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000
FXUS66 KPDT 231133
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
332 AM PST Tue Jan 23 2024

.Updated Aviation Discussion.


.SHORT TERM...Today through Thursday night...The active weather
pattern continues with frequent short wave weather disturbances
in an overall progressive westerly flow aloft. The main difference
will be the much milder temperatures in most areas with high snow
levels and little in the way of winter weather concerns.

An upper trough axis will move across the forecast area today with
chance to likely probabilities (40-70%) of rain and high mountain
snow this morning. The probabilities of precipitation will
decrease in the lower elevations, but will persist in the Cascades
and northeast mountains through the afternoon. Snow levels early
this morning will be around 5000-5500 ft MSL in all most areas,
except extreme northwest Kittitas County where they will be
4500-5000 ft MSL. However, with such high freezing levels,
precipitation in the higher elevations will be mainly rain, or
mixed rain and snow. By afternoon, precipitation type will be all
rain, except the highest peaks, such as the Eagle Cap Wilderness
and the volcanic peaks along the Cascade crest. The lower
elevations will become dry by this afternoon. Then most areas
will be dry this evening as a very temporary transient upper ridge
moves across the region. Snow levels will come down some this
afternoon and overnight tonight to 4000-4500 ft MSL in most
areas, except 2000-3000 ft MSL in the far northwest CWA, and
3500-4000 ft MSL east and south of the Blue Mountains.

Another weather disturbance will then move in off the Pacific and
bring another round of precipitation late tonight and on
Wednesday, with a Pacific cold front. The main upper low with
this system will be off the coast of SW Canada, with the base of
the upper trough moving into the region overnight and on
Wednesday. At the start, snow levels will be low enough (2000-2500
ft MSL) in the WA Cascades for mainly snow in the morning. The
precipitation will be a mix of rain and snow over most areas
before 18Z (10 AM PST). Even the lower elevations could see a mix
of rain and snow early on, but becoming all rain by afternoon as
snow levels rise. There will be little or no snow accumulation in
the lower elevations. In the WA Cascades there will be a 50-80%
probability of 2 inches or more snow between 4 AM and 10 AM PST
Wednesday, and a 5-15% probability over the OR Cascades. In the
northeast mountains, there will be a 10-30% chance of 2 inches or
more snow between 10 AM and 4 PM Wednesday. Snow levels will rise
again, to 5000-5500 ft MSL by late afternoon over most of the
forecast area, except 4500-5000 ft MSL over the northern portion
of the WA Lower Columbia Basin and east of the Blue Mountains. In
the northwest corner of the CWA, snow levels will only rise to
around 3000 ft MSL by late afternoon...which is why there will be
a greater probability of 2 inches or more snow in the WA Cascades.

The main upper trough will move slowly, and as a result,
precipitation will persist through the day Wednesday at all
elevations, and then mainly just over the Cascades and
eastern/northeast mountains Wednesday evening. Then a minor
secondary embedded shortwave trough will move across the CWA early
Thursday morning. This will bring back a chance of precipitation
again to the lower elevations as well as the mountains. Some of
this may be a mix of rain and snow again, with mainly snow over
the Cascades and the eastern and northeast mountains. Then by
Thursday late afternoon and evening, the lower elevations will
become mostly dry again, with lingering rain and higher mountain
snow over the Cascades and the northeast mountains. By early
Friday morning into the early extended period, most areas will
have another short break in the weather as a drier northwest flow
moves into the region behind the departing upper trough.

Temperatures will be back to near normal, with highs in the mid
30s to lower 40s in the lower elevations, and upper 20s to mid
30s in the mountains. There will be some minor fluctuation of
temperatures by a couple degrees at most each day Tuesday through
Thursday.

Winds will be light in all areas until Wednesday morning, and then
they will become breezy to windy over the ridges of the southern
half of the forecast area, as well as the Grande Ronde Valley and
the Wallowa Valley. At this time, they are expected to remain
below advisory speeds. Winds will then become mostly light again
in all areas by early Thursday morning. 88


.LONG TERM...Friday through Tuesday...The extended period is
characterized by several weak systems that will continue to bring
precipitation across the Cascades, with chances waning across the
Basin through the weekend and into the workweek. These systems will
be approaching from the southwest to allow both high and low
temperatures to warm about 10 degrees through the period. This will
bring highs into the upper 40s to low 50s across lower elevations of
the Basin and in the mid 50s over Central Oregon by Monday, which is
about 5 degrees above normal for the Basin and 10 degrees above
normal for Central Oregon. Snow levels will also be on the rise and
into the 6000-7000 foot range by Friday evening. However, areas
along the east slopes of the Washington Cascades and the
Kittitas/Yakima Valleys will be slightly delayed, reaching around
6000 feet Saturday afternoon. This will allow for the potential of a
wintry mix and freezing rain across these areas Friday and early
Saturday, but current confidence is rather low (20-30%) due to the
variability in guidance.

