Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 282236

Area Forecast Discussion
National Weather Service Spokane WA
235 PM PST FRI NOV 28 2014

A showery and breezy weather pattern will continue over far
eastern Washington and the Idaho Panhandle tonight, but snow
levels will remain above pass levels for motorists traveling. A
strong push of cold arctic air will bring very windy conditions
to the Okanogan Valley tonight and Saturday. Cold conditions, with
a few minor snow bearing weather disturbances, are forecast for
the upcoming work week.





A complex weather scenario will unfold during the overnight hours
across the Inland Northwest. The main gist of tonight`s weather
will revolve around the southeast plunge of a strong surface low
currently over extreme southeast British Columbia and the arctic
cold front associated with it.

As of 2pm...the front was located generally north of the
Washington/BC noted by surface observations and isallobaric
analysis...however the one exception was noted over the extreme
northern tip of the Okanogan Valley...mainly the Oroville area.
Based on recent trends...we fully expect the front to continue
dropping southward overnight...with precipitation coverage
steadily increasing ahead of it based on deep frotogenetic
circulations combined with a good pool of available moisture. We
expect most locations too see some precipitation overnight. While
most of it will be fairly light based on a steady frontal
progression...good westerly mid-level flow will add to the
orographic ascent resulting in higher precipitation amounts over
the Idaho Panhandle and near the Cascade Crest. Precipitation
totals could near an inch over these favored locations with much
lighter amounts elsewhere. We expect most of the precipitation to
fall ahead of the cold air which will limit potential snowfall
amounts. Even in the mountains this will be the case...with
current snow levels between 5-6k feet. Near the Cascades...the
snow levels will be 1-2k feet lower so some snow will be possible
there. We will also need to keep our eye out for locally heavy
snow late tonight near the Crest due to the formation of a post-
frontal Puget Sound Convergence Zone. Most model solutions place
this feature across the western Chelan County as opposed to
western Okanogan this could definitely impact travel
later tonight as snow rates could get quite high for a brief time.

The other story for tonight will be the strong south to southwest winds
blowing into the BC low. These winds will likely remain fairly
strong into the evening...and then will begin to lax overnight as
the arctic front begins to pass. Based on guidance from the UW MM5
model...we will continue with the wind advisory for the
southeastern third of Washington and a small part of the southern
Idaho Panhandle. Still looks like wind gusts of 40-45 mph will be
possible through the evening. By midnight...winds should be well
below criteria...however after that...we expect things to pick up
dramatically through the Okanogan Valley as the arctic air rushes
in. We will also issue a wind highlight for this event.

Temperature forecasts for tonight will be quite tough. Typically the model
guidance goes too cool with lows when we are faced with an
incoming arctic front as the winds actually keep temperatures up
to a moderate extent. We will go with that notion and either
forecast using the mildest mos guidance or go slightly warmer than
even that. fx

Saturday through Sunday...The transition from the current wet
maritime air mass to a bone dry arctic air mass will dominate the
weather issues for this period. An upper level trough currently
diving through British Columbia will clip the northern and eastern
reaches of the forecast area over the next 24 hours. This trough
will drag a shot of arctic air in the form of a strong surface
high pressure into the forecast area from the north overnight
tonight and through Saturday. The best conduit for this cold
dense air will be through the narrow and constricted Okanogan
Valley. Starting tonight very windy conditions will develop down
the Okanogan Valley and onto the Waterville Plateau. A Wind
Advisory is well justified for this veritable fire hose of frigid
air howling into the Columbia Basin over the next 24 hours.
Previous case histories...all the available model guidance and
surface observations from upstream Canadian stations suggests only
Wind Advisory criteria (30 to 39 mph sustained with gusts to 57
mph) will be reached...but the magnitude of this arctic push and
the geographic orientation of the gradient is somewhat ominously
unprecedented and Subsequent shifts will need to monitor this
closely for the possibility of upgrading to a High Wind Warning
and/or potentially broadening out the areal extent of the advisory
to include the Moses Lake area.

The arriving upper trough and arctic front will form such a stark
air mass boundary that even though moisture is quickly decreasing
frontal lift may be enough to squeeze out an area of snow showers
or at least flurries over the northern and eastern zones as far
south as the Pullman area during the day Saturday. Minor
accumulations are possible on area roads across the eastern basin
and valleys of the northeast mountains. In the Shoshone County
mountains these new snow showers and residual upslope rain and
snow showers from the current Pacific moisture feed late tonight
and early tomorrow could create some slippery and hazardous
driving conditions across Lookout Pass from rain soaked roads
freezing overnight followed by a dusting to an inch or two of snow
on top...enough to concern holiday travelers and thus warranting
continuing the current Winter Weather Advisory.

