Heavy Snow in Washington, December 1 to 3, 2023

Area Forecast Discussion
National Weather Service Pendleton OR
1004 PM PST Thu Nov 30 2023

.EVENING UPDATE...The forecast remains on track this evening as a
band of light snow associated with our first upper-level
disturbance continues to progress through the inland Pacific
Northwest. So far, reported amounts have been fairly light with
generally inch or less across the lower Columbia Basin into the
base of the Blue Mountains. Going forward, high resolution
guidance like the HRRR and HREF is in good agreement in a brief
reprieve from the light snowfall outside of the mountain areas
overnight before a second stronger shortwave trough quickly
increases precipitation Friday morning. It still appears
developing west-southwesterly winds Friday will help to score out
much of the cold air entrenched in the lower Columbia Basin but
forecast confidence in regards to precipitation timing and type is
only low to moderate at this time, especially during the morning
hours when the transition begins to take place. Otherwise, the
impacts are still focused on rather heavy mountain snowfall
through the weekend. If you plan on traveling over the mountain
passes the next several days, please be prepared for potentially
difficult winter driving conditions! The previous discussion
remains below. Schuldt/99

.PREV DISCUSSION... /issued 309 PM PST Thu Nov 30 2023/

SHORT TERM...Tonight through Saturday...Weather pattern continues
to undergo a change from the dry ridging pattern to a more active
and wet pattern through the weekend. This is currently taking
place as the first in a series of systems is slowly moving into
the forecast area switching us to a northwest flow pattern. This
system is weakening as it pushes into the Pacific Northwest but
will still produce some light precipitation as it moves across the
forecast area late today and overnight before exiting Friday
morning. Low snow levels combined with the cold air pool trapped
in the Columbia Basin means the precipitation type will be
primarily snow but with just light accumulations expected. Expect
a dusting to up around an inch in the lower elevations while
mountains could see 2 to 4 inches.

The next system in the northwest flow arrives Friday with more
abundant moisture and increasing winds. These winds will aid in
scoring out the cold air pool trapped in the lower elevations
resulting in a change from snow to rain during the day. The
mountains will begin seeing more significant snow accumulations
associated with this system with snow levels 2000 to 3000 feet.

Yet another system is expected to pass over the area on Saturday
into early Sunday as a warm front. In addition to the expected wet
conditions this system will begin to raise snow levels through the
day and overnight to around 3000 to 4500 feet Saturday afternoon and
then 4000 to 6000 feet Saturday night. This will lift snow levels
above most pass levels in the forecast area. Will also see some
breezy to windy conditions which will approach advisory levels and
will be monitored for the need for a possible advisory. Also,
numerous Winter Storm Warnings and Winter Weather Advisories remain
in effect through Sunday morning.

LONG TERM...Sunday through Thursday night...The long term will be
characterized by a active weather pattern. Two separate
atmospheric river events will bring plenty of precipitation to the
area. These systems will also bring warmer temperatures back to
the region with snow levels rising back to above 7000ft by
midweek. This will allow the precipitation to transition back to
rain with the exception of the higher peaks. With the rain on snow
occurring, there may be some hydrologic concerns that arise. This
will continue to be monitored over the course of the two
atmospheric river events.

Sunday through Monday models are in firm agreement with the
continuation of the atmospheric river event. The continuation of the
event will bring with it more precipitation as well as warmer
temperatures. Total QPF Saturday to Sunday will near two inches or
more along the Cascades with 0.3 inches along the Cascades
foothills, 0.10 inches through the Columbia Basin and increasing to
0.5-1 inches along the foothills of the Blues and Blue Mountains.
Probabilities of these accumulation amounts are above 85% for the
Cascades and foothills of the Cascades, 75% through the Columbia
Basin, and 50-80% for the foothills of the Blues and Blue Mountains.
The beginning of the period will see a rain snow mix along the
Cascades, transitioning to rain as you head farther east then a
rain/snow mix through the Blues as the snow levels will be around
4000ft and lifting to above 7000ft through Monday. EFI is showing
the precipitation amounts to be well above seasonal (0.9-0.93) and
signaling temperatures to be above average temperatures for this
time of year (0.79-0.8) with the 39% of the ensembles showing
temperatures in the 50-55 degree range and 26% showing temperatures
of 55-60 degrees. These temperatures are 10-15 degrees above the
average across the region.

