036
FXUS66 KPDT 202239
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
239 PM PST Thu Nov 20 2025

.DISCUSSION...Water vapor imagery reveals an elongated 500-hPa
closed low stretching from the Pacific Northwest southeast to
southern California. Within the broader trough, one vorticity
maximum is present off the southern California coast, and a
vorticity lobe is placed along the coast from Oregon to northern
California. Ensemble NWP guidance takes this low south-southeast
out of the Pacific Northwest through the overnight hours tonight,
leaving a flat 500-hPa ridge in its wake for Friday through
Saturday and likely into Sunday morning (90 percent confidence).
Aside from a slight chance to chance (15-40 percent) of rain along
the Washington Cascade crest, dry conditions are forecast through
Saturday. With the ridge overhead, have put patchy fog in the
forecast for valley locations, including the Columbia Basin.

Sunday through Monday, a shortwave trough and surface cold front
will very likely (95 percent confidence) sweep across the Pacific
Northwest, though ensemble clusters do indicate some variance
among solutions regarding the amplitude of the trough and
resulting precipitation. Little to no precipitation (a trace to a
tenth of an inch) is forecast for the lower elevations, while
favored mountain locations (Cascade crest and northern Blue
Mountains) are forecast to receive higher totals (0.10-0.40 inches
for the northern Blues and Oregon Cascades, with higher totals up
to an inch focused along the Washington Cascades near Snoqualmie
Pass). Snow levels will lower to pass elevations Sunday evening
through Monday morning, with light snowfall (trace to 3 inches)
currently anticipated for major mountain passes. Additionally,
breezy westerly winds are forecast Sunday into Monday.

Tuesday through Thursday, uncertainty in forecast details burgeons
as ensemble guidance struggles to resolve features of a progressive
weather pattern, including the magnitude and axis of moisture
transport into the Pacific Northwest, focusing mechanisms for
precipitation (fronts, surface lows, etc.), and any wintry
precipitation. Will note the NBM suggests a low probability (up
to 10 percent) of 1 inch or greater snowfall across the Columbia
Basin for the 48-hr period ending 4 PM PST Thursday, with higher
chances (10-30 percent) for the Yakima, Kittitas, and Grande Ronde
valleys. Farther south in central Oregon, guidance is more
confident in a warmer air mass, resulting in very low
probabilities of measurable snowfall (less than 5 percent).

&&

.AVIATION...00Z TAFS...VFR conditions currently across all sites,
which will stay the course through the period. Ceilings will
improve this evening and overnight as light winds persist below 6
kts. This is a result of high pressure moving into the area from
the west, which will lead to a slight chance (15%) of fog between
9-16Z for KYKM, KBDN, and KRDM. 75

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  29  49  30  48 /   0   0   0   0
ALW  34  48  34  47 /  10  10   0   0
PSC  29  48  29  43 /   0   0   0   0
YKM  28  48  29  47 /   0   0   0   0
HRI  29  47  29  45 /   0   0   0   0
ELN  26  47  30  46 /   0  10  10   0
RDM  20  51  23  56 /   0   0   0   0
LGD  30  51  30  53 /  10   0   0   0
GCD  30  50  29  56 /   0   0   0   0
DLS  34  50  35  50 /   0   0   0   0

&&

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

$$

DISCUSSION...86
AVIATION...75