000
FXUS66 KPDT 291059
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
259 AM PST Sun Jan 29 2023

.SHORT TERM...Today through Tuesday night...Frigid arctic air
continues to spill into the forecast area this morning, as an
upper trough that is oriented southwest to northeast continues to
sag southward across the CWA. There are still a few light snow
showers over extreme southwest Deschutes County, however, these
should be ending soon. A low end Winter Weather Advisory will
remain in effect until 4 AM today for central OR. NBM
temperatures look reasonable for this morning, with lows in the
teens to near 20 over the lower elevations, and single digits to
the lower teens in the mountains. Winds are still a bit breezy
over much of the forecast area. This will keep the atmosphere
more mixed resulting in temperatures being a bit warmer than they
would be with calm winds. Dry conditions will persist, with cold
high temperatures in the 20s to near 30 lower elevations and
mostly teens in the mountains today.

Tonight (Sunday night) will be colder than this morning`s lows
due to skies being more clear, with light winds. This will allow
good radiational cooling to take place. Tonight`s lows will range
from 5 below to 5 above zero in the mountains, and mostly 10 to 20
degrees in the lower elevations. Conditions will continue to be
very dry with dew points mostly from -10 to +5 degrees F across
the CWA. There is some variance in the NBM forecast highs and
lows today and tonight, with ranges between the 25th and 75th
percentiles about 5 to 10 degrees. While this decreases confidence
in forecast of both highs and lows a bit for today and tonight,
the temperatures near the 50th percentile seems reasonable.

An upper ridge will begin to push into the region from the west on
Monday, with a continued northerly flow aloft. However, with
mostly clear skies, solar heating, and light winds, temperatures
will moderate more. High temperatures are forecast to be not as
cold, and in the upper teens to mid 20s mountains, and upper 20s
to lower 30s lower elevations. It will remain very dry on Monday
under this high pressure ridge. There could be some local patchy
freezing fog or low clouds Monday morning, especially in favored
upslope areas in central OR, and in the Lower Columbia
Basin/Yakima Valley.

On Tuesday, the upper ridge will weaken, flatten and therefore
allowing more of a northwest flow rather than a northerly flow.
This will allow temperatures to continue warming back up, with
highs in the 20s to lower 30s mountains, and mid 30s to mid 40s
central OR and the lower elevations around the Lower Columbia
Basin and adjacent lower elevation valleys.

There will be a weak disturbance in the flow on Tuesday, with
just enough moisture for possible very light snow over the
northern Blue Mountains, far eastern Wallowa County, and over the
southern WA Cascades. Total snow amounts will be mostly less than
an inch, but there could be isolated locations over the highest
peaks and ridges with 1 to 2 inches of snow. This light snow will
persist into early Tuesday evening, but then it will end and
become dry again in all areas overnight. Low temperatures by
early Wednesday morning will be in the teens mountains, and 20s
elsewhere.

Breezy winds this morning will diminish and become light, and
then remain light through the short term period. The exception
will be over The Grande Ronde Valley, where southerly winds will
increase to the breezy to windy category by early Tuesday
morning. It is possible that the winds in the Grande Ronde Valley
may approach advisory speeds as time progresses, but at this
time, winds are expected to remain below advisory criteria in the
Grande Ronde Valley. The NBM 4.1 WSUP viewer shows this increase
in winds over the Grande Ronde Valley, but keeps the wind speeds
below advisory criteria. 88

.LONG TERM...Wednesday through Sunday...The extended period is
characterized by a return to a progressive weather pattern as the
ridge of high pressure overhead breaks down Wednesday ushering in a
weather disturbance Thursday night into Friday followed by a second
late Saturday into Sunday.  Mountain snowfall appears to be the
primary impact associated with the two aforementioned disturbances,
although breezy southerly winds through the Grande Ronde Valley and
along the Blue Mountain foothills may come into play. Overall,
conditions remain close to climatologist norms as depicted by the
ECMWF Extreme Forecast Index; lingering cold temperatures trend back
to normal by Thursday and Friday.

