Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 221323
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
505 AM PST Thu Feb 22 2018

.SYNOPSIS...
Cold and generally dry conditions will persist through Friday
morning except for some passing light snow showers or flurries at
times. A series of storm systems will impact the Inland Northwest
this weekend into early next week with the potential for more
snow, breezy conditions and continued below normal temperatures for
most locations.

&&

.DISCUSSION...
Today and tonight...Cold weather to continue today as 500 mb low
is expected to remain focused over the Inland NW. Water vapor
imagery suggests the center of the low was just east of Moses Lake
and it is expected to meander ever so slowly toward NE Oregon by
this afternoon. Despite its presence there isn`t a whole lot of
sensible weather with it as the core of the atmospheric ascent is
focused well west of our forecast area. Nonetheless the weather is
somewhat troublesome as we will maintain a highly moistened
dendritic layer. This has resulted in periods of snow flurries
through much last evening and into the early morning hours. The
models are not handling this situation very well and really see no
reason for significant changes through today. As the low
continues to sag SSE through the day it will eventually pull drier
air in from the NW. This should decrease the flurry threat at
least over central Washington. Meanwhile the best location for
appreciable snow today will occur over SE Washington and NC
Idaho, including the Blue Mountains and Camas Prairie. Both of
these locations should see an uptick in snow activity by this
afternoon and evening as the saturated dendritic layer combines
with increasing W-NW winds in the mid-atmosphere. This in turn
will lead to orographic ascent through the moist dendritic layer
and should increase the coverage and intensity of the snow.
Neither location should see heavy amounts of snow, but anywhere
from 1-3 inches will be possible. How long this activity lingers
is questionable, however it looks like it should persist until
about midnight.

As the drier air moves into the region, we are looking for another
chilly night with clearing skies and light winds expected almost
across the entire forecast area. We suspect single digit below
zero temperatures are possible again over the valleys of NC/NE
Washington and possibly over the northern Idaho Panhandle. While
it won`t be quite as cold as it was yesterday morning it could be
close for some sheltered locations such as Republic, Winthrop, and
Deer Park.

Friday and Friday night...the clearing will be short-lived as yet
another weak shortwave trough sweeps and cold front sweeps in from
the NW. Although this one will be a progressive feature it will
feature much stronger forcing than the current system and thus
we`d expect to see measurable snow for a much broader area. It
looks like the threat of snow will begin in the Cascades by mid-
morning and toward the WA/ID border by afternoon. During the
overnight hours most of the threat will shift into north Idaho
however the atmosphere will be very unstable as 500 mb
temperatures plummet to -38c or colder which should support
spurious post frontal snow showers. QPF totals for this period
will range from 0.15-0.30 from extreme NE Washington to the ID
Clearwater Mountains which will translate to 2-4 of snow with
locally heavier amounts over the mountains. Totals near the
Cascade Crest will be even higher with values ranging from
0.30-0.60 which should equate to snow amounts ranging from 4-8
inches. For locations in between the two extremes, snow amounts
will be fairly light. Locations from Colville to Spokane to
Pullman can expect values between 1-2 with much lighter amounts
over the Columbia Basin and into the Wenatchee area. We don`t not
anticipate issuing winter weather advisories for snow at this time
however it may become necessary as it may impact the Friday
evening commute over the eastern third of Washington and the Idaho
Panhandle. fx

Saturday through Monday: A progressive cold pattern continues
with the upper flow from the northwest. The Inland NW will remain
on the cold side of the jet stream as a series of waves move
through the region. By Saturday morning, a surface low will track
from the northern Cascades to the southern ID Panhandle. A cold
front drops in from the north by Saturday afternoon and evening
with a drier air mass. Breezy southwest winds will become more
northerly in the afternoon. Temperatures will warm slightly with
valley locations experiencing above freezing temperatures. Despite
the surface warming, snow levels will rise less than 1k ft which
would only bring a rain snow mix to the LC valley. Looks to be a
good chance of snow in the Cascades and north Idaho, with snow
persisting in the central Panhandle into Saturday evening. The
region experiences a brief break in precipitation Saturday night,
but another system quickly arrives by Sunday morning. This one
looks to be slightly warmer and moister while the main surface low
tracks across southern Canada. Temperatures will rise a couple
more degrees, while snow levels rise 1-2K ft by afternoon leading
to a better chance of a rain/snow mix across the southern Basin
and LC valley. The Cascades and Panhandle mountains stand to see
the potential for significant snow, while light accumulations will
be found in the eastern third of Washington. Windy conditions
develop Sunday afternoon and night. The Sunday system exits Sunday
evening. By Monday, an upper level trough remains across the
region with a chance of showers and slightly cooler temperatures.

Tuesday through Wednesday: 00z Models lose their agreement and
confidence lowers. While the GFS carves another trough over the
region by mid-week, the ECMWF delays the trough arrival and builds
a ridge over the region. Will lean toward a more persistence
forecast, under climatology with little change from the previous
forecast. Temperatures should remain cool and continue to be below
normal through the end of February. /rfox.

&&

.AVIATION...
12Z TAFS: An upper trough will remain fixed over eastern WA and NC
Idaho through most of the day. For the morning hours this scenario
will result in a chance of light snow showers or flurries.
Occasionally these flurries will deliver MVFR conditions. Hard to
pinpoint how widespread the showers will be since radar is
overshooting a good portion of them, however we suspect that most of
the showers will concentrate around PUW and LWS after 18z and
persist into the early evening. It is these two locations which
stand the greatest chance for accumulating snow, but both airports
should see an inch or less of snow today. For the overnight hours
expect VFR conditions with clear or clearing skies. This could lead
to some fog late in the period near bodies of water. Not confident
enough to put as a prevailing conditions in the forecasts, but did
mention patchy fog at SFF due to the proximity to the Spokane River.
fx

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        27   8  28  23  36  23 /  10   0  30  70  80  10
Coeur d`Alene  27   4  29  21  35  20 /  10   0  30  80  90  10
Pullman        27  10  28  24  35  24 /  20  10  10  80  90  10
Lewiston       34  18  35  29  42  28 /  30  10  10  50  70  20
Colville       28   7  28  20  36  19 /   0   0  50  60  60  10
Sandpoint      28   5  27  22  34  21 /  10   0  30  90  70  20
Kellogg        26  11  26  21  31  21 /  10  10  20  90  90  30
Moses Lake     33  13  34  26  44  26 /   0   0  20  20  20  10
Wenatchee      30  14  32  24  43  26 /  10   0  20  30  40  10
Omak           29   8  29  19  38  20 /   0   0  40  20  40  10

&&

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

&&

$$


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