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afdotx

Area forecast discussion
National Weather Service Spokane Washington
403 PM PST Mon Dec 5 2016

Synopsis...
light snow is forecast for the Columbia Basin, Palouse, and
southern Idaho Panhandle tonight as a low pressure system moves
through the area, otherwise expect dry weather with some rather
colder temperatures Tuesday afternoon through Wednesday night.
More snow will be on the way Thursday into Friday, and this snowy
pattern may continue into the weekend.

&&

Discussion...

Tonight through Tuesday night...a small and compact closed area of
low pressure drifting across the cold airmass over the area will
spread snow slowly from west to east across a portion of eastern
Washington and northern Idaho tonight and into Tuesday morning...
primarily south of a line from Omak to Deer Park to Prichard. The
overall trend is for this compact low and associated bands of snow
slowly rotating counterclockwise around it to all translate west to
east tonight and Tuesday morning. 700-500mb lapse rates of the cold
and quite conditionally unstable airmass are quite steep and in the
8.0-8.0 deg c/km range which will aid in allowing for moderate snow
ratios generally near 15 to 1. Have favored the numerous high
resolution models coming in more frequently such as the hrrr which
have reasonable consistency in the positioning and movement of the
low and its associated snow bands. Greatest intensity snowfall
expected in the ongoing winter weather advisories during the early
afternoon and evening and highlights were issued to address the
higher intensity snowfall occurrence impact during the afternoon and
early evening commute rather than the overall quantity of snow.
Afterword snowfall intensity should decrease substantially but not
stop until the low and its associated snow bands move out of the
area and into northeast Oregon late Tuesday morning. Otherwise
behind the exiting low continually cold northerly flow of a dry and
cold Continental airmass allows the cooling trend to continue with
decreasing cloud cover and brisk northerly winds in the north to
south oriented northern valleys such as the Purcell Trench and the
Okanogan Valley down to the Waterville Plateau. /Pelatti

Wed through Mon nt: Wed and Wed nt will be the coldest
temperatures we've had the pleasure of enduring this "winter" so
far, with low and high temps generally ranging from the single
digits to mid teens, and mid teens to mid 20s, respectively. With
this antecedent very cold Continental polar air mass in place, it
will set the stage for the next round of snow and potentially
mixed pcpn thurs through Fri nt. The only significant changes to
the fcst were to follow the consistent model guidance trends of
slowing down the front for thurs and decreasing pcpn amnts.

Most model guidance is still trending closer to the European model (ecmwf) as far
as timing of the inbound front. This moist cool front (more like a
warm front from a sfc perspective) overtakes the region by thurs,
with plenty of isentropic ascent, steep lapse rates aloft, and
orographics helping to generate sufficient lift. Even though we
reduced pcpn amnts for the thurs to sun nt period, the very cold,
deep air mass in place will allow for 20-25:1 swe ratios in
general. Thus, it won't take much pcpn to produce significant
powdery snow accumulations. Confidence in our current predicted
snow accumulations are low to moderate. It will snow, however.
There's even the chance of mixed pcpn in the form of sleet or
freezing rain...but confidence is low. We'll focus heavily on
narrowing down the specifics of the fcst the next couple days. Bz

&&

Aviation...
00z tafs: the next wave will track by the Cascades, then through
srn Washington/nrn Oregon this evening through tues. Moderate snow has began
to fall at keat, with snow just beginning to reach the ground at
kmwh. Expect this snow to expand slowly east through tues morning,
lowering cigs and vsbys to IFR for keat/kmwh/kpuw through Tuesday
morning. The ern taf sites may not see steady snow until after
03-06z. The precipitation threat gradually starts to wane
overnight and especially Tuesday morning from the west. The
Spokane area tafs will likely fall to MVFR.

&&

Preliminary point temps/pops...
Spokane 18 24 9 20 11 23 / 50 20 0 0 10 50
Coeur D'Alene 16 23 6 20 8 22 / 20 20 0 0 10 50
Pullman 21 26 12 25 15 26 / 80 50 10 0 10 60
Lewiston 23 30 16 26 16 30 / 80 40 10 0 10 60
Colville 16 24 9 20 9 22 / 20 0 0 0 0 40
Sandpoint 13 20 3 16 5 21 / 10 0 0 0 10 30
Kellogg 10 18 3 15 4 21 / 30 20 10 10 10 50
Moses Lake 22 31 13 26 15 26 / 80 40 0 0 10 70
Wenatchee 23 30 13 24 15 24 / 80 20 0 0 10 70
Omak 18 26 9 22 11 23 / 60 0 0 0 0 60

&&

Otx watches/warnings/advisories...
Idaho...none.
Washington...Winter Weather Advisory until 7 am PST Tuesday for Waterville
Plateau-Wenatchee area.

Winter Weather Advisory until 10 am PST Tuesday for Moses Lake

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