Heavy Snow in Oregon, February 11 to 13, 2021

Area Forecast Discussion
National Weather Service Pendleton OR
758 AM PST Thu Feb 11 2021

.UPDATE...Snow has already begun over most of OR, and portions of
southeast and south-central WA. This the beginning of the first
in a series of winter storms that will move across the region
today through the weekend and into early next week. Many winter
weather highlights are currently in effect. The only areas that
so far do not have any warnings or advisories are the WA Lower
Columbia Basin, and the Yakima/Kittitas Valleys. These may be
added later as new guidance arrives and confidence is increased
whether or not a highlight is needed in these none-highlighted
areas. May also need to upgrade some or all Winter Weather
Advisory areas when new data arrives and confidence is increased.

Very cold temperatures with locally breezy north to northeast
winds will drop southward into the forecast area with an arctic
cold front. The convergence zone with warmer moist air over-
running the colder air to the north is what is causing the current
snow that is falling. Wind chill values will become very cold,
with readings around zero +/- a few degrees. Highs will likely
not get above freezing today in most of the CWA, with the warmest
areas being in Central OR and also the Lower Columbia Basin. Then
on Friday all areas will be below freezing with highs in the teens
to 20s. At night temperatures will plummet to the single digits
to teens. Current forecast is on track, but may need to adjust a
little for the earlier than expected start of the snow over
eastern OR/southeast WA. 88

&&

.PREV DISCUSSION... /issued 333 AM PST Thu Feb 11 2021/

SHORT TERM...
Today through Friday night...The first in a series of winter
storms that will affect the area will begin to impact the region
today. An area of low pressure in the Pacific ill move southeast
toward the northern California coat by tonight. As it progresses,
its associated warm front will extend across northern oregon and
southern Washington and moisture will increase. Snow will begin to
develop first possibly along the front, then along the Cascades
as the low advances and then over a good portion of the region by
afternoon.

Winter headlines have been expanded to include the Grand Ronde
valley and Wallowa County, and the Southern Blue Mountains have
been upgraded from an advisory to a warning. The Yakima Valley,
Kittitas Valley and even northern portions of the Columbia Basin
look to receive the least amounts of snow from this system. Snow
will continue into tonight before decreasing and possibly ending,
especially across the Basin on Friday before precip chances
increase again going into Friday night in advance of the next
system.

As it gets closer, we will have to look at extending headlines
further in time and further upgrades as an additional round of
snow could cause areas currently in advisories to reach warning
criteria or necessitate advisories or warnings for locations with
no headlines, but there is enough time right now to see how
things develop, how much snow is received from the first system
and what changes additional guidance may bring.

High temperatures today look marginal especially across central
Oregon and portions of the Basin and this will have to be watched
to see if these temperatures near freezing verify, as these can
affect daytime accumulations. Today will see most areas in the 20s
to lower 30s for highs, which will be 8 to 10 degrees warmer than
Friday's high temperatures.

Friday still looks to be the coldest day with many areas not
getting out of the teens and most struggling to reach into the
20s. Overnight lows will be in the single digits and teens.

It still looks like there will be breezy winds, mainly across
portions of the Basin and Gorge and higher terrain, which could
cause blowing and drifting of snow and very cold wind chills.

LONG TERM...
Saturday through Thursday...The extended will see the continued
pattern of very active weather as a series of moist systems pass
through the PacNW, each potentially bringing impactful snow
amounts across the CWA. After another very cold day on Saturday,
expect a warming trend into next week.

For the weekend onward, models are coming into better agreement.
General trend is for the large polar low over south-central Canada
to slowly drift east, allowing for a return or more onshore flow
aloft over the PacNW. This will allow for snow chances each day as a
series of potent shortwaves work onshore. First shortwave low/trough
is expected to work onshore Friday night into Saturday as shortwave
ridging passes overhead and shifts east of the region, allowing for
southwest flow to pull ribbon of deeper Pacific moisture into
eastern OR/WA. Plenty of cold, Canadian air in place through the
column that will allow all snow across the CWA. Stratiform snow will
transition to showers later Saturday into Saturday night as the
low/trough work overhead. Cold through Saturday night with highs Sat
in the 20s to low 30s with lows Sat night in the upper single digits
to mid teens. Expect a lull in activity Sunday with region under
northwest flow aloft, but could see a few upslope snow showers
showers. Slightly warmer as well on Sunday as the arctic airmass
shifts east and we begin to see a return of more moderate Pacific
air with highs in the mid 20s to mid 30s expected. The next
shortwave expected early next week with more snow and possible low
elevation rain. Models again in good agreement with this system with
upper low/trough working onshore near the WA/BC border and plume of
Pacific moisture overrunning the CWA late Sunday night into Monday.
One thing to watch will be if cold air near the surface lingers, esp
if ground is covered in abundant snow. If it is persistent, could
see potential for freezing rain, esp Sunday night into Monday
morning. Best potential for light ice accumulations looks to be
central OR. As of now, expecting warmer temps Monday with highs in
the low 30s to low 40s. Lingering upslope showers expected on
Tuesday with moist northwest flow behind the departing low/trough.
Models hint at precip diminishing Wednesday with ridging trying to
build onshore. Yet another moist Pacific shortwave is projected to
work onshore and spread yet more snow and possible low elevation
rain across eastern OR/WA on Thursday. Warmer Tue through Thu with
highs in the mid 30s to mid 40s. For the extended, trended highs and
low down 2-3 degrees each day from NBM, as ensemble guidance remains
colder and with abundant snow cover expected, which may inhibit
abundant warming.

AVIATION...12Z TAFs...conditions will deteriorate today as a
winter storm approaches from the west and begins a prolonged
period of snowfall, with worse conditions at OR sites and ALW.
DLS/PDT/ALW will see snow beginning early to mid morning, with
BDN/RDM beginning late morning to midday. YKM/PSC may see
occasional light snow, especially later tonight. IFR or lower
VIS/CIGs a strong possibility under the snowfall. MVFR and brief
IFR conditions possible at PSC/YKM tonight. Winds increasing to
10-15kts today, slightly higher at PSC.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  23  11  18  14 /  70  80  40  60
ALW  27  14  24  17 /  50  70  40  50
PSC  29  16  26  17 /  40  50  20  50
YKM  27  18  24  17 /  40  30  30  60
HRI  33  16  24  17 /  60  60  30  60
ELN  24  14  23  14 /  30  20  30  50
RDM  33   8  19  11 /  70  90  40  70
LGD  27  16  22  16 /  70  70  40  60
GCD  35  18  27  23 /  60  80  30  70
DLS  33  19  23  16 /  70  80  60  80

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning from noon today to 10 PM PST Friday for
     ORZ041-049-502-503-507>511.

     Winter Weather Advisory from noon today to 10 PM PST Friday for
     ORZ044-050-505-506.

WA...Winter Storm Warning from noon today to 10 PM PST Friday for
     WAZ024-029-030-520.

     Winter Weather Advisory from noon today to 10 PM PST Friday for
     WAZ521.

&&

SHORT TERM...88
LONG TERM...84
AVIATION...84