Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 192354

Area Forecast Discussion
National Weather Service Spokane WA
354 PM PST Thu Jan 19 2017

Rain and high elevation will linger over the southern Idaho
Panhandle through this evening. Then a disturbance moving in from
the southwest will spread the risk of snow late tonight and into
Friday near the Cascades. Chances for light snow will continue
through the weekend as a couple additional systems sweep in from
the southwest. A drier pattern is expected Monday and this will
persist through much of next week.


Tonight: Another round of precipitation comes to some of the
Inland NW, this time focused toward the Cascades and mountains,
mainly clipping the western and northern Columbia Basin. The area
remains in a southwest flow littered with small scale
disturbances: one is shifting through the ID Panhandle and a
second is coming into the Cascades. The first will exit this
afternoon/early evening, keeping some snow shower threat alive
across the Blues Mountains and the central and southern Panhandle.
The second will be moving across the region through the evening
and early overnight, before exiting to the north and east. This
feature is not very strong in terms of lift, but the best moisture
and instability (with lapse rates of 7-8 C/km) lays toward the
Cascades/northern mountains with weak upslope flow toward that
region. This will expand a threat of mainly light snow across that
region. There is still a weak tongue of warm air across the
deeper Columbia Basin and in the vicinity of Wenatchee and the
Waterville Plateau. This may support some mixed precipitation this
evening, including sleet and/or freezing rain and perhaps general
rain. Chances will also spread across the northern WA/north ID
mountains and around the northern rim of the Columbia Basin,
generally north of Highway 2. These areas will see mainly snow or
a rain/snow mix. In general snow accumulations tonight are
expected to be less than an inch, save for the Cascade Mountains
and higher Cascade valleys and some of the higher elevations
across the northern mountains where a couple inches or so are
possible. As far as icing from any freezing rain, if anything
falls as freezing rain at this time amounts are expected to be
trace amounts to a couple hundredths of an inch or so. Fog has
been plaguing much of the deeper Columbia Basin through the day.
The southeast flow and the incoming shortwave may help to mix this
up some, but in general this will be possible throughout the west
and northern Columbia Basin into the Waterville Plateau,
expanding around the northern valleys and some of the Spokane/C`dA
area, especially northern and western areas and around bodies of
water. As far as road conditions, there is the potential for more
slick conditions regardless of precipitation falling or not. With
areas of black ice, melting and refreezing and a lot of compact
snow and ice on areas roads/surfaces we have the potential for
more slick roads. /J. Cote`

Friday through Saturday night...The region will remain on the
unsettled and cold side of the upper level jet through this
period. The overall pattern will feature a deep trough off the WA
coast which leaves our area subject to various shortwave troughs
which are expected to pivot in from the southwest. During this
period there will be at least two such features. The first
significant one will feature an elongated occluded from which is
expected to hit the central Cascades by late morning and then
sweep and then into the northern Cascades and Okanogan Highlands
by evening. This is a good setup for fairly high precipitation
chances as the 850-700 mb flow will be ESE into the offshore
low/trough. This spells prolonged upslope flow into the Cascade
Crest. Despite the good setup, this should not be a heavy
precipitation event as the precipitable water values are quite a
bit lower than the latest event. Generally this looks like a
prolonged light snow event for most of the area near the Cascades,
including the Wenatchee area, Cascade Valleys, Okanogan Valley,
and possibly the Waterville Plateau. Snow amounts will generally
range from 1 to 3 inches with locally heavier amounts toward the
crest. Although we can`t entirely rule out a brief chance of
freezing rain model soundings suggest the odds are quite small.
The second disturbance comes into the area from the southwest on
Saturday into Saturday night. This feature is a little more
diffuse and features weaker upslope flow. Consequently we expect
to an even lesser chance of significant precipitation. Unlike the
first system, this one has a better chance of spreading the
precipitation threat farther to the east as the moist isentropic
ascent spills toward the Idaho Panhandle by late afternoon and
into the night. The precipitation type forecast could prove a
little trickier across the SE portion of the forecast area as the
NAM is showing a melting layer aloft with surface temperatures
around freezing. If this were to happen we could either see some
rain or a rain snow mix with a slight chance of freezing rain. The
SREF isn`t too excited about freezing rain or sleet, nor are the
GFS and EC models. Thus we won`t harp on that threat at this
point, but it bears watching. Nonetheless precipitation amounts
should be quite light.. fx

Sunday and Sunday Night: The upper level low pressure system
situated off of the WA/OR coast will send across a deformation
band on Sunday. The parent low will remain well offshore at around
45N 133W. Dynamics with this wave will be weak as the vorticity
maximum will stretch and shear apart. The most favored areas for
precipitation will be across the east slopes of the northern
Cascades. Easterly upslope flow will provide additional lift
across this area orographic effects. Snow is expected to be the
dominant precipitation type with between 2 to 5 inches expected
over the east slopes of the northern Cascades. Mostly snow
expected for the northern mountain valleys of northeast WA and the
central to northern Panhandle, but amounts will be light.

Precipitation type could be a little bit trickier over portions
of the basin and into the Spokane/Coeur d`Alene area and Palouse.
The GFS model shows a warm tongue nosing in over these areas.
This warmer air would erode as the temperature profile moistens
due to evaporative cooling effects. The potential issue is that
precipitation may start out light enough where falling snow melts
and then encounters freezing temperatures at the surface. The
good news is that the deformation band looks to arrive closer to
late morning or afternoon. An earlier arrival around the early
morning hours would increase the chances for light freezing rain.

Monday through Thursday: The upper level low pressure system
offshore will dive south into southern OR/northern CA on
Monday/Monday Nt. This will pave the wave for a building ridge of
high pressure that will shift in over the region by mid week.
Strong subsidence will take place that will increase low level
inversions with light or calm winds. Fog or low stratus will
likely blossom over the region. Mixing potential will likely start
out poor as well as fog and stratus first develops, but may
improve slightly each day if the stratus slowly lifts through the
latter half of the week. An Air Stagnation Advisory will need to
be considered as we head into next week. /SVH


00Z TAFS: Areas of dense fog and low stratus will impact Central
WA including Moses Lake and Wenatchee. There may be some brief
improvement btwn 00-02z but expect any breaks will be short-lived
and quickly fill back in. Look for a band of light wintry pcpn
to spread over these terminals aft 20z Fri. Snow looks to be the
main pcpn type for Wenatchee with a light wintry mix for Moses
Lake. Further east, the main concern will be whether stratus or
fog develops this evening and early overnight before east winds
incr. Confidence is moderate for east winds to deliver VFR skies
for much of Ern WA and N ID but timing carries moderate
uncertainty and thus lower confidence. Light S/SW boundary layer
winds prior to the wind switch will favor stratus for Spokane-Cd`A
but cannot rule out areas of fog at this point either, especially
around the more sheltered Felts Field. /sb


Spokane        28  35  25  32  25  34 /  10   0  10  30  20  70
Coeur d`Alene  28  36  24  34  24  35 /  20   0  10  30  40  60
Pullman        28  38  27  34  27  35 /  10   0  10  40  30  70
Lewiston       31  40  28  36  27  38 /  20   0  10  30  30  50
Colville       29  36  25  32  27  34 /  50  10  10  40  50  60
Sandpoint      28  36  25  32  26  34 /  40  10  10  30  50  40
Kellogg        27  35  25  32  25  32 /  30  10  10  30  40  60
Moses Lake     28  35  24  33  24  35 /  20  40  50  20  20  80
Wenatchee      27  33  26  32  24  32 /  50  50  70  50  50  80
Omak           29  35  26  34  25  35 /  50  30  40  60  60  70



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