As we move into Tuesday, models begin to diverge slightly but mostly
with the amplitude of the next incoming system. Models show a brief
ridge will move over the region however, precipitation is still
expected with this system as another atmospheric river has set
itself up to flow across the region. The western and northern
sections of the CWA will be the areas that will see the highest
chances of rain. Again, warmer temperatures will accompany the
system and snow levels will crest 8000ft, so all precipitation will
fall as rain with the exception of the peaks of Wallowa Mts. Total
24 hour QPF amounts show that there is an 82% chance that the WA
Cascades will see an  additional 2 inches of rain, 25-35% chance of
the Columbia Basin and Gorge will see an additional 0.25 inches and
40-6-% chance that the foothills of the Blues and Blue Mts will also
see an additional 0.25 inches. EFI continues to show above average
QPF amounts as well as temperatures. 54% of the ensembles show that
temperatures will be in the 50-55 degree range again on Tuesday.

Wednesday through Thursday night, models continue to diverge ever so
slightly. These slight discrepancies are not enough to drop
confidence in the forecast. The models show the continuation of the
atmospheric river flowing into the region with the main variance
being the positioning of the axis. Even with these variances, all
the models as well as the ensembles show the continuation of
precipitation. Precipitation will continue to be rain as the snow
levels remain above 6000-8000ft with warm temperatures across the
region. EFI is continuing to show above average temperatures as well
as above average QPF for this time of year. However, rain amounts do
dwindle and remains primarily locked along the mountains where there
is above 50% chances the Cascades and above 40% chances the Blues
could see an additional 0.25 inches of rain. Temperatures continue
to be above normal however slightly cooler than previous days with
42% of the ensembles in agreement of 40-45 degrees. Snow levels will
decrease again to 3000-5000ft so the precipitation along the
Cascades will fall as a rain/snow mix while remaining rain
elsewhere. Bennese/90

&&

.AVIATION DISCUSSION...06Z TAFs...A band of light snow continues
to produce a mix of LIFR to MVFR conditions at PDT, ALW, and PSC
for the next couple of hours before we enter a quick lull between
weather disturbances headed into early Friday morning. However
confidence is high degraded cigs and vis will persist overnight
for all sites excluding RDM and BDN. Expect another round of
wintry precipitation to arrive Friday morning before this second
disturbance exits the region in the afternoon - again categories
will mainly range from MVFR to IFR although brief periods of LIFR
can't be ruled out in heavier precipitation. BDN, RDM and PDT will
see winds increase for the afternoon hours with gusts of 20-30
kts likely. 99

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  28  41  34  47 /  90  90  50  80
ALW  29  39  35  46 /  80 100  60  90
PSC  30  39  35  50 /  80  90  20  60
YKM  27  41  33  49 /  70 100  30  60
HRI  30  44  36  51 /  90  90  30  60
ELN  26  38  30  43 /  60  90  50  70
RDM  26  42  31  46 /  80  70  40  70
LGD  28  39  34  42 /  80 100  90 100
GCD  27  40  30  43 / 100 100  70  90
DLS  32  47  37  50 /  90 100  60  90

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning from 4 AM Friday to 10 AM PST Sunday for
     ORZ502.

     Winter Weather Advisory from 4 AM Friday to 4 AM PST Sunday for
     ORZ503-506.

     Winter Weather Advisory until 4 AM PST Friday for ORZ509.

     Winter Storm Warning from 4 AM Friday to 4 PM PST Saturday for
     ORZ509.

WA...Winter Storm Warning from 4 AM Friday to 10 AM PST Sunday for
     WAZ030.

     Winter Storm Warning until 10 AM PST Sunday for WAZ522.

     Winter Storm Warning from 4 AM Friday to 4 AM PST Sunday for
     WAZ523.

&&

UPDATE...99
SHORT TERM...91
LONG TERM....90
AVIATION...99

Preliminary Local Storm Report
National Weather Service Pendleton OR
1015 PM PST Thu Nov 30 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0800 PM     Snow             11 ESE Lonerock         45.05N 119.67W
11/30/2023  E2.0 inch        Morrow             OR   Dept of Highways

            2 inches of snow since 4 pm. elevation 4000
            feet.

&&

Preliminary Local Storm Report
National Weather Service Pendleton OR
1018 PM PST Thu Nov 30 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1000 PM     Snow             2 NW Pendleton          45.70N 118.86W
11/30/2023  M1.3 inch        Umatilla           OR   Official NWS Obs

            1.3 inches of snow since 6 pm. elevation
            1492 ft.

&&

Preliminary Local Storm Report
National Weather Service Pendleton OR
1024 PM PST Thu Nov 30 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0700 PM     Snow             9 NW Seneca             44.24N 119.09W
11/30/2023  E1.5 inch        Grant              OR   Public

            1.5 inches of new snow since 5 pm. Elevation
            4800 ft. Social media report.

&&