Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 020025

Area Forecast Discussion
National Weather Service Spokane WA
425 PM PST TUE DEC 1 2015

A series of systems will moves into the Inland Northwest. A
somewhat weaker system moves in tonight into early Wednesday.
This will carry snow and the potential for some freezing rain,
with accompanying potential impacts to travel. A second system
arrives from the west late Wednesday night into Thursday, with the
potential for mountain snow and valley rain, and impacts from
freezing rain across north-central Washington. The pattern
remains active late this week into early next week, with several
opportunities for precipitation. However the pattern looks mild,
with valley rain and mountain snow.




Tonight and Wednesday: the first in a series of storm systems
moves in, expanding precipitation from west to east overnight into
Wednesday morning before dissipating. With cold air in place at
the surface and some milder air nosing in aloft there may be some
varying precipitation types around the Columbia Basin and perhaps
toward southwest Chelan county in the Cascades.

* Timing: First an occluded front pushing toward the Pacific NW
  coast this afternoon will come toward the Cascades by later this
  evening. Moisture along and ahead of this feature will allow
  precipitation to expand into the Cascades through this, toward
  the Okanogan Valley through Wenatchee Area/Waterville Plateau
  toward late evening/midnight, toward the western Columbia Basin
  and Okanogan Highlands between midnight and the overnight and on
  toward the eastern third of WA and the ID border overnight into
  the pre-dawn hours.

* Precipitation Types and Amounts: For much of the region
  precipitation is expected to stay in the form of snow. The
  threat of a mixture of freezing rain and snow, however, will
  nose into portions of southwest Chelan County and across the
  deeper Columbia Basin, mainly south of I-90 where some milder
  air noses in aloft. The warm air aloft is deeper south of our
  forecast area, i.e. toward Franklin/Benton county, so the
  forecast calls for a mix rather than straight freezing rain. In
  terms of precipitation amounts models continue to point to the
  Cascades east to Okanogan and western Columbia Basin as having
  the higher amounts. Amounts further east are comparatively
  lighter as the system stretches and moves by relatively quickly.

* Highlights and Impacts: So the ongoing winter weather advisories
  seem to have the main risk areas covered well for now. The
  possible area to monitor for expansion will be toward the
  Palouse and near the Blue Mountains, should the milder air surge
  and more of wintry mix occurs. Either way areas from the
  Cascades through the Idaho Panhandle, including but not limited
  to the I-90/Highway 2 and 395 corridor, travel impacts are
  possible. This would include the potential for treacherous roads
  for the morning commute. This is especially in areas under
  winter weather advisories but any one traveling on untreated
  roads should exercise caution, especially if there is black ice
  underneath any freshly fallen snow regardless of amounts.

Going through Wednesday morning to midday the precipitation
threat is expected to wane from the west, so that by afternoon the
main threat will be in the Idaho Panhandle as the occluded front
pushes east. As this occurs the middle to upper atmosphere dries
out. However the lower atmosphere remains relatively moist and the
stratus is expected to linger. With this stratus the low level
southeast flow and any upslope flow will may be enough to squeeze
some flurries or freezing drizzle across the Basin and sheltered
valleys. So the forecast continues to hold onto this threat
through the afternoon. Amounts at this time are not expected to be
significant, but this will have to be monitored too. If any of the
freezing drizzle becomes more developed and focused in any one
locations there may be concerns, but confidence leans toward it
not being a big impact by afternoon. /J. Cote`


Wednesday night through Friday: Another winter storm will push
into the region during this period. The forecast is quite messy
with precipitation types ranging from rain to freezing rain and
snow. Precipitation will increase across the region late
Wednesday with a warm front. Strong isentropic ascent combined
with orographic enhancement will place much of the precipitation
across the east slopes of the northern Cascades over to the
Waterville Plateau and into Okanogan Highlands. Isentropics and
orographics will be weaker further east into the ID Panhandle.
Although there will be some light precipitation along the warm
front out across this portion of the forecast area, impacts are
not expected to be as significant.

High impacts will be possible across portions of north-
central WA including the lee of the Cascades, Wenatchee Area,
Waterville Plateau and the Okanogan Highlands. These areas will
see the potential for moderate to heavy snowfall and/or freezing
rain or sleet. The NAM and SREF show a good potential for freezing
precipitation in the Wenatchee Area and up Highway 2 into Cashmere
and Leavenworth; Plain, the Entiat River Valley and communities
along Lake Chelan will also see the possibility for freezing rain
or sleet. Models are consistent with showing enough liquid to
produce between a tenth and a quarter of an inch of ice with
localized ice accumulations of over a quarter of inch possible.
This may lead to broken tree limbs or sagging power lines with
power outages possible. Ice will also result in the possibility
for treacherous travel conditions.

Moderate to heavy snows will be more likely for valley locations
north of Lake Chelan. This will include Twisp, Winthrop and
Mazama up the Methow Valley. Snowfall accumulations of 6-12 inches
will be possible across these areas. The Okanogan Valley and the
Okanogan Highlands will also see snow, but amounts are a bit more
uncertain. Snow levels may increase enough in the Okanogan Valley
with lighter accumulations near the river and along Highway 97. We
are expecting snowfall accumulations closer to 1-3 inches across
this area, but the higher benches could receive a bit more. The
Okanogan Highlands are expected to receive around 2-4 inches in
the valleys and 4-8 inches in the mountains.

The cold front will eventually sweep across Thursday night.
Temperatures should moderate out by Thursday afternoon across the
valleys of the Northeast Mtns and in the ID Panhandle. The
precipitation along the cold front will likely fall as mountain
snow with moderate accumulations possible and valley rain. Travel
will likely be impacted over Lookout Pass by early Friday morning
as snow levels drop with cold front passage. Although winds will
increase with the front Thursday night and Friday, we are only
expecting gusts up to around 30 mph. /SVH

...Very wet and active weather pattern to continue through early
next week...

