Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 081215
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
415 AM PST Thu Dec 8 2016

.SYNOPSIS...
Dry and cool weather will linger through the day. Then widespread
snow is expected this evening likely producing light to moderate
valley accumulations and heavy mountain accumulations through
Friday. This snowy pattern will is expected to continue through
early next week. Another shot of much colder air is likely by
Tuesday with lows in the single digits.


&&

.DISCUSSION...
Today...Northwest flow is over the region early this morning as
weak high pressure builds into the region. This will be the last
totally dry day for a while. Some mid-level stratus across the
northwest zones will clear out before sunrise but will be replaced
by high clouds moving into the region ahead of the next weather
system. A cold air mass is currently in place and temperatures are
only expected to rebound into the mid 20s this afternoon. Winds
down the Okanogan valley and into the basin will remain elevated
and will make temperatures feel 5-10 degrees cooler.

Tonight through Friday night...The well advertised winter storm
will then become the weather producer with active weather
expected through the weekend. Model guidance has been trending
slower and weaker with this storm. And it now looks like the main
energy and deeper moisture will stay to the south of our area. In
addition it looks like the strong warm front is weakening and will
likely get sheared apart tonight into Saturday. As the warm front
tracks through the region there will still be enough isentropic
lift for widespread light snow, but with the weaker lift and less
moisture snow accumulations will be less than expected. East-
southeast flow into the Cascades will enhance the lift and that is
the area that looks like it will get a pretty good accumulation
of snow. Friday night a weak cold front will move through the
region. The forcing is increased, but at the same time we loose
what moisture tap we had. But we can still expect a second shot of
slightly heavier precipitation to the forecast area, and mainly
across the Cascade crest and the central Panhandle mountains. So,
while all areas will see measurable snow the amounts have been
lowered quite a bit.

* Snow Accumulations: Thursday night through early Saturday
  morning we can expect 1-2 inches for most of the Columbia basin,
  the Palouse and north into the northern mountains. The Cascades,
  and the lower east slopes will have the best chance for heavier
  snow and we expect 2-5 inches for the lower east slopes...5-12
  inches for the Cascades. For the central panhandle mountains 5-8
  inches in the valley and 8-12 inches in the mountains.

* Impacts: The winter Storm Watch for the Cascades and Wenatchee
  area was updated to a Snow Advisory. The best chance for
  moderate to heavy snow for the central Idaho Panhandle will not
  be until Friday night and Saturday. As such we will leave that
  Winter Storm Watch in effect, so we can get a better idea on
  snow accumulations. Tobin


Saturday through Monday night: A low pressure system will track
over the region from off of Vancouver Island on Saturday. The 00Z
GFS and NAM are tracking the surface low a bit further north than
previous runs and more across central to northern WA. All models
are in agreement that the low will fill as it crosses the
Cascades. The 00Z ECMWF guidance is the weakest model without
much discernible cyclogenesis occurring east of the Cascades. If
this solution verifies then generally only light precipitation
is expected. The exception will be in the Central Panhandle Mtns
as northwest flow will create favorable upslope flow into this
area. The GFS, NAM and Canadian models all indicate a better
potential for wetter condtions with a surface low tracking across
the region. They just don`t agree on the exact track at this time.
If the GFS, NAM or Canadian solutions pan out, then we could see
a band of moderate to heavy snow form where the surface low tracks
across in extreme eastern WA and in the ID Panhandle. I cannot
discount the ECMWF solution though the surface low does look to
fill and would indicate a weakening trend as it cross east of the
Cascades. The Central Panhandle Mtns stands the best chances for
heavy snowfall Saturday afternoon into Saturday night. The Winter
Storm Watch has been extended in time to cover this potential
threat.

There will be the potential for windy conditions on the southern
flank of the low for Saturday afternoon. How strong the winds
will get will of course depend on how deep the low will remain
over the Inland Northwest. The NAM model is the deepest with winds
up to 60 kts up at 850 mbs into the Blue Mtns. The pressure
gradient off of the GFS is considerably weaker with 850 mb winds
around 40 kts. Winds would be most likely off of the Northeast
Blue Mtns, on the Camas Prairie, into the Palouse and possibly as
far north as the Spokane Area if the GFS is correct. The NAM
appears to be too strong, so sustained winds of 15-20 mph with
gusts up to 30 mph will be possible. Confidence is low, but if the
stronger winds do pan out, then we could see some blowing snow as
well.

The battle between the drier and cold Polar Continental air to the
north and more milder and moist Pacific Maritime air to the south
will continue over the region into Monday. Models show another
quick hitting shortwave trough of lower pressure for Sunday
night. Confidence for where snow will develop is low as it will
depend on where the boundary between these two air mass come
together. The general consensus is for the best chances of snow
being across southeast WA and over the southern to central ID
Panhandle.

Temperatures will moderate out a little bit heading into Sunday
with highs in the upper 20s and 30s. Models are hinting at a
resurgence of modified Arctic air spilling in over the region
Monday night. Winds could be quite breezy down the Okanogan Valley
with below zero wind chills possible into Tuesday morning. The
morning lows on Tuesday could be quite chilly as well, especially
in areas with a fresh snowpack with single temperatures possible.
/SVH

Tuesday and Wednesday: Tuesday we transition to a dry north-
northwest flow around a low to our northeast in Canada. This will
keep us dry Tuesday and most of Wednesday but bring another shot
of frigid air for Tuesday morning into Tuesday night. The EC and
Canadian models have slowly come into better agreement with the
GFS in bringing highs in the teens and lows in the single digits
on Tuesday, so have dropped temps quite a bit. They should
regulate by Wednesday as sfc winds shift out of the south bringing
in some milder air. Late Wednesday into Thursday our next
potential snow system moves in from the southwest with snow levels
at the valley floors. /BW


&&

.AVIATION...
12Z TAFS: light easterly flow up against the Cascades is
resulting in a mid level stratus deck...mainly effecting KEAT but
this should break up between 16-19z. Otherwise high level clouds
will move into the region through the day for mainly VFR cigs/vsby.
A tight northeast pressure gradient will also result in gusty
winds developing at KCOE and to a lesser degree near KMWH.
Snow from the next weather disturbance will make it to KEAT by
00-02z...KLWS-KPUW 02-04z and the KCOE-KGEG corridor closer to
07-08z. VFR conditions will lower to MVFR/IFR cigs/vsby with the
onset of the snow. Tobin




&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        24  19  27  24  32  26 /   0  70  50  60  80  40
Coeur d`Alene  25  19  27  24  33  27 /   0  60  50  70  90  60
Pullman        25  23  31  27  34  29 /  10  90  70  90  80  70
Lewiston       31  27  36  30  39  32 /  10  80  60  80  40  50
Colville       26  19  26  22  31  26 /   0  50  40  30  80  40
Sandpoint      25  17  26  24  30  27 /   0  50  40  60  80  60
Kellogg        22  17  27  24  31  27 /   0  80  70  90  90  80
Moses Lake     25  20  29  19  32  20 /  10  70  50  70  50  10
Wenatchee      23  21  27  22  31  24 /  10  80  60  70  50  10
Omak           23  20  25  20  30  21 /   0  70  60  30  50  10

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Winter Storm Watch from Friday afternoon through Saturday
     evening for Central Panhandle Mountains.

WA...Winter Weather Advisory from 6 PM this evening to 4 PM PST
     Saturday for East Slopes Northern Cascades.

     Winter Weather Advisory from 6 PM this evening to 4 AM PST
     Friday for Waterville Plateau-Wenatchee Area.

&&

$$


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