Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 140005
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
405 PM PST SAT FEB 13 2016

.SYNOPSIS...
Rain and mountain snow will spread into eastern Washington and
north Idaho tonight. Accumulating snow will likely impact travel
over Lookout Pass on Interstate 90 tonight as well as Stevens Pass
on Highway 2 over the Cascades. Snow levels will rise Sunday night
into Monday and temperatures will seem more like early April than
mid February. Many locations will experience highs in the 50s to
low 60s on Monday and Tuesday. More wet weather will arrive for
middle to late week with occasionally windy conditions.

&&

.DISCUSSION...
Tonight and Sunday: Clouds will thicken this evening as the next
weather system moves across the Pacific Northwest. Precipitation
will push east of the Cascades as a warm front lifts northeast
toward the Canadian border. Snow levels will range from 3-4k feet.
With lack of any lingering cold air, not expecting much lowering
of snow levels overnight. Precipitation will be in the form of low
elevation rain and mountain snow starting this evening and
tapering off overnight into Sunday morning. The heaviest snow
amounts will be found near in the higher mountains of the Cascade
crest and in the central Idaho mountains near Lookout Pass. The
snow in the Cascades will be found at and above 4k ft which bring
minor impacts due to a few inches of snow to Stevens Pass, yet
more significant snow will have a better impact to travel at
Lookout Pass overnight with around 4 to 8 inches possible. Due the
timing of snow and travel concerns, will issue a winter weather
advisory for snow that includes Lookout Pass for tonight into
Sunday morning. By midday, road temperatures will warm and any
additional snow should leave roadways mostly wet despite
orographic showers continuing into the afternoon hours in the
Panhandle mountains. By Sunday afternoon, the next in a series of
weather systems arrives and a secondary warm front pushes across
the region with another round of precipitation. With the warm air
advection and the arrival of the front during the afternoon,
anticipate the precipitation to fall as rain in most areas with
the exception of the higher mountains. Meanwhile, low elevation
southwest winds will be on the increase and help create a rain
shadow in the lee of the Cascades. This will lead to breezy winds
across the Waterville Plateau and western Columbia Basin with
little if any precipitation. The Cascade valleys will be tricky
as winds pick up on the ridge tops, although valleys may still see
little mixing and winds leading to the fog and stratus to linger.
This supports the continuation of the Air Stagnation Advisory for
the Cascade valleys. Overall, expect mild temperatures through
the remainder of the weekend. /rfox

Sunday night and Monday: A moist low pressure system will move
over the top of a flat high pressure ridge Sunday night into
Monday morning. The weather models forecast the 300mb jet axis
will be over southern British Columbia as the surface low tracks
through northern B.C. into central Alberta overnight Sunday.

* Precipitation: Our region will be in the mild warm sector as a
  rich tap of subtropical moisture spills into the region. With
  the main frontal forcing well north of the Canadian border, the
  primary forcing mechanism for precipitation will be orographic
  in nature with the Cascades and Idaho Panhandle mountains
  receiving the most significant precipitation which will be in
  the form of rain below 5000-6000 feet. The high terrain of the
  Cascades and Idaho Panhandle will likely receive in excess of an
  inch of rain Sunday night into Monday.

* Gusty Winds: With the strong west to east jet stream over
  southern BC, there will be a good deal of strong winds aloft
  Sunday night into Monday. The SW orientation of the pressure
  gradient will favor the windiest conditions over southeast
  Washington, the Upper Columbia Basin, and West Plains. With the
  passage of the surface low well to our north, we don`t expect a
  strong frontal passage, so we may not completely realize our
  850mb gust potential (45-50kt), but 40 mph gusts look like a
  good bet for places like Pomeroy, Ritzville, Pullman, Cheney,
  and Spokane.

* Hydrology: The rain and mild temperatures will lead to increased
  run-off as we lose some of our mid-elevation snow pack. Modest
  rises are expected on our mid-large sized rivers of the Idaho
  Panhandle including the Coeur d`Alene system, the St Joe, and
  Palouse. At this time it does not look as if the combination of
  rain and melting snow will be sufficient for significant
  flooding, but we may see some minor small stream flooding in the
  Panhandle. Melting snow on the Waterville Plateau may also
  produce some minor field flooding in areas with poor drainage.

Tuesday and Wednesday: Locally breezy and unusually warm weather
will continue Tuesday into Wednesday as our mild high pressure
ridge remains over the region. Mountain rain showers will continue
to be a possibility as the remnants of Monday`s moist frontal
system languish over the region providing the potential for
orographic precipitation. /GKoch

Wednesday night through Saturday: Periods of rain and mountain
snow, as well as occasionally breezy to windy conditions will mark
this period as a parade of systems comes into the Pacific
Northwest. Expect a cooling trend too, though average values are
still projected to be above normal.

