Winter Storm in Washington, November 12 to 14, 2020

Area Forecast Discussion
National Weather Service Pendleton OR
630 PM PST Thu Nov 12 2020

.UPDATE...Changes were made to the forecast to add wind
advisories for the Lower Columbia Basin, Blue Mtn Foothills, the
Kittitas and Yakima Valleys, Simcoe Highlands, the eastern
Columbia River Gorge, and North Central Oregon. Areas of blowing
and drifting of snow was also added to the mountain zones.
Although models have backed off on the winds compared to several
days ago, the tight pressure gradients will cause very windy
conditions to develop in the advisory areas. HRRR and other high
resolution models are showing 10m winds around 25 kts, and there
are hints of isentropic descent that could bring 50kt winds at
850mb near the surface. No changes were made to the current winter
storm warnings and winter advisories. Wister

&&

.PREV DISCUSSION... /issued 223 PM PST Thu Nov 12 2020/

SHORT TERM...Tonight through Saturday...A weather system will move
into the area overnight initially as a weak warm front followed by a
cold frontal passage on Friday. This system will bring some
significant snow accumulations in the mountains where winter warning
and advisories will be in effect. Will also see some increasing
southerly winds overnight warranting a windy advisory for the
southern Grande Ronde Valley. There could be some blowing and
drifting snow issues late tonight and early Friday in the southern
Grande Ronde valley. The cold frontal passage Friday will bring some
breezy to windy conditions across the entire forecast area through
Friday night then decreasing slowly Saturday. Precipitation will
taper off to showers Saturday mainly over the mountains. Another
system will arrive from the west late Saturday and overnight as a
warm front. Precipitation will be snow above 2000-3000 feet
initially Saturday afternoon and then rising overnight especially in
central Oregon. Still there will be some more snow accumulation
before changing to rain going into Sunday. More highlights may be
needed for this event but will hold off on any until current
highlights have ended.

LONG TERM...Sunday through Thursday...models in fairly good
agreement for the long term. Moist westerly flow for Sunday with
mountain rain/snow showers expected with mainly dry and locally
breezy conditions in the basin. Highs 40s to low 50s. Rain and high
mountain snow chances increase Sunday night into Monday with strong
warm air advection over the region and plume of deep Pac moisture
working onshore and lifting north. Precip expected to end south to
north Monday night as ridging builds in. Flow turns more southerly
on Tuesday as the ridge shifts east as another upper level
trough/low approaches the coast. Mild temps for Mon/Tue with highs
generally in the 50s. Some breezy southerly winds possible, esp in
the Grande Ronde valley and the foothills of the Blues/Wallowas. The
next system is expected to impact the region Wednesday into
Thursday. Models showing mainly rain and high mountain snow
overspreading the region Wednesday afternoon as deep Pacific
moisture working from the south and front passing through. Precip
expected to become more showery Thursday into Friday as the upper
trough is expected to work overhead. Snow levels remain fairly high
only expected to drop down to 4000-5500ft. Slightly cooler Wed/Thu
with highs in the mid 40s to lower 50s.

AVIATION...00Z TAFs...VFR conditions with increasing mid to
high level cloudiness today. Winds will be less than 10kts as well.
An approaching system will overspread showers tonight with light to
moderate rain expected for most of tomorrow. MVFR conditions
possible, esp as CIGS drop down to 2-3kft. Mountain obscuration
likely. Winds will increase 16-22g24-30kt tomorrow. Pockets of
strong low-level wind shear possible starting late tonight, esp near
BDN/RDM and PDT.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  37  51  35  47 /  60  90  30  10
ALW  38  51  37  49 /  60  90  40  10
PSC  39  54  39  52 /  50  60  20  10
YKM  32  51  32  50 /  30  60  20  10
HRI  38  55  39  52 /  40  70  20  10
ELN  29  45  31  44 /  40  60  30  10
RDM  35  49  30  44 /  20  90  60  40
LGD  32  42  31  40 /  40 100  70  30
GCD  35  43  30  38 /  30 100  70  40
DLS  39  51  39  50 /  50  90  50  30

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning until 4 AM PST Saturday for ORZ509.

     Wind Advisory from 4 PM Friday to 4 AM PST Saturday for ORZ041-
     044-507-508-510.

     Winter Weather Advisory from 10 PM this evening to 4 AM PST
     Saturday for ORZ049-050-506.

     Winter Storm Warning from 10 PM this evening to 4 AM PST
     Saturday for ORZ502-503.

     Wind Advisory from midnight tonight to 3 PM PST Friday for
     ORZ049.

WA...Winter Storm Warning until 4 AM PST Saturday for WAZ520.

     Wind Advisory from 4 PM Friday to 4 AM PST Saturday for WAZ024-
     026>029-521.

     Winter Storm Warning from 10 PM this evening to 4 AM PST
     Saturday for WAZ030.

&&

SHORT TERM...85
LONG TERM....84
AVIATION...84