839
FXUS66 KPDT 090659
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1059 PM PST Mon Dec 8 2025

.AVIATION...06Z TAFs...Predominantly VFR conditions are forecast
into the overnight hours for all sites as a strong rain shadow is
keeping rain pinned to the mountains. Early Tuesday morning
through Tuesday evening, rain is forecast for most sites as a band
of rain sets up over the region. Some MVFR conditions are
forecast within the band. Breezy to windy southwest to west winds
are expected to be strongest this evening.

&&

.PREV DISCUSSION... /issued 328 PM PST Mon Dec 8 2025/

DISCUSSION...

Key Messages:

1. Windy conditions tonight, returning late Tuesday night.
   *Wind Advisories Active*

2. Significant rainfall leading to flood concerns through week.
   *Flood Watch Issued*

Current radar and visible satellite imagery showing moderate to
heavy rainfall west of the Cascades, with isolated showers passing
through the Columbia Basin under mostly cloudy to cloudy skies.
This is in response to an upper level shortwave and associated
cold front that is passing through the area this evening. The
passing cold front will also tighten the pressure gradient across
the Columbia Basin, providing breezy winds tonight through the
morning hours on Tuesday. The RAP, GFS, and NAM advertise a
gradient of 9.5-11.5 mb between Portland (PDX) and Spokane (GEG),
which relates to sustained southwest winds of 25-35 mph with gusts
of up to 60 mph, peaking between 8PM tonight and 3AM Tuesday
morning. Confidence in these wind speeds is high (75-90%) as the
NBM advertises a 70-95% chance of 50 mph gusts or greater across
the Simcoe Highlands and southern Blue Mountain foothills and a
50-80% chance of 45 mph gusts or greater through the northern Blue
Mountain foothills, Yakima Valley, and the Lower Columbia Basin.
Thus, a Wind Advisory has been issued through 7 AM Tuesday for the
aforementioned areas. Winds will slowly taper off through Tuesday
morning and afternoon before a second cold front passage occurs
late Tuesday night through Wednesday morning, promoting a return
to breezy conditions. This second frontal system is not as strong
as tonight`s, as the GFS and NAM indicates a pressure gradient of
8-10 mb. These values relate to sustained southwest winds of 15-25
mph and gusts up to 40 mph, which is below the wind advisory
threshold (45 mph or greater).

The early week system will not only bring windy conditions across
the region, but substantial and widespread rainfall is anticipated
through Wednesday associated with a strong atmospheric river (AR)
event. The ECMWF and GFS ensembles show Precipitable Water (PW)
amounts of around 1 inch, which is 225-250% above normal. The
fetch associated with this AR extends to the date line (~170W),
effectively tapping into subtropical moisture and temperatures. As
a result, snow levels have soared into the 7000-9000 foot range
as high temperatures are breaking into the upper 50s to mid-60s
through the week, which is about 15-25 degrees above normal for
this time of year. These warm temperatures and rain-on-snow
potential due to elevated snow levels will allow rivers and
streams to reach flood levels, especially for the Naches and
Yakima Rivers as the current forecast reaches action stage over
the next 24 hours and peaks in moderate flood stage for the Naches
River near Naches and the Yakima River near Parker and Kiona
Thursday and Friday. Reaches further upstream are expected to
reach minor flood stage over the next 24 hours. As a result, a
Flood Watch has been issued and is in effect through Friday
morning. The Yakima River at Easton has already reached into
action stage, with low-lying and flood prone areas already
experiencing localized flooding. This has warranted the issuance
of a Flood Advisory for the Elk Meadows area near Cle Elum as
flooding is ongoing via contact with the public. In addition to
the Yakima and Naches Rivers, the Klickitat River near Pitt and
the Umatilla River near Gibbon both reach action (bankfull) stage
Wednesday morning. River levels are expected to subside slowly
over the weekend with all reaches currently forecast to fall below
minor flood stage by early Monday morning. 75

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  48  56  45  65 /  70 100  90  40
ALW  50  54  46  64 /  80  90  90  60
PSC  49  56  44  66 /  40  80  90  20
YKM  41  50  39  60 /  80  80 100  60
HRI  50  57  46  66 /  50  90  90  20
ELN  37  46  35  56 /  90  70 100  70
RDM  43  57  44  63 /  50  70  50  10
LGD  45  51  41  58 /  90 100 100  50
GCD  45  51  45  58 /  50  90  70  10
DLS  51  56  48  63 /  90 100 100  70

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory until 7 AM PST Tuesday for ORZ041-044-507-508-510.

WA...Wind Advisory until 7 AM PST Tuesday for WAZ024-027>029-521.

     Flood Watch through Friday morning for WAZ026-027-521>523.

&&

$$

DISCUSSION...75
AVIATION...86