.Friday night through Tuesday...There is little reason to think the
active weather pattern will cease during this period. Extended
model guidance is quite insistent that a relentless parade of
storms will in drift through the Gulf of Alaska before crossing
the Cascades and Inland Northwest. Each storm will be relatively
mild with the most of the snow falling over the mountains. The
main exception will be in the lee of the Cascades mainly impacting
valleys over western portions of Chelan and Okanogan Counties due
to persistent cold air damming.

So now onto the specifics. Most of the model guidance suggests this
period will begin on a dry note as the upper level trough
associated with the Thursday/early Friday system moves into
Montana. Meanwhile a shortwave ridge will briefly develop over the
region. How long it lasts is questionable. The GFS and ECMWF
suggest an impressive atmospheric river and warm front will push
over the Cascades by Saturday afternoon spreading a broad plume of
precipitation over the entire forecast area through Sunday.
Meanwhile the Canadian is about 6 to 12 hours quicker. We will
lean toward the GFS/EC solutions which both support dry weather
from Friday night through midday Saturday. Precipitation amounts
could be significant as it will be tied to a slow moving
atmospheric river with precipitable water values ranging from
0.60-0.80. The values are generally too moist to support
widespread valley snow with the main exception in the immediate
lee of the Cascades and perhaps near the Canadian
border...otherwise snow levels will generally range from 3-4k in
the northern part of the forecast area and nearing 5k in the
south. Heavy snows will certainly be possible and another round of
winter highlights will likely be needed for the east slopes of the
Cascades. The threat of freezing rain and or sleet should be
minimal by that time as the polar air mass currently over the
region will likely scrub out by then. Precipitation totals could
easily range from 1 to 2 inches near the Cascade Crest with a
large swath of up to an inch over the northern third of Washington
and the northern Idaho Panhandle.

All of the extended guidance suggests the atmospheric river feed
will temporarily end as a shortwave trough crosses the Cascades
sometime on Sunday. This will lead to a growing drying trend in
the lee of the Cascades as mid-level warm air advection turns to
cold air advection and consequently leads to downslope flow.
Meanwhile the threat of showers would likely continue at the crest
and over the Idaho Panhandle due to orographic ascent and fair
mid-level lapse rates. Again this should be a brief break as all
extended guidance suggests the next atmospheric will hit the
Pacific Northwest by early Monday morning. This system will likely
be warmer and wetter than the one earlier in the weekend, however
we still could see some significant snows developing in the lee of
the Cascades. A trailing cold front will lower the threat of
precipitation by Monday evening and this will be accompanied by
some moderate west to southwest winds with gusts to 40 mph
certainly possible. Yet another plume of sub-tropical moisture
looks like it could impact the area by late Tuesday with another
threat of moderate to locally heavy precipitation. fx


00Z TAFS: Expect MVFR conditions for most TAF sites with brief
VFR conditions for KEAT/KLWS/KPUW this evening. A winter weather
system will affect the Cascades this evening then spread east
overnight before exiting Wednesday. This will bring a threat of
-FZRA/SN around KEAT/KMWH, especially toward 07-09Z continuing
through 13-14Z. After 10-12Z the precip threat expands toward the
eastern TAF sites. KGEG to KCOE is expected to see mainly snow,
but there may be some freezing rain mixed in on the tail end
around 16-18Z. KPUW/KLWS is expected to see a mix of freezing
rain/snow during this time frame. Light icing is possible in any
locations that see freezing rain, but the speed of the system
should keep any ice amounts less than 0.05 inches. After 20Z
precipitation will end at eastern TAF sites but will continue
for the central panhandle mountains of Idaho through 00Z Thursday.


Spokane        24  32  32  42  35  40 /  50  90  20  60  60  30
Coeur d`Alene  26  32  32  43  36  41 /  20  90  10  60  70  40
Pullman        27  38  38  45  36  42 /  20  90  10  30  60  30
Lewiston       29  38  38  49  38  47 /  20  50  10  10  40  30
Colville       22  31  31  40  34  39 /  80  80  50  80  70  30
Sandpoint      22  30  30  40  35  40 /  20  90  20  70  80  50
Kellogg        22  31  30  42  34  38 /  10  80  10  50  80  60
Moses Lake     23  31  31  41  34  42 / 100  30  60  60  50  10
Wenatchee      24  30  30  36  32  40 / 100  10  80  70  50  20
Omak           24  30  30  37  31  38 / 100  20  90  90  70  20


ID...Air Stagnation Advisory until noon PST Friday for Central
     Panhandle Mountains-Coeur d`Alene Area-Idaho Palouse-Lewis
     and Southern Nez Perce Counties-Lewiston Area-Northern

WA...Air Stagnation Advisory until noon PST Friday for East Slopes
     Northern Cascades-Lower Garfield and Asotin Counties-Moses
     Lake Area-Northeast Blue Mountains-Northeast Mountains-
     Okanogan Highlands-Okanogan Valley-Spokane Area-Upper
     Columbia Basin-Washington Palouse-Waterville Plateau-
     Wenatchee Area.

     Winter Weather Advisory from 10 PM this evening to 7 AM PST
     Wednesday for Moses Lake Area-Okanogan Highlands-Okanogan
     Valley-Upper Columbia Basin-Waterville Plateau-Wenatchee

     Winter Weather Advisory from 7 PM this evening to 7 AM PST
     Wednesday for East Slopes Northern Cascades.


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