Both deterministic and ensemble guidance are in fairly good
agreement resolving the large scale pattern to begin the extended
period as the ridge of high pressure responsible for maintaining the
calm, drier, and colder than normal conditions gradually shifts
eastward through Wednesday night and Thursday, amplifying slightly
due to an approaching trough off the coast. It appears this ridge
may not be robust enough to completely ward off precipitation
however, as ensembles and some deterministic guidance(GFS) support
light snow bleeding southward into central Washington Cascades
through this time period.  Otherwise, completely dry conditions
prevail elsewhere albeit with added high clouds streaming overhead
due to the proximity of weather disturbances to our north.
Increasingly southerly flow will also help temperatures eventually
rebound closer to normal.

Come late Thursday and Friday a tough of low pressure is expected to
swing into the Pacific Northwest  increasing chances for
precipitation primarily over the mountains terrain. Some model
uncertainty does exist in the timing and depth of this trough, best
highlighted by the differences between the deterministic ECMWF and
GFS which are nearly 12-18 hours out of phase; the former model is
the slower of the two. This is also reflected in the Ensemble
Cluster Analysis, with 45% of the totals members more closely
resembling the earlier GFS, while 38% of members delay the arrival
of the trough until Friday. The remaining 17% present an even
earlier arrival scenario but is certainly more of an outlier.
Fortunately, in either case, snowfall impacts appear limited to the
Cascade crests and don’t appear significant or climatologically
abnormal. The NBM projects a 55-75% probability to exceed 2 inches
along the Washington Cascades(a little lower for the Oregon Cascades
and Blue Mountains) through Friday night, falling to only a 10-25%
percent probability of exceedance when the total is raised to 6
inches. Thus the overall snow totals are likely bound between these
two thresholds and impacts geographically limited. We’ll also have
to keep our eye on a decent surface pressure and thermal gradient
across the Eastern Mountains which would facilitate breezy to gusty
winds through the Grande Ronde Valley and potentially the Blue
Mountain foothills during this time period as well.

Saturday the trough continues to progress eastward into the Rockies
while ensembles and deterministic guidance show a transient ridge of
high pressure moving overhead. Expect a trend towards drier
conditions Friday night and Saturday outside of some possible
lingering precipitation near the mountain crests. While there was
some notable disagreement resolving of the placement and amplitude
of the previous trough feature, models seem to better agree on how
to handle the ridge; an interesting development. Our attention then
quickly turns to a robust, closed upper-level low over the eastern
Pacific, slated to march into the region Saturday night and Sunday.
Ensembles and the deterministic ECMWF/GFS are in fairly good
agreement overall in the eventual arrival of this disturbance,
especially given it’s out at day seven, likely bringing another
round of mountain snowfall impacts. The magnitude of these impacts
will become clearer as we get closer, but at the very least they
should be more significant than the first Thursday night/Friday
disturbance. Schuldt/99

&&

.AVIATION...12Z TAFS...VFR conditions are expected through the TAF
period at all sites as the weather disturbances responsible for the
earlier degraded conditions has now progresses southward out of the
BDN and RDM area. Confidence is high(75-90%) persistent northerly
winds prevent fog formation this morning. Expect these winds to
generally stay 10-15 knots or less, except at BDN/RDM and
potentially PSC, which may see some gusts in the 20 to 25 kt range
on Sunday. 99

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  26  11  30  20 /   0   0   0  20
ALW  29  11  31  21 /   0   0   0  20
PSC  30  14  30  22 /   0   0   0   0
YKM  29   9  29  17 /   0   0   0   0
HRI  30  14  31  22 /   0   0   0   0
ELN  26   8  27  17 /   0   0   0   0
RDM  23   3  35  19 /   0   0   0   0
LGD  22   5  27  16 /   0   0   0   0
GCD  24   2  30  13 /   0   0   0   0
DLS  33  15  34  25 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 4 AM PST early this morning for
     ORZ511.

WA...None.

&&

$$

SHORT TERM...88
LONG TERM....99
AVIATION...99

NWS PDT Office Area Forecast Discussion