Between Wednesday night into Thursday an occluding frontal system
moves through. This will spread a threat of precipitation across
most of the region Wednesday night. Some models suggest a dry slot
wrapping into the system, which could put a wrinkle in determining
whether the threat of precipitation will be held back or not in
some locations. Right now the typical areas over the deeper
Columbia Basin and the L-C Valley appear have a lower risk in the
forecast, but it could also be surging further into southeast WA.
Thursday the front continues to shift east while the supporting
unstable upper trough follows on its heels. The steadier
precipitation shifts into the mountains, while the rest of the
area sees lingering shower chances.

* Snow Levels and precipitation-types: Snow levels Wednesday
  night linger between 4500-6500 feet, lowest near the Cascades.
  By Thursday they fall to between 3500-5000 feet. So some snow
  may fall in the mountain passes and cause impacts. One area to
  watch will be near the Cascades. Snow levels could be locally
  lower near the passes and perhaps the Methow Valley, allowing
  for more snow impacts. Confidence right now leans toward the
  main impacts remaining near to just above pass level Wednesday
  night before they lower Thursday. As for the remaining valleys,
  largely rain is expected but in the instability with the upper
  trough I would be surprised to see some snow pellets or graupel
  mixed in Thursday afternoon. In fact some models show CAPE in
  afternoon (with a few pockets averaging near 300 J/kg) somewhere
  over the eastern CWA. This puts the idea of isolated
  thunderstorms in my head, but there is far too much disagreement
  over its placement to have confidence to put in the forecast.
  However it will be monitored.

* Winds: a more widespread impact will be winds. There appears to
  be two opportunities for breezy to windy conditions. The first
  comes with the occluded front passage Wednesday night. Some
  guidance shows 850mb winds in the 30 to 50 kt range punching
  across the Columbia Basin into the Palouse and Spokane areas
  late Wednesday night into Thursday morning. This the isn`t best
  time of day for the most efficient mixing but the passing front
  may be enough to capture at least a portion of those winds. If
  the winds don`t mix down to the surface then some locations
  could be in for some wind shear, which could be an aviation
  concern. As the front pushes east on Thursday models show a
  secondary low deepening over southern AB/northwest MT. This will
  be the second impetus for continued breezy to windy conditions
  with a tightening southwest-northeast gradient and decent mixing
  within the upper trough. Bottom line: everything points to
  breezy to windy conditions this period. It is not yet known
  whether they will be strong enough to necessitate any
  highlights, but it will be monitored as well.

Between Thursday night and Friday night most models show another
frontal wave pushes through the region, with a possible third
approaching Saturday. However model differences begin to show up.
With the Thursday night to Friday night system there is a 6-12
hour difference regarding the start and end of the precipitation
threat. There is even one model that does not bring much of system
in at all and keeps the area drier, but it`s an outlier. As it
stands now models show the leading warm front starts in late
Thursday night into Friday, expanding precipitation across the
region from west to east. The lowest risk will be in the deeper
Columbia Basin and L-C Valley. The cold front then pushes through
sometime between Friday afternoon into Friday night which once
again shifts the steadier precipitation into the mountains. The
primary precipitation chances linger around the mountains
Saturday, but some risk will also be found over the southeastern
CWA with that possible third system. Snow levels average between
3000 and 4000 feet this period, meaning more possible snow impacts
in the passes. As for winds, speeds subside Thursday night into
Friday morning but increase again Friday afternoon into Friday
night with the incoming front. Values may again cause impacts but
in a relative sense models show speeds a bit less than with the
Wednesday night/Thursday system. However this may change as well
as models come into better agreement. /J. Cote`

&&

.AVIATION...
00Z TAFS: Warm frontal precipitation will spread into eastern
Washington and north Idaho early this evening. A strong mid-level
westerly wind is expected to enhance the rain shadow east of the
Cascades limiting rain amount for Wenatchee, Moses Lake, and Omak.
These airports may only get a hundredth or two of rain this
evening. The wet ground may be sufficient to produce fog in
Wenatchee. Further east, Spokane, Coeur d`Alene, and Pullman
should get higher precipitation amounts, but the rain will be
accompanied by more wind. Odds are good that some low stratus will
develop following the rain, and possibly some fog. For now, it
looks like there will be enough wind to limit fog development.
/GKoch

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        37  47  42  53  41  53 /  70  60  30  20  20  20
Coeur d`Alene  36  46  41  52  41  51 /  70  60  70  60  20  30
Pullman        38  50  44  56  43  55 /  70  60  70  30  10  20
Lewiston       41  55  47  63  45  61 / 100  60  60  20  10  10
Colville       33  42  37  51  40  50 /  70  60  30  30  30  20
Sandpoint      35  41  37  49  40  47 /  90  60  70  60  50  40
Kellogg        34  40  38  46  39  45 / 100  80 100  90  70  40
Moses Lake     36  51  43  60  42  57 /  60  20  10   0  10  10
Wenatchee      33  47  39  56  41  54 /  60  10  10  10  10  10
Omak           33  40  35  48  36  50 /  60  20  10  10  10  20

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Winter Weather Advisory from 7 PM this evening to 10 AM PST
     Sunday for Central Panhandle Mountains.

WA...Air Stagnation Advisory until 10 AM PST Tuesday for East Slopes
     Northern Cascades-Okanogan Valley.

&&

$$


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