NWS Pendleton, OR wrote its last area forecast discussion before the first event at Dec 13, 3:14 PM PST, 46 minutes earlier.
Area forecast discussion, Dec 13, 3:14 PM PST, 46 minutes before
Area Forecast Discussion
National Weather Service Pendleton OR
314 PM PST Fri Dec 13 2024
.SHORT TERM...Tonight through Sunday...Radar and satellite imagery
this afternoon show a wide swath of light precipitation pushing
northeast across the inland PacNW, with mostly mid to high level
cloud cover. So far, this precipitation has been mostly made up of
a mix of rain and snow with no reports or indications of freezing
rain.
Through tonight, this band of precipitation associated with a
frontal system lifting across the PacNW will continue to bring a
mix of rain and snow across the forecast area. However, areas
that have seen a persistent cold-pooling with temperatures below
freezing will see a few hours of freezing rain potentially mix in
with the snow, especially along the Cascade east slopes and
adjacent valleys, north central OR, Simcoe Highlands, and the
Southern Blue mountain foothills. In the areas where freezing rain
does develop, confidence is mod-high(70-80%) that ice
accumulations will be up to 0.05 inches, making for a light glaze
in some areas. Otherwise, light snow accumulations of 1-2 inches
will also be possible in the Kittitas/Yakima valleys this evening
with mainly light rain or rain/snow mix elsewhere. Early Saturday
morning, an upper trough offshore will slide across the PacNW,
reinforcing rain and mountain snow chances across the forecast
area. By the end of Saturday night, areas of the Cascades below
5kft will see snow accumulations between 4-6 inches, with 1-3
inches in the Blues and eastern mountains below 4.5kft. Above
4.5kft in the northern Blues, snow amounts are forecast to be
between 4-10 inches (confidence 80-90%). Besides precipitation,
the incoming frontal system will tighten the pressure gradient
between the Columbia Basin and the Lower Treasure Valley through
tomorrow morning, resulting in strong winds developing above
2.5kft along the Blue Mountain foothills and through the southern
Grande Ronde valley. Confidence is high (80-90%) that wind gusts
will be between 45-55mph with sustained winds between 25-35mph
through tomorrow morning.
Drier conditions will develop across the forecast area Saturday
night as the upper trough continues to swing into the northern
Rockies. However, northwest flow aloft behind the trough axis will
result in light upslope snow flurries along the Cascades and
Northern Blues into early Sunday morning before tapering off by
the afternoon. Throughout Sunday, the forecast area will be under
a transitory ridge, which will keep conditions dry and quiet.
Though the ridge will quickly move over the region and exit by
Sunday night, there is still a concern for pockets of dense fog or
freezing fog to develop across portions of the Columbia Basin and
adjacent valleys, however due to low confidence (20-30%), have
left mention of fog out in the afternoon forecast package. By late
Sunday night, the next frontal system will arrive to the Cascades
with another round of mountain to mid elevation valley snow and
rain elsewhere. Lawhorn/82
.LONG TERM...Monday through Friday...
Key Messages:
1. Widespread rain and mountain snow Monday.
2. Breezy winds across the Grande Ronde Valley Monday.
3. Warming trend through Wednesday before cooling through Friday.
The extended period is characterized by an upper level trough and
attendant frontal system that passes through the area early in the
period, ahead of upper level ridging that will develop through the
remainder of the week. The ridge does suppress slightly on Wednesday
as a weak embedded shortwave passes over Washington before the ridge
rebuilds. This weak feature only looks to bring light rainfall along
the Washington Cascade crest and mountain snow to higher elevations.
The main concern through the period will reside with the early
workweek system as significant mountain snow is anticipated. Due to
an incoming warm front, snow levels will be rising to 2500-4500 feet
north-to-south by Monday afternoon. 5-9 inches of snowfall is
currently forecast over the Cascade Passes, with the highest amounts
expected at Snoqualmie (8-9") and Santiam (7-8") Passes. The
northern Blue Mountains also looks to pick up 4-10" above 3500 feet.
