Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 211222
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
422 AM PST Tue Nov 21 2017

.SYNOPSIS...
Expect wet and mild weather through the week as a series of storm
systems brings several rounds of precipitation to the region.
Snow levels will be quite high Today through Thursday allowing
motorists to travel over mountain passes on wet pavement. Snow
levels will begin to lower just as travelers are heading home
Friday into the weekend. Breezy to windy conditions are expected
Thursday across the exposed areas of the basin and into the
Spokane Area and Palouse.

&&

.DISCUSSION...
Today through Wednesday...Currently a ridge of high pressure is
over the Pacific northwest, with a deep low pressure system just
moving inside of 150W early this morning. This puts the region in
a very warm and moist southwest flow that will remain over the
area into Thursday. Moderate to strong isentropic up glide and
several weak embedded waves will interact with the deep moisture
for widespread moderate to heavy precipitation. The precipitation
will begin late this morning across the western zones and move
across the area this afternoon. The heaviest precipitation will be
from late this evening through Wednesday morning, then decrease
slightly Wednesday afternoon. Snow levels will be high enough that
most of the precipitation will be as rain. Temperatures in the
40s today will increase to the upper 40s to mid 50s on Wednesday.

The potential hazard today will be for the possibility of freezing
precipitation for Blewett Pass and some of the mid slope areas
south of Lake Chelan this morning. Then for Loup Loup Pass and
some of the higher valleys and mid slopes for the Methow valley
this afternoon. This is far from a slam dunk forecast and for now
we will get the word out with social media and Nowcasts.

Precipitation amounts will be quite impressive the couple of days.
The forecast area will see from a third of an inch to over three
quarters of an inch by Wednesday afternoon. The Cascades from a
half inch to well over 2 inches near the crest. For the Panhandle
from three quarters of an inch in the valleys with 1-2 inches
possible in the mountains. Warm temperatures and moderate rain on
an existing snow pack will result in heavy runoff into area
streams and rivers. Right now rapid rises are expected on all
small rivers and streams across the region, but no flooding is
expected. Paradise Creek near Moscow Idaho was able to make it up
to just about action stage Monday afternoon. The precipitation
for Moscow mountain will not be quite as much today and Wednesday,
but expect Paradise Creek to remain very high through the week.
Tobin

Wednesday Night through Thanksgiving Night: A cold front will
push across the region during the day on Thanksgiving. The front
will bring another round of precip along the front, but the more
significant impact with this frontal passage may be the winds.

* Winds: Strongest wind gusts are expected in the afternoon.
  Models are in good agreement with 850 mb wind speeds up around
  45 to 50 kts. Due to the timing of the front in the afternoon
  and precip winding down across the basin and into the Spokane-
  Coeur d`Alene Area and Palouse, this should result in a period
  of favorable mixing with the potential for these 45-50 kt winds
  to mix down to the surface. The surface gradient doesn`t look to
  be particularly strong with sustained winds more likely in the
  20-25 mph range and below wind advisory criteria. However,
  advisory wind gust criteria of up to 45 mph does look promising.
  Light weight objects may become airborne with these winds and
  difficult travel of high profile vehicles will be possible,
  particularly those traveling along I-90, US-2, US-395 and
  US-195. Will add these potential impacts into our HWO, but it is
  a bit early for a Wind Advisory at this time.

* Precipitation: Looking at a 6 hour or so period of light to
  moderate rain across extreme eastern WA and into the ID
  Panhandle. There is enough westerly flow that models show a
  substantial rain shadow in the lee of the Cascades. The
  westerly orientation of the winds will also result in more
  moderate rainfall intensity to be more favored in the Panhandle.
  Snow levels will be lowering over the Cascades with showers
  changing over to snow at Stevens Pass late in the afternoon with
  some slush accumulating for the evening hours. This will just
  be a pass issue with snow not expected to be much of an impact
  during this period.

Friday through Monday: Shortwave ridging building into the region
will result in a short break between weather systems on Friday.
This looks to be a lone relatively dry day with a potential for
some lingering mountain showers. The forecast turns wet again for
next weekend. Snow levels may begin low enough for a little bit of
snow to start out with Saturday morning down as low as 3000 feet.
Then snow levels will increase rapidly through the day Saturday
with even the highest mountain peaks changing over to rain. Models
disagree with when the trough will swing late in the weekend with
another potential for breezy to windy conditions during this
time. /SVH

&&

.AVIATION...
12Z TAFs: Increasing clouds ahead of the next weather system has
allowed the fog/stratus to lift out of the region this morning.
The exception is KLWS where cigs/vsby remain LIFR, but expect
conditions down there to improve after sunrise. And there may be
some brief periods of IFR/LIFR cigs/vsby vcnty KGEG/KSFF/KCOE,
otherwise conditions are VFR across the region this morning. VFR
conditons will not last long as moisture is rapidly moving into
the region from the southwest. We expect an extended period of
moderate to heavy precipitation through Wednesday afternoon.
Cigs/vsby will fluctuate between IFR/LIFR as bands of heavier
precipitation move through an area. Finally there may be a wintry
mix for the Cascade East Slopes this morning into early afternoon
including the Methow Valley Airport. Tobin


&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        46  41  53  45  57  35 / 100 100  70  40  80  30
Coeur d`Alene  45  40  53  44  57  35 / 100 100  70  50  90  40
Pullman        49  44  56  47  59  37 / 100 100  70  30  90  70
Lewiston       53  46  59  48  62  40 /  80  80  70  20  80  80
Colville       41  36  47  40  54  33 / 100 100  60  40  90  10
Sandpoint      41  37  47  41  53  34 / 100 100  70  70 100  30
Kellogg        42  39  49  43  53  34 / 100 100  80  40 100  80
Moses Lake     44  38  52  42  60  34 / 100  80  60  30  20  10
Wenatchee      40  36  48  40  55  35 / 100  40  60  40  50  10
Omak           42  36  47  42  56  34 / 100  70  50  50  70  10

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...None.

&&

$$



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