An upper level ridge slowly moves onshore and across the area on
Friday as the axis eventually pushes to our east by the evening.
Flow aloft will proceed to come in from the southwest as a shortwave
and associated warm front rides up the backside of the ridge,
bringing precipitation chances (30-50%) east of the Cascades and
into the Basin by the afternoon. This shortwave will dissipate as it
attempts to move onshore over the Washington/Oregon coasts late
Friday as temperatures cool overnight into the low to mid 30s from
earlier highs in the upper 30s to low 40s across lower elevations of
the Basin and Central Oregon. Minimal snow amounts are expected,
with the Cascades only picking up 1-2 inches above 4500 feet and the
Eastern Mountains accumulating about an inch at higher elevations.
Rain amounts of 0.25-0.35 of an inch are currently expected along
the east slopes of the Cascades, 0.10-0.20 of an inch along the
northern Blue Mountains, and around 0.05 of an inch for lower
elevations of the Basin, Blue Mountain foothills, and across Central
Oregon on Friday. Widespread precipitation chances (40-80%) will
extend into early Saturday afternoon, with another weak shortwave
approaching to keep precipitation chances (20-40%) lingering across
the Basin into early Sunday morning. Snow levels will be between
6500-8000 feet north-to-south on Saturday, which will persist
through Tuesday. This will keep snowfall below 1 inch daily at
higher elevations across the Cascades and Eastern Mountains Saturday
through Monday. Rain amounts will be even more lacking on Saturday
as 0.15-0.25 of an inch is currently forecast along the east slopes
of the Cascades, 0.10-0.20 of an inch over the northern Blue
Mountains, and 0.05 of an inch or less across lower elevations of
the Basin and Central Oregon - with some areas likely staying dry.
The upper level ridge continues to slowly track east over the
northern Rockies Monday and Tuesday as daily shortwaves erode the
backside of the ridge. These systems will have a tough time
overcoming the ridge through the workweek, keeping light
precipitation confined mainly along the Cascades both days.

Guidance is in good agreement regarding the overall pattern of a
slowly passing upper level ridge followed by subsequent shortwaves
through the period, but differences in ridge strength exist. These
discrepancies are noted in the 500mb EOF Patterns as an ensemble
variance between 46-53% is present Friday through Tuesday regarding
the ridge`s strength and correlating shortwave efficiency. The GFS
incurs the stronger ridge and keeps the ridge axis more overhead
than to the east as with the ECMWF. This would protect our area from
the incoming shortwaves, allowing a drier and warmer scenario than
with the ECMWF. Ensembles seem to align much better with the ECMWF
outcome as the cluster 500mb heights show the predominant feature of
a weaker and more east ridge, and the cluster phase space highlights
better representation of the ensemble mean by the deterministic
solution. Thus, the NBM was utilized through the extended period to
provide an applicably weighted scenario, with the expectation of a
slight trend toward warmer and wetter conditions over future model
runs. 75

&&

.AVIATION...12Z TAFS...Variable conditions across all sites, with
VFR at PDT/RDM/BDN, MVFR at YKM, IFR at DLS/ALW, and LIFR at PSC due
to reduced ceilings of OVC003. This is in response to a weak weather
system that has been bringing periods of light showers across our
Basin sites. These showers will be tapering off shortly as
conditions will continue to degrade to LIFR for DLS/YKM/PSC and MVFR
for PDT/ALW due to reduced ceilings of OVC004 and OVC020
respectively. Fog is also anticipated to develop through the morning
for PSC/DLS as 1/2SM visibilities are anticipated. Conditions will
improve across all sites by late morning, with the exception of YKM
as ceilings of OVC003-009 will linger through the period.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  40  29  39  32 /  30   0  70  60
ALW  41  32  39  34 /  40   0  60  70
PSC  39  32  37  33 /  20   0  60  40
YKM  41  30  38  30 /  20   0  80  20
HRI  40  30  38  31 /  30   0  70  40
ELN  41  31  36  29 /  30   0  90  20
RDM  44  29  46  29 /  10   0  90  20
LGD  42  32  40  31 /  50   0  70  80
GCD  43  28  41  29 /  20   0  80  60
DLS  42  35  39  36 /  30  10 100  60

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.

&&

$$

SHORT TERM...88
LONG TERM....75
AVIATION...75