National Weather Service Spokane WA
510 AM PDT Fri Mar 13 2020

.SYNOPSIS...
Big weather changes arrive later today and last through the
weekend with blustery north winds, snow, and very cold
temperatures as an arctic front moves into the region. Poor travel
conditons expected Friday evening into Saturday. The work week
will start off dry and cold, but temperatures return to normal by
mid week.

&&

.DISCUSSION...

...MODERATE TO HEAVY SNOWFALL AND BLOWING SNOW EXPECTED ACROSS THE
INLAND NORTHWEST Friday INTO SATURDAY WITH FRIDAY EVENING COMMUTE
TO BE IMPACTED...

Today through Saturday night: This morning, a deepening low pressure
system dropping south from southern BC and then southeast across
the Gulf of Alaska and further deepen as it pushes closer towards
the Pacific Northwest. IR satellite imagery shows a deep moisture
field ahead of the upper trough that is advancing southeast then
extends east into the Inland Northwest. This system will exert its
influence across the region into the weekend.

The most favorable dynamics aloft will advance in force with a
deepening upper level low that will begin to exert its greatest
influence by this evening and continue to provide the necessary
combination of forcing and moisture flux across the region through
Saturday morning. That said, we will see continued low and mid-
level moistening across much of the northern half of the CWA with
an accompanying increase in isentropic ascent across northeast
Washington and the Northern Idaho Panhandle. As this plays out, a
tightening of temperature and pressure gradients will signal the
entrance of a strong arctic frontal boundary that increases a low-
level easterly downslope flow off of the higher terrain across
the northern zones. As the northeast winds ramp up across the
basin, orographics will then play an increasing role with regards
to snow production along the east slopes of the northern Cascades
which will continue through overnight hours. These favorable
dynamics aloft have the potential to overcome the drying affect
of the downslope flow off of the Panhandle Mountains and further
bring the best chances for snow further east across the basin
tonight into Saturday morning.

The Northerly winds will also increase today and peak overnight
into Saturday morning. The combination of strong winds and
fresh, relatively dry snow, will create the potential for patchy
blowing snow especially in the northern Idaho. A transient low
pressure circulation appearing as far down as 850 mbs in HIRes
model guidance will linger over the basin Saturday morning but
quickly fill during the subsequent afternoon. Snow will then
decrease with time with less accumulations by the afternoon hours
and surface convergence slows as well. Winds will continue to be
breezy adding to the threat of blowing snow into the afternoon as
cold Arctic air continues to filter down across the region.
Temperatures will be much colder than average with highs only
expected to top out in the mid 20s to lower 30s for many locations
on Saturday.

* Snow Amounts: Storm total of 3 to 5 inches of snow accumulation
  across the northern rim of the basin and into the mountains from
  the Cascades to the Northern Panhandle. Snow amounts will decrease
  further south across the basin into the Spokane/Coeur
  d`Alene corridor, Palouse and Central Panhandle Mountains with 1
  to 3 inches expected.

* Winds: Strongest wind gusts of 40 to 45 mph will be possible
  down the Purcell Trench with peak gusts occurring tonight
  into Saturday morning. The Okanogan Valley, Waterville Plateau
  and the basin will see wind gusts in the range of 35 to 45 mph.

* Impacts: A wide variety of impacts are possible with this late
  winter storm with possible impacts to this evening commute. Snow
  amounts alone will not be as large a concern than the increasing
  winds ablating fresh dry snow over parts of the CWA. The basin
  should see at least a couple of inches of snow, and that Coupled
  with the increasing winds could raise concerns for reduced
  visibility due to patchy blowing snow. The areas at most risk
  will still be down the Purcell Trench and down the Okanogan
  Valley and onto the Waterville Plateau. The potential for near .
  Blizzard like conditions are possible for those areas. Moderate
  to heavy snow is still expected across the Okanogan Highlands
  and Northeast Mountains as well. A winter storm warning has been
  Issued for these areas as well. Much of the rest of the Inland
  Northwest is covered with a winter weather advisory. A wind
  advisory has also been issued to address the windy conditions
  over northern Idaho Panhandle that continues west and southwest
  into the Columbia Basin.

* Wind Chills: Winds will be blustery with wind chills still
  expected To drop below zero across the Northern Panhandle both
  Saturday morning and again Sunday morning even though winds
  start winding down but ambient temperatures will remain mostly
  in the single digits. Single digit wind chills are possible down
  the Okanogan Valley and out across the basin through the weekend
  as well. /ayoung

Saturday night through Sunday:  With the passage of the arctic
front, brisk northeast to north winds continue, especially from
the Purcell Trench of north Idaho and across the Columbia Basin.
Patchy blowing and drifting snow will be a concern mainly through
Saturday evening. While the gustiness decreases late Saturday
night into Sunday morning, daytime mixing will bring a return of
the gusts for Sunday afternoon. Skies gradually clear Saturday
night from the north as snow showers chances remain on the upslope
side of the northern Blue mountains. Strong cold air advection
and radiational cooling will lead to one of the coldest nights of
the season, even more so where the winds are able to decouple.
Many record low temperatures may be broken by Sunday morning. The
track of the cutoff low looks slow to depart the region, and
theres the potential of wrap around moisture with an embedded
shortwave to brush southeast Washington Sunday. This would elevate
the snow shower chances into the lower Columbia Basin and
Palouse. Any additional snow amounts will be light given the drier
air mass. Daytime temperatures are expected to rise above
freezing in most areas.

Monday through Thursday: The cutoff low continues to sink further
south from Oregon into California, allowing a drier northerly flow
to invade the region. Winds aloft become light as a col sets up
over the region, while surface winds become light as well.
Temperatures profiles gradually moderate and should experience
warming each day. While daytime high return to normal by mid week,
the dry airmass will yield to sub freezing overnight lows for much
of the week. The low elevation snow pack will slowly erode. It
will remain dry for Monday and Tuesday, although could see a
return of patchy fog in the northern valleys. By mid week, the
models show the cutoff low opening into an upper trough as it
swings inland. Moisture creeps northward into southern Idaho and
Oregon and could see the potential of afternoon convection
developing as close as southeast Washington into the southern ID
panhandle on Wednesday and Thursday. /rfox.

&&