A warm and cold front passage followed by a deep Pacific low moving across the region starting during the day on Dec. 29th through Dec.
Every event with a place to put it; 1 filed by county or forecast zone are drawn at its center, in a lighter ring. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS PDT wrote it up.
A warm and cold front passage followed by a deep Pacific low moving across the region starting during the day on Dec. 29th through Dec. 31st produced heavy snow along the Washington Cascade east slopes and along the Northwest Blue Mountains of Washington, which resulted in accumulations of 12 inches and more being reported in those areas.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Pendleton, OR wrote its last area forecast discussion before the first event at Dec 30, 3:46 AM PST, 1 hour 14 minutes earlier.
Area Forecast Discussion
National Weather Service Pendleton OR
346 AM PST Wed Dec 30 2020
.SHORT TERM...Today through New Years Day...A warm front has
brought a wintry mix of snow, sleet, and light freezing rain across
the Lower Columbia Basin and the Blue Mtn Foothills at this time.
Both road Temperatures and air temperatures range from 30-35F,
therefore the ice accumulation on roads will be spotty. As the air
mass continues to warm later in the morning, the wintry mix will
change over to rain. The Yakima Valley will likely observe a
wintry mix in the next several hours, and this was included in the
forecast. Deeper colder air in the Kittitas Valley and the east
slopes of the WA Cascades will keep precipitation in the form of
snow.
Heavy snow has fallen along the WA Cascades and will likely
continue for most of the day. A cold front will arrive to the
Cascades this evening, and snow will decrease in intensity after
the frontal passage. The current Winter Weather Advisory for the
WA Cascade east slopes will continue with accumulations of 6-12
inches expected. A Winter Weather Advisory was extended to include
the OR side of the eastern Cascades for 4-8 inches. The advisory
for the northern Blues will be upgraded to a Winter Storm Warning
and will be extended through Thursday afternoon. A northwest flow
behind the cold front will bring moderate to heavy snow to the
Blue Mountains on Thursday. An upper level ridge building over
WA/OR on Thursday will bring drier conditions to most of the
forecast area by the afternoon with the exception of the Blue
Mountains and foothills, Wallowa County, and the WA/OR Cascades.
Precipitation will end on New Years Eve. Although there will be
increasing subsidence during this time, overcast skies will keep
fog and freezing fog from developing.
New Years Day is looking mostly quiet weather wise except for the
increasing chance of snow along the Cascades, mainly north of Mt.
Jefferson. The Grande Ronde Valley will be very windy, and the
Blue Mtn Foothills east-southeast of Pendleton may also have
breezy to windy conditions. Afternoon temperatures on New Years
Day will be in the mid-30s to mid-40s. Wister
.LONG TERM...Friday night through Tuesday. Active pattern as a
strong jet over the Pacific will continue to push weather systems
across the region. Models are in reasonable agreement with the
overall pattern with some differences in timing. Upper level
trough moves across the area Friday night followed by a stronger
system Sat night into Sunday. Another strong trough then moves
through the Pacific northwest Mon into Tue. Most of the extended
period will be cloudy with periods of rain and mountain snow.
Breezy to windy conditions likely as well with surface lows
resulting in gusty southwesterly winds at times. Fairly mild
airmass with snow levels 3000-5000 feet and temperatures 5-10
degrees above normal levels. 94
.AVIATION...12Z TAFS. Mixed precipitation this morning at PDT ALW
PSC YKM DLS with light snow, freezing rain and rain likely.
Warming temperatures this afternoon with mostly rain. RDM BDN warm
enough for all rain today. Ceilings 020-050. Winds 5-10kt except
RDM BDN with stronger southwest winds 10-20kt. 94
&&
.PRELIMINARY POINT TEMPS/POPS...
PDT 41 34 45 31 / 70 50 50 10
ALW 41 35 46 33 / 80 60 60 10
PSC 40 35 45 33 / 70 30 20 0
YKM 36 28 41 30 / 60 30 20 10
HRI 40 35 48 33 / 60 40 30 0
ELN 34 27 40 30 / 70 30 20 10
RDM 46 31 46 29 / 60 60 10 0
LGD 33 29 40 26 / 70 70 70 20
GCD 42 32 42 28 / 40 70 40 10
DLS 40 36 48 35 / 80 60 40 10
&&
.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning until 3 PM PST Thursday for ORZ502.
Winter Weather Advisory until 4 AM PST Thursday for ORZ509.
WA...Winter Storm Warning until 3 PM PST Thursday for WAZ030.
Winter Weather Advisory until 4 AM PST Thursday for WAZ520.
&&
SHORT TERM...85
LONG TERM....94
AVIATION...94The national radar mosaic for the moment the first event began.
2 heavy snow. Local standard time, as filed.
A trained spotter 1 mile WSW of Ski Bluewood, WA recorded 12 inches of new snow between 5am PST on 12/30 and 5am PST on 12/31.
A co-op observer 2 miles SSE of Snoqualmie Pass, WA recorded 14 inches of new snow between 6am PST on 12/30 and 6am PST on 12/31. A Cocorahs observer 6 miles NW of Easton, WA measured 14 inches of new snow between 7am PST on 12/30 and 7am PST on 12/31.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Washington in 2020Every state in 2020This day in weather historyWarnings in Washington, December 2020
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 928909, 928908.