Heavy Snow in Oregon, November 27 to 28, 2022

Area Forecast Discussion
National Weather Service Pendleton OR
246 AM PST Sun Nov 27 2022

.SHORT TERM...Today through Tuesday night...Snow is beginning to
fall over the central WA Cascades at this time as a strong
Pacific weather system begins to move into the region. There are
currently a variety of winter weather highlights across the CWA
from Winter Weather Advisories to Winter Storm Warnings. Winter
Storm Warnings are in effect for both the OR and WA Cascades/east
slopes, and for the Blue Mountains of both OR and WA (both
northern and southern Blue Mountains). These highlights all look
good with heavy snow likely reaching warning or advisory criteria.

The NBM v4.1 24 hour snowfall probabilities for both the OR and
WA Cascades show between an 80-100 percent chance of snowfall
amounts exceeding 10 inches. Then for the Northern Blue Mountains,
it shows between an 80 to 100 percent chance of snowfall amounts
exceeding 8 inches for the Northern Blue Mountains, and between an
80 to 100 percent chance for snowfall amounts exceeding 6 inches
for the Southern Blue Mountains. Storm total snow amounts for all
these areas are expected to reach warning criteria by mid morning
to mid afternoon Monday. Will therefore continue the Winter Storm
Warnings in place for these areas.

For Wallowa County and the Ochoco-John Day Highlands, the NBM
v4.1 snowfall probabilities of exceeding greater than 4 inches is
between 60 and 90 percent. However, the storm total snowfall
amounts are expected to reach advisory amounts for these areas.
Will therefore leave the Winter Weather Advisory in place for
Wallowa County, and will upgrade the Winter Storm Watch to a
Winter Weather Advisory for the Ochoco-John Day Highlands.

Snowfall amounts for the Grande Ronde Valley are marginal by the
NBM v4.1 24 24 hour snowfall probabilities, with it showing an
80-100 percent chance of only exceeding 2 inches, but not 4
inches. However, the storm total snow amounts for the Grande Ronde
Valley do reach advisory amount for parts of that zone. Will
therefore leave the advisory in place for the Grande Ronde Valley
for now. However, the next couple of shifts will need to watch
the Grande Ronde Valley in case the advisory needs to be
cancelled. For the John Day Basin, the NBM v4.1 24 hour snowfall
amount probabilities only reach a high percentage for 2 inches of
snow, but a very low percentage of 4 or more inches. Also, the
storm total snowfall amounts are expected to only reach between 1
to 2 inches. Will therefore cancel the Winter Storm Watch for the
John Day Basin, and not issue an advisory for that zone.

For central/north central OR, and the Blue Mountain Foothills
(both north and south) there will likely be some light snow
accumulations of a half inch to as much as 2 inches possible.
Elsewhere, there will likely be some snow mixed with rain, but
with no snow accumulations expected. This is in line with the NBM
v4.1 24 hour snowfall probabilities for all these areas and also
with the storm total snowfall amounts expected. Therefore, will
not consider any winter weather highlights at this time for these
areas, as amounts are not expected to reach advisory criteria.
This could change though with just a small change in temperatures
or precipitation distribution or a combination of both, and will
therefore need to keep a close eye on these areas. This is
mainly the case for central OR and the Blue Mountain Foothills
where some light snow accumulations are expected.

The next concern will be increasing winds today and tonight, with
wind speeds increasing to the breezy to windy category across
most of the forecast area. This will cause blowing and drifting of
snow, especially where temperatures are colder in the advisory
and warning areas. Elsewhere, the snow will likely be too wet for
much blowing or drifting until colder air moves in. However, by
that time the winds will likely have decreased too much to cause
any blowing or drifting snow. The strongest winds will be on
exposed ridge tops across the entire forecast area.

Temperatures today will start out mild with highs in the 40s to
lower 50s in the lower elevations, and in the 30s in the
mountains. Much colder temperatures are expected on Monday with
highs ranging from the mid to upper 30s in the lowest elevations
and 20s in the mountains. On Tuesday, temperatures will be even
colder with highs ranging from the upper 20s to mid 30s lower
elevations and teens and 20s in the mountains.

