359
FXUS66 KPDT 032229
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
229 PM PST Wed Dec 3 2025

.DISCUSSION...Rest of Today through Thursday Morning: A weak
shortwave moving over the region has helped to lift fog impacting
areas of the Columbia Basin and adjacent valleys/foothills this
afternoon, though cloud decks remain fairly low across these
areas. This reprieve is expected to be brief, as there is mod-high
confidence (55-85%) that dense fog will redevelop in these areas
by this evening. That said, confidence is low(25-40%) in the
exact timing and areal extent of the fog redeveloping. Regardless,
opted to extend the current fog advisories through 12PM PST
tomorrow for the Kittitas/Yakima valleys and the Northern Blue
Mountain Foothills of OR/WA.

By Thursday morning, the first in a series of coupled shortwaves
and surface lows will arrive to the PacNW, while a plume of
moisture originating from the Hawaii region will aid precipitation
amounts with each incoming system. As for impacts Thursday
morning, dense fog is expected to persist across portions of the
Columbia Basin and adjacent foothills/valleys, with rain and
high mountain snow or rain/snow mix starting developing along the
Cascade Crest, then spreading southeast to the remainder of the
forecast area. One concern with incoming precipitation will be the
potential for freezing rain to develop tomorrow morning if the
cold pool strengthens and drops to or below freezing tonight.
Areas that would be most likely impacted would be portions of the
Kittitas/Yakima valleys, and along the northern Blue Mountain
foothills. That said, confidence is very low (<15%) in this
scenario playing out with onset of precipitation tomorrow.


Thursday Afternoon through Saturday: Ensemble guidance is in
great agreement through Saturday night that a persistent upper
level ridge in the northeast Pacific will force several surface
lows and attendant shortwave troughs to dive southeast across the
forecast area. While each system will be approaching from the
northwest, which would generally mean their airmasses would be
fairly cool/cold, a plume of tropical moisture with IVTs in the
150 to 250 kg/m/s range will ride up and over the ridge into the
PacNW, keeping the airmass generally saturated with well above
normal temperatures than normal this time of year (confidence
70-90%). As the airmass infiltrates the PacNW, snow levels will
slowly rise above pass level, keeping any snow or wintry mixes to
the higher mountain ridges and peaks through this period.

By Thursday afternoon, the first in the series of systems will
glide over the forecast area, resulting in widespread rain and
high mountain snow chances the remainder of Thursday. By Friday, a
more potent surface low with a strong cold front will approach the
PacNW, resulting in a tightening of pressure gradients at the
surface. Westerly winds will increase as early as Friday morning,
with winds forecast to peak Friday evening as the surface low and
cold front move across the forecast area. Wind gusts will
increase to 25-40mph across much of the area(confidence 60-80%),
with a 50-80% chance that wind gusts will exceed 45mph across
portions of the OR Columbia Basin, Blue Mountain Foothills, North
Central OR, Simcoe Highlands, as well as ridges in the WA Columbia
Basin to the WA Cascade east slopes. Winds will gradually decrease
through Saturday as the low departs the region to the east.

Otherwise, during this period, there is a 60-85% chance of the
Cascade crest and east slopes receiving 1.5 inches of QPF while
the northern Blues will only see a 30-55% chance. Looking at 0.5
inches of QPF, the northern Blue Mountain foothills and the
remainder of the eastern mountains will see a 35-70%. For the
remainder of the areas, there is generally a 20-60% chance for
0.25 inches of precipitation, with the lowest chances in the WA
Columbia Basin and across portions of central OR.


Sunday through Tuesday: Guidance continues to be in good
agreement Sunday through Tuesday as all but about 10-13% of
ensemble cluster members show the ridge axis moving inland and
flattening Sunday and Monday, with zonal flow developing over the
PacNW. Meanwhile, ensemble members of the GFS and ECMWF continue
to depict a 60-80% chance of at least a weak atmospheric river
(AR) with IVT of 150 kg/m/s filtering into the area. Under the
zonal flow, weak shortwaves with surface fronts will continue to
impact the forecast area through Tuesday, and with the
accompanying AR, this will result in periods of moderate to heavy
precipitation in the mountains with lighter amounts further down
along the Cascade east slopes and the northern Blue Mountain
foothills. Of the ensemble cluster members, the solutions made up
of the earlier mentioned 10-13% favor lower 500mb heights over
the PacNW, which would translate to lower snow levels and
potentially snow at or just above pass level in the mountains.
Otherwise, the remaining members favor mostly rain with high
mountain snow during this period. Lawhorn/82



&&

.AVIATION...00Z TAFS...Variable conditions across all sites, with
VFR at KRDM/KBDN, MVFR at KPSC/KDLS, and IFR at KYKM/KPDT/KALW due
to reduced ceilings of 500-600 feet. These conditions are expected
to persist through the afternoon and early evening before degrading
tonight. LIFR conditions are expected for KYKM/KPDT/KALW as fog
redevelops, leading to visibilities of 1/4SM into the early morning
hours on Thursday. IFR conditions are anticipated at KPSC due to
reduced ceilings of 900 feet tonight, with visibilities decreasing
to around 2SM by late Thursday morning.

A weak incoming system late Thursday morning will bring the
potential for light snow rain mix for KALW/KPDT before transitioning
to rain into the early afternoon. Winds will stay light and below 8
kts. for all sites. 75


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  28  43  37  53 /   0  80  70  90
ALW  31  42  37  49 /  10  80  80  90
PSC  28  42  32  51 /  10  70  50  60
YKM  27  42  31  52 /  10  40  40  70
HRI  29  43  36  53 /   0  80  60  70
ELN  27  43  31  46 /  20  50  50  80
RDM  25  47  35  55 /   0  50  40  60
LGD  26  40  35  48 /   0  90  90 100
GCD  26  42  37  49 /   0  80  90  90
DLS  36  48  42  56 /  10  70  70  90

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Dense Fog Advisory until 11 AM PST Thursday for ORZ507.

WA...Dense Fog Advisory until 11 AM PST Thursday for WAZ029.

     Freezing Fog Advisory until 11 AM PST Thursday for WAZ026-027.

&&

$$

DISCUSSION...82
AVIATION...75