150
FXUS66 KPDT 292242
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
242 PM PST Mon Dec 29 2025

.DISCUSSION...

Key Messages:

1. High pressure dominates through much of the week.
   *Air Stagnation Advisories Active*

2. Below normal temperatures and dry conditions through mid-week.


3. Precipitation chances return Thursday onward.


Current radar and visible satellite imagery showing dry conditions
as a thin veil of high elevation cirrus clouds stream in from the
west. This is in response to a weak upper level shortwave riding
over the building upper level ridge to our south. This ridge will
continue to build Tuesday and early Wednesday before slowly
shifting east into Thursday. Surface high pressure is also
present, leading to sinking air and low winds over the Lower
Columbia Basin, Blue Mountain foothills, Central & North Central
Oregon, and the Yakima/Kittitas Valleys through Thursday morning.
The combination of sinking air and low winds will inhibit mixing
over these areas, allowing for pollutants to be trapped near the
surface and may lead to degrading air quality through the week.
Moderate air quality is already being observed across Central
Oregon, North-Central Oregon, and portions of the Yakima Valley,
with conditions expected to deteriorate through early Thursday. As
a result, an Air Stagnation Advisory has been issued across the
aforementioned areas until noon on Thursday. Confidence in these
stagnant conditions is high (80-90%) as the NBM advertises less
than a 30% chance of wind gusts reaching 10 mph (the majority of
areas stay below a 10-15% chance) and mixing heights below 1500
feet through this timeframe.

As high pressure dominates all levels of the atmosphere, dry
conditions and mostly clear, hazy skies will prevail. Inversions
will set up across portions of the Basin, foothills, and
protected valleys to promote the development of fog and low level
stratus, with the best chances along the northern Blue Mountain
foothills and the Tri-Cities, Prosser, and Hermiston areas.
Overnight temperatures will be dipping into the low-to mid-20s, so
freezing fog will be of concern that may lead to slick roadways
during the morning hours. Be sure to check area roadway conditions
and weather updates if planning to travel during morning hours,
in case any freezing fog products are active. Temperatures will be
slow to warm, with highs only expected to break into the mid-to
upper 30s through Thursday.

The upper level ridge begins to suppress and shift east early
Thursday as an upper level trough drops along the British Columbia
coast from the Gulf of Alaska. Southwest flow aloft will enhance
with the incoming trough to allow for light rain to move in from
the south Thursday morning, becoming more widespread by late
afternoon. Snow levels are expected to be between 5000-6000 feet
Thursday, 4000-5000 feet Friday, and 3000-4000 feet Saturday
associated with a slowly encroaching cold front struggling to
push into the Pacific Northwest due to the strong surface high
pressure located in the Great Basin. This struggle between these
two synoptic features is prevalent in ensembles as 54% of members
hint at a later arrival, less overall precipitation Thursday and
Friday, and warmer temperatures as the cold front delays. This
uncertainty is also present when viewing the spread (difference
between the 75th-25th percentile) in snow levels of 2000-5000 feet
Thursday and 2000-3000 feet on Friday via the NBM. Mountain
precipitation is likely to start out as rain Thursday before
transitioning to snow into the weekend as the snow levels drop. 75


&&

.AVIATION...0z TAFs...Low stratus finally eroded at the RDM
terminal around 20z. A return of this aviation hazard is possible
by as early as 04 UTC with 20- 50% percent confidence in the
vicinity for sub 1000ft ceilings and sub 1 mile vsby being
indicated by the HREF ensemble mean overnight. Stratus is also
prone in the foothills of the Blues and in vicinity of the ALW
terminal, but confidence here is far lower (10-20% approaching the
vicinity at this time and we will leave it out of the TAF for the
time being. Outside of these couple of areas of concern,
confidence is high (90%) the other terminals remain in the VFR
camp overnight with high cirrus overhead and light terrain (under
5 knots) influenced winds. Russell/71


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  22  36  21  35 /   0   0   0   0
ALW  25  35  25  34 /   0   0   0   0
PSC  25  36  21  33 /   0   0   0   0
YKM  22  37  21  36 /   0   0   0   0
HRI  25  35  22  33 /   0   0   0   0
ELN  21  34  20  35 /   0   0   0   0
RDM  21  46  20  48 /   0   0   0   0
LGD  27  41  29  43 /   0   0   0   0
GCD  26  46  26  48 /   0   0   0   0
DLS  28  38  26  38 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Air Stagnation Advisory until noon PST Thursday for ORZ044-507-
     508-510-511.

WA...Air Stagnation Advisory until noon PST Thursday for WAZ026>029.

&&

$$

DISCUSSION...75
AVIATION...71