This weather system will begin moving out of the forecast area by
Monday evening and on Tuesday. However, there will be lingering
upslope snowfall over the Blue Mountains and Foothills, which may
add to accumulations. However, the snow will end in all areas,
except for some additional upslope snow over the northern OR
Cascades, by late Monday evening. On Tuesday, the forecast area
will be under a dry northwest flow aloft. Then snow will increase
again over the Cascades by late Tuesday afternoon/evening and
continue overnight as the next weather system begins to move into
the region going into the extended forecast period. 88

.LONG TERM...Wednesday through Sunday...Messy pattern in store for
the forecast area much of the period as ensembles continue to
suggest a prolonged period of NW flow aloft, accompanied by a train
of upper-level systems that will provide for a continuous chance of
precipitation. Whether and how much of this precip reaches outside
the mountains remains to be seen, as well as how cold it well get
and if it'll be cold enough to support lower-elevation snow.

Both ensemble and deterministic guidance seem to agree that a system
much of the day Wednesday will be characterized by a west to
southwesterly flow aloft as the trough develops a bit of a positive
tilt as it digs down from off the coast of BC. Such a scenario would
support heavy mountain snow with some distinct shadowing across the
Basin, maybe a bit more low-elevation QPF with more SW flow, but
guidance either way suggesting no more than two tenths in the
foothills, and less than a tenth across the lower Basin. Even then,
a more SW flow solution would cut greatly into potential snow
amounts in the lower elevations by increasing snow levels up into
the 2000-3000 ft range. The Yakima/Kittitas Valleys may be more
sheltered by this WAA, but even then, QPFs across guidance aren't
too bullish on precip outside of the mountains. Overall messaging
for Wednesday's system for now will thus be more for a heavy
mountain snow event, with light precip possible elsewhere, either in
the form of light snow or rain.

Synoptic pattern then becomes a bit more complex after the midweek,
but judging by deterministic guidance, looking like potential
sequence would be Wednesday's system, still centered just off the
Pacific coast, would be absorbed by another oncoming system from the
Gulf of Alaska, before this newly formed system moves inland and
deepens overhead, which would lead to widespread cold air advection
by the end of the work week. Models had previously been out of sync
after Wednesday, but they seem to be trending more towards this
solution based on ensemble clustering, so looking like a cold air
outbreak is likely by Thursday night into Friday morning. That
being said, models seem to have backed off on the magnitude of the
CAA, favoring teens into lower 20s for overnight lows Friday
morning rather than widespread single digits that had been seen
across earlier runs. Such an outcome would also result in limited
QPF, even in the mountains, however the persistence of the precip
would still lead to impacts across our area's mountain passes.

Heading into next weekend, ensembles suggest yet another system that
could bring another round of precip to the CWA, but they disagree
on the exact timing. Won't speculate too much on the impacts of
this system that far out, but looking like the forecast area,
especially our mountain zones, will see little in the way of
relief when it comes to precipitation chances through the week.
Evans/74

&&

.AVIATION...12z TAFs...Expecting mostly VFR conditions through the
period, however an upper-level system will bring two rounds of
precip to the forecast area, with the second round more likely to
bring impacts to TAF sites. Models suggesting this first round of
precip, in the form of rain, will impact primarily ALW/PDT, with all
sites seeing gusty conditions through the day Sunday. Expect W/SW
winds 15-20 kts gusting up to 35 kts at times. Mid-level sct-bkn
cigs most of the day, except for lower cigs under rain for PDT/ALW,
before widespread bkn-ovc cigs less than 7 kft by Sunday night as
the second round of precip brings a chance of rain or snow to all
sites. Evans/74

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  45  27  35  20 /  50  50  60  20
ALW  46  29  36  20 /  60  60  60  20
PSC  48  29  38  19 /  20  20  40  10
YKM  46  20  38  14 /  30  20  30  10
HRI  50  30  39  20 /  30  30  50  10
ELN  42  21  35  13 /  40  30  30  10
RDM  46  25  36  16 /  30  50  80  10
LGD  40  24  32  14 /  80  70  80  20
GCD  43  24  32  14 /  50  80  80  20
DLS  48  34  43  25 /  60  70  70  20

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 7 AM this morning to 4 PM PST
     Monday for ORZ049-050.

     Winter Storm Warning from 7 AM this morning to 4 PM PST Monday
     for ORZ502-503.

     Winter Weather Advisory from 10 PM this evening to 4 PM PST
     Monday for ORZ506.

     Winter Storm Warning until 10 AM PST Monday for ORZ509.

WA...Winter Storm Warning from 7 AM this morning to 4 PM PST Monday
     for WAZ030.

     Winter Storm Warning until 10 AM PST Monday for WAZ520.

&&

SHORT TERM...88
LONG TERM....74
AVIATION...74