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Area forecast discussion 
National Weather Service Spokane Washington
237 PM PST Monday Mar 2 2015

Synopsis...
a weather system will pass through this evening, with potential
for scattered snow showers along with breezy conditions across
much of the region. The snow shower threat and winds abate
overnight into Tuesday. Much colder air moves in behind this
system, with temperatures well below average for the middle of the
week. A dry period, with moderating temperature will follow for
the end of the week.

&&

Discussion...

Tonight and tuesday: the snow shower threat will dissipate
across the inland northwest tonight on a breezy north wind, leading to a
drier and colder Tuesday. This evening a mid-level shortwave is
tracking across the eastern third of Washington and north id; it is
projected to sag toward central and southern Idaho late this evening
into the overnight. As that shortwave tracks by US it is
accompanied by a pool of colder 500mb temperatures, in the -30 to
-35 c range. This leads to some unstable lapse rates and so the
accompanying potential for convective showers. Furthermore, a
deformation axis/surface trough lingers across the eastern third
of Washington through the night, though it does lose some of its
resolution overnight.

All of this means a threat of scattered to locally numerous snow
showers through, especially early this evening, between 00-04z (4
to 8 pm), before the drier northerly flow take hold and the
precipitation threat begins to wind down. The deformation
axis/trough itself should help to focus those showers and right
now that been somewhat consistently across Northeast Mountain and
Spokane/c'da area.

The convective nature of the system makes pinning down the
precise snow accumulations difficult in the valleys. Values could
range from nothing or a trace to as much as 1 inch or so under the
heaviest bands. The Road and air temperatures, however, should
keep the snow from sticking to the roads and other surfaces
initially. However later this evening and overnight any
precipitation that falls would have every opportunity to freeze on
surface. How much of an impact that would have later this evening
and for the Tuesday morning commute is less certain, because areas
that would see little to no accumulation may have no problems but
if you happen to be that spot that gets more precipitation you be
more prone to slick spots. As for the mountains, snow amounts
could be in the 1 to 3 inch range, when all is said and done for
the entire system, with isolated higher amounts.

Late this evening and overnight, the main shower threat is
expected to shift toward the central Panhandle and Camas Prairie,
and possibly The Blues. A secondary threat will linger within the
northeastern mountains, with a weak secondary wave dropping by and
the weakening deformation axis in the region. Tuesday looks mainly
dry and partly cloudy to clear.

Winds will be an issue this evening with the shortwave pushing in
coupling with a tightening pressure gradients. The strongest winds
are expected through the Okanogan Valley, where some channeling is
expected. However speeds will also be up across the remainder of
eastern Washington and the north Idaho valleys. On average we are looking for
10 to 20 mph, with gusts to 30 mph. However in the Okanogan
Valley speeds of 15 to 30 mph with gusts to 40 mph are possible.
Going into the overnight and Tuesday the gradients begin to
slacken and the atmosphere decouples, so overall speeds should
begin to abate.

Temperatures tonight are expected to drop below normal thanks to
the push of colder, drier air from the north. Values could be some
10 or so degrees below average. This carries into Tuesday
afternoon with highs expected to remain below normal by 5 to 10
degrees. /J. Cote'

Tuesday night through monday: a ridge of high pressure will be the
dominate weather feature through the period delivering mostly dry
conditions and a gradual warming trend. The only exception is
within the Friday time-frame. Models suggest the ridge will
flatten enough allowing a weak disturbance to skirt the
international border before dropping southeast down the Montana/Idaho
border. The European model (ecmwf) is an outlier at this time indicating the
potential for a tenth or so of liquid while the GFS/Canadian/ensemble
means only bring through trace amounts of liquid. Needless to say,
confidence is low regarding this feature and precipitation chances
but this looks to be the only chance for precipitation through
Monday. Temperatures will start off cool with 850mb temperatures
on the order of -5c but warm near 3-8c by the weekend. This will
equate to warming temperatures each day with temperatures expected
to warm well above normal by the weekend.

There are signs in the extended models that a large scale pattern
change will be possible which features the reversal of the eastern
US trough and western ridge. There are some timing differences but
looks to occur around mid to late next week. Climate prediction
center looks to favor this change as well with the 8-14 day
outlook predicting the potential for warmer and wetter conditions
on the West Coast... conducive of the incoming trof and
mild/wetter southwest flow. /Sb




&&

Aviation...
18z tafs: incoming shortwave will increase snow showers this
afternoon, around the geg to Coe to puw/lws area. Some locally
heavier bands of snow are possible in the convective pattern and
incoming vort Max. Primarily VFR conditions are expected, but
local MVFR conditions are possible in the heavier bands. The
snow shower threat and clouds will dissipate after 03 to 07z.
Western taf sites are expected to see little other than flurries.
Winds will be on the increase through the afternoon and values
remain breezy through the evening, before abating overnight.
/J. Cote'




&&

Preliminary point temps/pops...
Spokane 18 37 17 43 23 48 / 50 0 0 0 0 0
Coeur D'Alene 16 37 14 43 21 49 / 50 0 0 0 0 0
Pullman 18 37 17 43 26 51 / 30 0 0 0 0 0
Lewiston 26 40 20 47 27 54 / 20 10 0 0 0 0
Colville 21 41 16 45 22 50 / 50 0 0 0 0 0
Sandpoint 17 34 12 41 21 44 / 50 0 0 0 0 0
Kellogg 17 32 13 40 22 45 / 70 0 0 0 0 0
Moses Lake 22 44 20 48 26 53 / 10 0 0 0 0 0
Wenatchee 27 45 20 50 29 54 / 0 0 0 0 0 0
Omak 22 45 19 47 24 52 / 0 0 0 0 0 0

&&

Otx watches/warnings/advisories...
Idaho...none.
Washington...none.
&&

$$

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