709
FXUS66 KPDT 270922
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
222 AM PDT Sat Apr 27 2024

.SHORT TERM...Today through Monday...Overnight satellite imagery
shows relatively clear skies around the forecast area outside of our
mountain zones. Synoptically, we find ourselves under the axis of a
low currently digging down towards the Four Corners region. Saturday
will serve as a transition day as the exiting low will invite mild
zonal flow over the forecast area, before a stronger low begins to
move towards the region by Sunday. Expecting a relatively quiet
Saturday, with precip chances isolated mainly to the Cascade crests,
however winds will pick up as amplified flow aloft mixes down into
the lower elevations ahead of the aforementioned low.

Weather becomes a bit more active Sunday into Monday as the oncoming
low`s influence becomes more apparent. Guidance depicts a jet max
overhead Sunday associated with the southern flank of the low, which
centers itself over BC, before easing southward and inviting more
WSW winds aloft and thus more moisture into the lower elevations by
Monday. End result will be precip chances both days, especially
across the Cascades, and breezy winds. Guidance generally shows sub-
advisory level winds, with Monday being the strongest day, however
the strongest belt of winds looks to shift just to our south that
day. Still, models suggest wind gusts up to 35 mph through the
Oregon Basin, Gorge, and Simcoe Highlands, with 40 mph gusts along
exposed ridgetops. CAMs may pick up Monday`s winds a bit better once
they go out long enough, but am not anticipating headlines at this
time.

As far as precip is concerned, these patterns of amplified zonal
flow are often conducive for heavy precip along the Cascade crests.
QPF guidance has picked up on this, and has trended upward over the
last 24 hours. With this oncoming low stemming from the Gulf of
Alaska, colder air will intrude in and drop snow levels down into
the 2500-3000 ft range overnight Sunday into Monday. This, combined
with the recent upward trend in precip amounts, suggest that
accumulating snow is possible for the Cascade passes. Confidence is
on the lower end, however (30-40%), as it is the last week of April
after all. Road temps will be warm, and the zonal flow preceding the
cold air intrusion will make for more mild conditions before snow
levels really begin to drop. Will see how guidance evolves over the
next 24 hours, but cannot rule out advisories so as to prevent
motorists from being caught off guard despite the time of year. The
more southerly component of the winds aloft Monday could also invite
storm chances across our mountain zones, however confidence in this
occurring is also low (10-15%).

High temps over the period will dip down towards the 50s as this
colder air advects into the region by Monday. Elevated winds may
keep the atmosphere well-mixed enough to prevent overnight lows from
bottoming out too much, but if this Alaskan low lingers about for
long enough, freezing temperatures could materialize (confidence 20-
30%) across some of our lower elevation Zones by the start of the
oncoming work week. Evans/74

.LONG TERM...Tuesday through Saturday...The long term forecast
will start off active as an upper trough continues to impact the
PacNW. However, there is much uncertainty in the progression of
the synoptic pattern Wednesday into next weekend, and therefore
uncertainty in temperature and precipitation forecasts.

Ensemble cluster solutions start in great agreement Tuesday that
an upper low will continue to slide east across the northern tier
states, resulting in continued mountain precipitation chances and
breezy west winds in the lower elevations. Across the mountains,
NBM probabilities for 0.1 inches of QPF peak between 50-60%
through Tuesday, and though much of this precip will be falling as
snow, there is moderate confidence (60%) in only up to 1 inch of
snow in the eastern mountains and WA Cascades and 2-3 inches along
the OR Cascade crest. While winds won`t be as strong as Monday,
NBM probabilities suggest there is 60-70% chance of wind gusts
exceeding 35mph in the OR Columbia Basin, and along the exposed
ridges across Klickitat, Yakima, and Kittitas counties.

Wednesday through Saturday, uncertainty in the synoptic pattern
grows into the weekend, as ensemble members disagree on the
progression of ridging/troughing over the PacNW during this
period. Late Tuesday into Wednesday, ~60% of ensemble cluster
members from the GFS, ECMWF, and Canadian ensemble suites want to
push a developing upper ridge over the PacNW, resulting in a
drying and warning trend; meanwhile the other 40% favor some form
of troughing into the PacNW, keeping persistent chances of
mountain precipitation and cooler afternoon temperatures(mid 50s
to mid 60s). Late Wednesday into Thursday, 36% of the cluster
members favor troughing moving into the area for increased
precipitation chances across the Cascades and eastern mountains,
while 30% have a weaker shortwave trough overriding a ridge over
the region for light mountain precip and comparatively mild
temperatures(upper 50s to mid 60s), and lastly 34% of members
favor the upper ridge over region with dry conditions and
comparatively warmer temperatures(60s to low 70s). Late Thursday
into Friday, 55% of members favor upper level ridging with a
continued dry and warm trend while the remainder 45% of members
bring a trough into the PacNW with cooler temperatures and
continued precip chances across the mountains. Finally, late
Friday into Saturday contains the most disagreement, with
solutions ranging from ridging offshore or centered over the
PacNW, and/or an upper level trough setting up offshore with
southwest flow aloft. Throughout the midweek to next weekend,
confidence is low (25-30%) in the going forecast due to high
uncertainty day to day, as well as great variability in shift to
shift trends. That said, NBM guidance depicts only a 20-40% chance
for 0.1 inches of QPF in the mountains each day; as for afternoon
temperatures, the NBM suggests that there is a 70-80% chance of
exceeding 70 degrees across the Columbia Basin/Gorge and adjacent
valleys Thursday through Friday, suggesting the ridging solution
is favorable. Lawhorn/82

&&

.AVIATION...Previous Discussion...06Z TAFS...VFR conditions
currently at all sites, which will stay the course through the
period. Breezy winds between 10- 15kts are still occurring across
most sites, which will linger overnight. The only exceptions are
KBDN/KYKM, which are below 6 kts and will persist into early
Saturday afternoon. Winds will stay elevated at all other sites
overnight and increase to between 20- 25kts as ceilings drop to
around OVC050 through the afternoon and evening. KPSC will be the
outlier and should stay OVC100 through the afternoon before
improving to BKN250 during the early evening. 75

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  59  40  61  38 /  10  10   0  10
ALW  64  45  63  41 /  10  10   0  10
PSC  66  46  66  45 /  10   0   0   0
YKM  62  38  63  37 /  10   0   0  10
HRI  64  43  65  41 /  10   0   0   0
ELN  58  38  58  35 /  10   0   0  10
RDM  54  34  58  32 /  10  10   0  10
LGD  57  39  59  37 /  10  10  10  10
GCD  57  36  61  34 /  10  20   0  10
DLS  61  45  62  41 /  10  10   0  20

&&

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

$$

SHORT TERM...74
LONG TERM....82
AVIATION...75