Saturday night and Sunday will be a benign but cold period with
decreasing winds and likely considerable sunshine as the arctic
air mass becomes established and the northerly pressure gradient
eases into a more placid high pressure over southern BC. /Fugazzi

Sunday night through Friday..Flattened ridge void of shortwaves
passage leaves a cloudy yet precipitation free Sunday night with
cool northeast winds to work with. Low confidence for Monday and
Monday night with the same model differences continuing. The
latest ECMWF is now closer to solution to the current GFS and a
number of old GFS runs which places a stalled wet frontal zone
much further north. Older ECMWF runs held the stalled frontal zone
further south and away from the forecast area. Current ECMWF
suggests Monday and Monday night placement of the stalled front
just south of a line from Moses Lake to Colfax to Lookout Pass. So
I should just state now with the low confidence in placement of
this stalled frontal zone the Monday and Monday night pops and
associated weather are highly likely to be changed and
concentrated on. On the bright side there still is some
consistency in the eventual appearance of a weakly amplifying
negatively tilted ridge that splits and erodes the stalled frontal
zone allowing for a brief drying and warming trend for Tuesday
into Tuesday night. Wednesday through Friday is another low
confidence interval as even the Consistent ECMWF is now bringing
in a wet low pressure system up from a warm Southwest trajectory
which would start precipitation out as snow in the beginning for
just about all elevations as early as Wednesday but snow level
would rise with temperatures and change it to rain for lowland and
valley locations. Older ECMWF solutions suggested the
Tuesday/Tuesday night ridge would stick around longer and the warm
wet system with the southwest trajectory would be over Washington
Thursday afternoon and beyond. So to summarize...I have more
confidence in the dry Tuesday/Tuesday night interval than I do
with the highly uncertain weather potential contained in the
Monday,Monday night and Wednesday through Friday portion of the
forecast. /Pelatti


18Z TAFS: Primarily VFR conditions are expected at all sites
through at least 00z. Main concern through that time will be with
winds and potential LLWS. The Spokane wind profiler showed winds
about 1500 feet above the ground with speeds nearing 50 kts and
given surface speeds of 20 kts or less will lead to potential LLWS
at least through 20-22z. After that time...the surface winds will
more closely resemble the winds aloft lowering the magnitude of
the shear. Overnight...the main forecast challenges will revolve
around the increasing chances of precipitation. Once the
precipitation arrives after 00z-02z the cigs will gradually lower.
The question and uncertainty is how soon? We forecast a very
short lag...but confidence isn`t high. Conditions should improve
late in the forecast once drier and much colder air moves in. fx

NOTE: The VIS sensor at EAT is out of service so amendments to
VIS are not scheduled. The offending sensor replacement is on
order and may be returned to service by Saturday.


Spokane        29  29  12  24  13  27 /  90  40   0   0   0  10
Coeur d`Alene  32  32  11  25  13  29 / 100  50  10   0   0  10
Pullman        33  33  15  26  17  32 / 100  40  10   0   0  30
Lewiston       40  40  20  29  20  37 /  80  50  20   0   0  30
Colville       22  24   3  20   9  26 /  80  20   0   0   0  10
Sandpoint      24  29   5  21   8  26 / 100  50  10   0   0  10
Kellogg        28  30   4  22  11  28 / 100  70  20   0   0  20
Moses Lake     32  32  12  25  13  27 /  30  10   0   0   0  10
Wenatchee      32  32  14  23  12  27 /  20  10   0   0  10  10
Omak           25  25   2  18   8  24 /  30  10   0   0   0   0


ID...Winter Weather Advisory from 4 AM to 4 PM PST Saturday for
     Central Panhandle Mountains.

     Wind Advisory until Midnight PST tonight for Idaho Palouse-
     Lewiston Area.

WA...Wind Advisory until Midnight PST tonight for Lower Garfield and
     Asotin Counties-Spokane Area-Upper Columbia Basin-
     Washington Palouse.

     Wind Advisory from Midnight tonight to 10 PM PST Saturday for
     Okanogan Valley-Waterville Plateau.


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