These snow amounts across the Cascade and northern Blue Mountains do
meet advisory criteria, but current confidence is lacking as the NBM
suggests a 20-50% chance of 7 inches of snowfall over Snoqualmie and
Santiam Passes on Monday. This is in response to variability in
guidance, which refers to 35% of ensemble clusters suggesting around
2 inches of less snow than the current forecast and 25% of ensemble
clusters hinting at 3 inches of more snow than the current forecast.
Thus, confidence is low to moderate (30-60%) in specific snow
amounts, but significant mountain snow is still anticipated due to the
synoptic pattern. Widespread rain chances (55-95%) are expected as
well on Monday, with up to 0.25" possible along the Blue Mountain
foothills, 0.10-0.20" across Central Oregon, lower elevations of the
Columbia Basin, and the Yakima/Kittitas Valley. Ensemble cluster
differences are also present when analyzing rain amounts, but are in
more agreement than with snow amounts. 57% of ensembles suggest
slightly less amounts (-0.03") than the current forecast, with only
25% hinting at slightly more rain amounts (+0.06").
The warm from will pass through the area from the southwest Monday
morning, followed by the cold front later in the day. These frontal
boundaries will attribute to a developing pressure gradient across
the northern Blue Mountains to allow breezy conditions to unfold
across the Grande Ronde Valley. The GFS, NAM, and SREF advertise a
pressure gradient of 5-6 mb between Baker City (KBKE) and Meacham
(MEH), which does hover near the normal advisory threshold of 5 mb.
Thus, confidence is moderate (50-60%) in the above mentioned wind
gusts.
The early workweek system will allow temperatures to increase due to
persistent southwest flow aloft that looks to continue into
Thursday. This is when the weak shortwave feature will be departing
the area as the upper level ridge rebuilds along the coast. The axis
of the ridge will slowly propagate east late Thursday into Friday to
provide a slight cool down as flow aloft becomes more from the
northwest. High temperatures are expected to stay above normal
through much of the week, likely dropping to below normal Thursday
and Friday. However, guidance is in disagreement in regards to the
ridge strength and location Wednesday onward. 51% of ensemble
members suggest warmer temperatures Thursday with 49% hinting at
slightly cooler temperatures which relates to a ridge axis more to
our east. Long-term guidance also hints at a deep upper level trough
approaching the northern California coast next Saturday. 75
&&
.AVIATION...00Z TAFs..FG is impacting DLS, impacting the terminal
with 1/4 mile visibility. It is expected to lift, but not improve
it enough to bring it out of MVFR conditions by the end of the
period. Precip is anticipated to impact all terminals some time in
the period. Most sites remain just above freezing, however it
hasn't pushed the snow to melt by the time it reaches the surface.
Given this, rain/snow will be the primary concern going into the
next 24 hours. Most terminals will remain at least in MVFR status
dropping to as far down as IFR in places like DLS/PDT/YKM. /95
&&
.PRELIMINARY POINT TEMPS/POPS...
PDT 35 48 31 45 / 80 30 30 0
ALW 37 51 34 46 / 80 30 40 0
PSC 35 48 34 45 / 90 20 20 0
YKM 27 41 23 38 / 100 10 10 0
HRI 34 46 33 45 / 90 20 20 0
ELN 30 39 25 38 / 90 10 10 0
RDM 31 43 22 39 / 70 30 30 0
LGD 35 44 28 40 / 90 60 70 10
GCD 35 43 23 38 / 90 60 60 0
DLS 35 44 33 42 / 100 30 40 10
&&
.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory until 10 AM PST Saturday for ORZ049-507.
Winter Weather Advisory from 8 PM this evening to 7 AM PST
Sunday for ORZ502.
Winter Weather Advisory until 10 AM PST Saturday for ORZ508>510.
WA...Winter Weather Advisory from 8 PM this evening to 7 AM PST
Sunday for WAZ030.
Winter Weather Advisory until 10 AM PST Saturday for WAZ026-027-
521>523.
&&
SHORT TERM...82
LONG TERM....75
AVIATION...95