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Area forecast discussion 
National Weather Service Spokane Washington
353 PM PST Thursday Dec 25 2014

Synopsis...
relatively quiet weather will persist into Friday following a
slight chance of snow in the Idaho Panhandle tonight. Confidence
is increasing that significant snow accumulations will occur
Saturday into Sunday over the Cascades, Idaho Panhandle, and the
eastern third of Washington. Much colder conditions will likely
arrive Monday and Tuesday.

&&

Discussion...

Tonight and Fri: a very weak short-wave trough will move southeast
across ern WA and north Idaho tonight... leading to light amnts of
stratiform pcpn for mainly the Idaho Panhandle, and the associated
cloud cover leading to a difficult low temp fcst. This transient
wave in northwest steering flow is far from impressive, with much of its
large-scale forcing for ascent from warm advection/shallow
isentropic ascent. This forcing combed with orographic northwest flow
should give the cntrl Idaho Panhandle mtns zone (shoshone county) the
highest chance of pcpn. Only minor snow amnts are expected. Some
of the pcpn may even be in the form of snow grains generated from
an extensive low cloud deck beneath a marginally dry dendritic
layer aloft. Low temps have been warmed due to cloud cover.Bz

..winter storm to bring heavy snow to portions of eastern
Washington and much of the Idaho Panhandle this weekend...

Friday night through Sunday night: a dynamic upper level low
pressure system will drop down out of British Columbia over this weekend. Good
upper level dynamics via a 125 kt jet streak and a strong
vorticity maximum at the base of the upper level trough will spin
up an area of low pressure down the British Columbia coastline. There is still
some uncertainty with where this low will track through Saturday
night. Models have the low tracking across anywhere from between
Pullman to the Canadian border. The favorable area for lift with
the surface low will be across its eastern and northern periphery
where warm and moist air will rise and wrap around the low
pressure circulation. This gives extreme eastern Washington and the Idaho
Panhandle the best shot at precip. A tightening pressure gradient
will also result in some breezy winds, especially south of the
surface low.

* Snow levels: snow is expected for the duration of this storm
system across most valley locations. The areas where snow
levels will be above valley floors will be across the southern
portion of the upper Columbia Basin and over into the l-c
valley; however, these areas will also transition over to snow
as colder air filters in from the north Saturday night.

* Snow amounts: heaviest snow accumulations are expected in the Idaho
Panhandle, especially in the central Panhandle mtns. These areas
are expected to see storm total accumulations of around 4-10
inches. The lesser amounts are more likely further north, in
the Coeur D'Alene area and on the Idaho Palouse. The higher amounts
are anticipated in the central Panhandle mtns where totals will
be closer to a foot across the higher elevations.

* Winds: will increase on Saturday as the low approaches and are
expected to be breezy by the evening into Saturday night.
Strongest winds will be over the higher terrain, especially
across the southeast portion of the forecast area with gusts
between 30-40 mph possible. We are also expecting gusts between
15-25 mph across the basin into the Spokane area, on the Palouse
and over the Camas Prairie.

* Hazards: confidence is high that heavy snow along with some
blowing snow will create treacherous travel conditions across
the Idaho Panhandle. This is especially a concern at this time of
the year during busy Holiday travel season. A Winter Storm Watch
is being issued to highlight these areas. Confidence is not as
high across eastern Washington as this system looks to be a bit more
progressive than previously thought. We are not as confident
that the northeast mountains, Spokane area, Washington Palouse and
northeast blue mtns will achieve warning criteria of 4 to 6
inches. There is a good chance that these areas will need to be
under an Snow Advisory as this event approaches. Messy travel is
also expected over the mountain passes, which includes Stevens
Pass, 4th of July pass and Lookout Pass.

* Temperatures: colder temps are anticipated through this weekend.
Overnight lows are expected to be quite chilly Sunday night,
especially across extreme eastern Washington and in the Idaho Panhandle
with fresh snow on the ground. /Svh

Monday through Thursday...somewhat positively tilted flop-over
ridge with eastern Washington and northern Idaho a bit on the east
side of its axis keeps a dry cold northerly flow going through
most of this forecast interval seemingly void of shortwaves. With
the expectation that there will be snow-cover on the ground from
storms expected to pass through this weekend (with Saturday being
the best day to bring it) both radiational cooling with snow-cover
and advective cooling from the cold northerly flow will keep
temperatures very cold. /Pelatti

&&

Aviation...
00z tafs: expect increasing clouds overnight from a quick-moving
wave that will likely produce mid-cloud ceilings 6-8k ft agl east
of a line from komk to kalw. Only light amnts of snow are possible
from this VFR cloud deck...limited to kcoe. The biggest challenge
will still be how far the vsbys and ground-based stratus/fog will
fall. MVFR seems likely for kpuw...even this evening. But once
the aforementioned wave passes and skies clear aloft, this should
be the time (around 09z-12z) after which dense fog expands and
produce IFR/LIFR conditions. The highest threat will be for the
Spokane area and the Palouse.Bz

&&

Preliminary point temps/pops...
Spokane 26 35 25 32 25 30 / 10 0 20 90 90 40
Coeur D'Alene 27 34 24 32 26 29 / 20 0 20 90 100 60
Pullman 29 35 25 33 28 30 / 20 10 10 90 100 70
Lewiston 34 39 26 37 30 36 / 20 10 10 50 90 70
Colville 25 34 23 33 23 31 / 0 0 20 90 80 40
Sandpoint 26 33 24 32 24 28 / 20 10 30 100 100 60
Kellogg 28 34 23 29 25 28 / 50 20 20 100 100 80
Moses Lake 27 39 24 36 27 36 / 0 0 10 20 30 20
Wenatchee 29 38 25 38 26 36 / 0 0 10 20 30 10
Omak 20 36 22 32 22 29 / 0 0 10 20 20 30

&&

Otx watches/warnings/advisories...
Idaho...Winter Storm Watch from late Friday night through Sunday morning
for central Panhandle mountains-Coeur D'Alene area-Idaho
Palouse-northern Panhandle.

Washington...none.
&&

$$

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