRMET SIERRA for mountain obscuration is posted for the Big
Island and Maui. This will be tailored as needed. AIRMET TANGO for
upper level turbulence is posted for all islands between
FL210-FL400. AIRMET ZULU is posted for light to moderated icing
possible in cloud between 140-FL410.

&&

.MARINE...
Light winds will persist through the weekend and into early next
week as a weak surface trough moves over the waters from west to
east. Some drier air will fill in across the waters early next week
as high pressure builds over the state. This will be short-lived due
to another low pressure that is forecast to move back into the area
from the northwest Wednesday through Thursday.

The latest surface analysis showed a small gale centered around 800
nm north of Oahu that was nearly stationary. The overnight ASCAT
pass showed a decent batch of 30 to 40 kt northerly winds associated
with it, favorable for Hawaii. The latest model guidance has
initialized well with this feature and generally depicts it
remaining stationary through the rest of the weekend before drifting
toward the south early next week and weakening. Wave models support
a short-period (10-13 sec) swell out of the north continuing into
the upcoming week. This northerly component will likely be the
largest between Oahu and Kauai due to the position of this feature
relative to the islands. This source should correlate to small to
moderate surf along the northern exposures through this time.

A new long-period west-northwest (310 deg) swell associated with a
gale currently approaching the Aleutians near the date line 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 west and northwest facing shores.

Models remain in good agreement and depict another gale developing
northeast of Japan Sunday and tracking toward the Aleutian Islands
early next week. A new long-period west-northwest swell will likely
fill in from this source Thursday and hold into the weekend before
trending down. Although confidence remains low this far out in the
forecast, advisory-level surf along north and west facing shores
will be possible by Friday. Plenty of uncertainty, however, is
introduced to the forecast through this period as the models depict
a strong low developing nearby or to the northwest of the marine
areas. If this verifies, moderate to strong 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.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until midnight HST tonight for Big Island
Summits.

&&

$$

DISCUSSION...Jelsema
AVIATION...Eaton
MARINE...Gibbs

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