353
FXUS66 KPDT 081107
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
307 AM PST Mon Dec 8 2025

.SYNOPSIS...

A potent atmospheric river will bring heavy mountain showers and
gusty winds to the forecast area over the next couple of days, with
the period beginning early this evening and lasting into Tuesday
morning looking to be the most impactful in terms of winds and rain.
Another, albeit weaker round will arrive during the overnight period
Tuesday into Wednesday, before precip chances gradually abate
heading into the end of the work week as high pressure slides in
over the region. Wind Advisories are out for much of the lowlands of
the forecast area starting late this afternoon, along with a Flood
Watch due to the potential of localized flooding along the WA
Cascade east slopes through the week.

.DISCUSSION...

Global models remain in agreement that the PacNW will find itself
locked into a persistent, strong WNW flow pattern aloft as a result
of blocked high pressure centered off the California coast. This
pattern will channel a potent atmospheric river right into our
region, with the WA Cascades in particular bearing the brunt of this
oncoming moisture. The latest QPF forecast from WPC suggests that
portions of the central WA Cascade crests may see upwards to 10
inches of liquid precip by the end of the week. While this pattern
more or less spares the mountain passes of any significant snow
threat, as well as the lowlands of heavy precip as 1) the main band
of IVT falls along the northern periphery of our CWA, save for
Tuesday when it briefly sags down to the Columbia Basin, and 2) the
mid-level winds remain westerly enough to support persistent rain
shadowing across the Columbia Basin and central Oregon, all that
rain expected to fall across the mountains will run down into the
valleys, spurring a flooding threat across primarily the east slopes
of the WA Cascades.

Multiple river gauges along the Klickitat, Naches, and Yakima Rivers
are forecasted to go into minor flood stage sometime around the
midweek, with Wednesday into Thursday being the most likely period.
Even some gauges along the foothills of the northern Blues may inch
up toward minor flood stage as the Blues get hammered with rain
around the same time. The RFC`s forecast has wobbled over the past
few days, as uncertainty in QPF amounts stems primarily from the
positioning of the strongest IVT axis over the PacNW, but both the
GFS and ECMWF are in good agreement in depicting IVTs in the 600-800
kg/m/s range slamming the WA Cascades. The one saving grace from
this otherwise strong AR event is that it`s still relatively early
in the winter, so mountain snow packs are on the lower end (SWE
below 50% across all basins), precluding any rain on snow excessive
runoff threat. Still, given the threat, opted to issue a Flood Watch
for our zones at and along the WA Cascades, in order to better
advertise the flooding threat over the Hydrologic Outlook that had
been in place over the last couple of days.

With the strong upper-level winds accompanying this AR, expect to
see surface winds ramping up later today, especially during the late
afternoon / early evening hours. Wind Advisories are currently in
effect for our Columbia Basin zones, as well as the foothills of the
Blue Mountains, Columbia River Gorge, and North Central Oregon
starting at 1 PM at lasting until sunrise Tuesday. Didn`t see too
much across guidance to suggest the need to deviate from ongoing
messaging, so the forecast continues to call for wind gusts in the
35-50 mph range across the advised areas, with the ridgetops along
the Simcoe Highlands seeing the biggest threat for gusts to exceed
60 mph. HREF suggests probabilities are too low (>5%) to warrant an
upgrade to a High Wind Warning, and will opt to keep the Advisory in
place for the Simcoe Highlands given that the populated areas within
the zone are not anticipated to receive Warning-level winds. Winds
abate during the day Tuesday, before picking back up again in the
late evening hours, but as of now, the forecasted wind gusts look to
remain below Advisory thresholds, save for along exposed ridgetops.

The precip threat finally starts to relent by Thursday as the
atmospheric river pattern shifts to our north and east in favor of
oncoming high pressure. This high pressure ridge pattern will be
`dirty` enough in the sense that models continue to suggest low-end
PoPs across the WA Cascades, but the bulk of the rain threat is
expected to end by early Thursday at the latest. Still, expect
elevated river levels to persist even into the weekend, especially
if the weather pattern continues to support even light shower
activity over the Cascades. 74

&&

.AVIATION...12Z TAFs...Primary concern through the forecast period
will be the winds. All TAF sites will be VFR through the period with
VSBY at P6SM and CIGs over 5kft. PDT/ALW/PSC are already seeing winds
10 kts and above while the remaining sites are below 6kts. Timing of
the winds will prove to be the challenge through the period.
However, model guidance shows the winds to begin to increase between
14-19Z with DLS coming in a bit later after 00Z or later. With that
said, winds are expected to be 15-25 kts sustained with gusts to 35
kts with some locally higher winds across PDT/ALW/PSC with sustained
wind reaching 30 kts (70-90%). 90


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  60  47  56  47 /  70  70  90  90
ALW  59  49  55  47 /  90  80  90 100
PSC  60  49  56  45 /  60  40  80  80
YKM  54  41  51  40 /  80  80  80  90
HRI  62  49  58  47 /  60  50  90  90
ELN  49  37  45  37 /  90  90  70  90
RDM  57  44  58  46 /  50  50  70  50
LGD  49  44  51  45 /  90  90 100 100
GCD  52  44  52  46 /  50  50  90  70
DLS  60  51  57  50 / 100  90 100 100

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 1 PM this afternoon to 7 AM PST Tuesday for
     ORZ041-510.

     Wind Advisory from 4 PM this afternoon to 7 AM PST Tuesday for
     ORZ044-507-508.

WA...Wind Advisory from 1 PM this afternoon to 7 AM PST Tuesday for
     WAZ024-521.

     Flood Watch from 7 AM PST this morning through Friday morning
     for WAZ026-027-521>523.

     Wind Advisory from 7 PM this evening to 7 AM PST Tuesday for
     WAZ027>029.

&&

$$

DISCUSSION...74
AVIATION...90