Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 241310 RRA

Area Forecast Discussion
National Weather Service Spokane WA
452 AM PST Tue Jan 24 2017

A pair of weak disturbances will move through the region today
through Wednesday and will result in light snow showers or snow
flurries. By midweek a strong ridge will build over the Pacific
Northwest. This will bring dry and cooler weather with an
abundance of low clouds and fog in the valleys.


Today through Wednesday...Satellite imagery this morning was showing
a large upper level low drifting southward off the central
California Coast with a ridge of high pressure folding into
central BC. Its the latter feature which will be of concern to the
Inland Northwest. This ridge is expected to slowly move eastward
through this period and should provide a stabilizing influence to
the weather. Generally speaking ridges bring dry weather to the
area, however it`s not that simple in this situation. Upon closer
inspection there was a weak shortwave trough moving dropping
across the NE corner of Washington and the northern ID Panhandle.
There isn`t a whole lot of precipitation associated with the
feature, however there was an influx of middle level cloudiness
and just a few flurries noted over SE BC per Canadian radar. This
shortwave is expected to move into NE Oregon by late afternoon
before weakening. Another disturbance is expected to take a
similar track late tonight and Wednesday. Neither system is
expected to be very strong, however they both could deepen the
boundary layer moisture found near the ground per the Spokane
sounding at 00z. At 00z, this layer was around 2500-3000` thick
and was responsible for the widespread stratus found over most of
the Inland Northwest this morning. During this period most of the
models deepen this moisture to nearly 9000 feet over the eastern
quarter of Washington and most of the Idaho Panhandle. That
suggest at a minimum we should see considerable cloud cover
through the period but we should also see an increasing chance of
snow as well. Most of this snow isn`t a product of the pair of
shortwaves as there is very little upward motion associated with
their presence. Rather the precipitation will be a result of
weak/moderate isentropic ascent between 280-285k. Most of this is
lower than the dendritic layer, however its cold enough to produce
light snow showers or flurries. Most of the models support this
scenario with the best chances likely to occur where NW low level
flow leads to the best orographic ascent. That suggests locations
such as central Idaho Panhandle and the Camas Prairie will see the
best chances but nearby areas such as the Palouse, extreme
eastern Columbia Basin, and LC Valley will see snow as well. Again
given the lack of strong ascent and moisture through the
dendritic layer, we don`t expect the precipitation to amount to
much. Most locations should see far less than an inch through the
entire period.

Aside from the snow threat the other story will once again revolve
around fog. Fog this morning was generally not as widespread as
yesterday morning, however patchy dense fog was noted over
portions of the West Plains, Columbia Basin, and Okanogan Valley.
As long as the ridge persists we will see more fog and low clouds,
but as the boundary layer moisture deepens we suspect the fog
chances will begin to decrease, especially over the eastern third
of the forecast area. fx

Wednesday night through Saturday night...Models continue to show
an upper ridge moving over the area late in the week and
persisting through Saturday night. Abundant boundary layer
moisture with a nearly saturated lower level atmosphere up to
800-850mb will result in low stratus clouds even in the mountains
through Friday...and then southeast boundary layer winds may
provide some clearing on the palouse, Lewiston area, and Central
Panhandle Mountains on Saturday. Also on Saturday boundary layer
moisture becomes more shallow with more opportunity for sun in the
mountains. Low clouds will limit diurnal temperature
ranges...with valley low temperatures mostly in the lower to mid
20s and highs in the upper 20s to lower 30s. The deep layer of
stratus should overall limit fog coverage except in the higher
elevations where the stratus intersects the terrain. JW

Saturday night through Tuesday: A quick moving system is expected
on Sunday bringing mountain snow and possible rain to rain/snow
mix to the Columbia Basin. Amounts with this system are not
expected to be significant as the speed of it will keep the any
accumulations to a minimum. A ridge is expected for Monday and
Tuesday bringing cooler and drier weather. Overnight freezing fog
could create hazardous driving during the morning commute. The
next major precip system is expected on Wednesday. Temperatures
for the period are expected to be in the low 30s to upper 20s for
highs and lower 20s to mid teens for lows. /JDC


12Z TAFS: Fog and stratus will be the main forecast challenge
through this period. Odds are good that most sites will see IFR
conditions through most of this period. The exceptions to this will
be EAT, where it is currently clear, and LWS where there are MVFR
conditions. The latest fog product shows low clouds near EAT so
conditions could deteriorate quickly at least through the morning
before a weak disturbance over NE Washington tracks southward
through the area. As this disturbance moves into SE WA by late
afternoon, we should see an increasing threat of light snow showers
for GEG SFF COE early this morning and spreading to LWS PUW later.
Cigs could rise as the snow moves in but this increase will likely
coincide with lower visibility values. There is also a very small
chance of seeing some brief freezing drizzle this morning but if the
models are correct that threat will mainly occur before 18z between
GEG and PUW. Overall confidence in this forecast is fair at least in
regards to IFR conditions at most sites. Confidence is much lower
for precipitation timing and type. Whatever falls should be light
though. For the latter portions of the forecast, after 06z, we
expect to see increasing chances for low cigs EAT, but confidence is
low. fx


Spokane        33  25  33  23  29  23 /  20  20  10  10  10  10
Coeur d`Alene  33  25  33  23  29  23 /  30  30  20  20  10  10
Pullman        32  25  33  22  30  21 /  40  30  20  10  20  10
Lewiston       35  28  35  24  34  25 /  30  30  20  10  20  10
Colville       32  25  32  20  30  23 /   0  10  10  10  10  10
Sandpoint      32  24  33  23  30  23 /  20  20  20  20  10  10
Kellogg        30  24  31  22  29  21 /  50  50  40  20  20  10
Moses Lake     35  21  34  20  30  22 /   0   0  10  10  10  10
Wenatchee      31  23  33  22  29  23 /   0   0   0  10  10  10
Omak           35  22  31  20  29  23 /   0   0  10  10  10  10



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