857
FXUS66 KPDT 231655
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
855 AM PST Sun Feb 23 2025

.MORNING UPDATE...Made some changes to PoPs and QPF this morning
as rainfall across the lowlands is pretty sparse compared to the
previous forecast, and even the mountains are only receiving light
precip as of 9 AM. Latest HREF suggests a surge in moisture
enveloping the forecast area later this afternoon into the
evening, consistent with the latest forecast, so kept that the
same, but indications thus far are that lowland precip may be a
bit under what the forecast had previously called for.

Fog has also been surprisingly persistent in the Kittitas Valley
given the current synoptic conditions, so added mention of patchy
fog in the forecast as well. Webcams show the sun peaking through
in spots, so not anticipating the fog to last too much longer,
especially when rain eventually arrives, but kept mention through
the early afternoon just to be safe. Evans/74

&&

.PREV DISCUSSION... /issued 355 AM PST Sun Feb 23 2025/

SHORT TERM...Today through Tuesday...Satellite and radar imagery
tonight shows an Atmospheric River (AR) event underway across the
PacNW, with moderate to heavy rainfall being reported in mountain
areas, with light locally moderate in the lower elevations. That
said, satellite imagery also shows rainshadowing effects taking
place across central OR, where mid to upper level cloud cover is
clearing and surface observations have stopped reporting rain.
Locally breezy to breezy winds have also developed tonight, with
strongest winds being observed in the Grande Ronde valley and
along the Blue mountain foothills.

The AR event will continue through this evening as a series of
shortwave troughs and a weak frontal boundary pass over the PacNW
throughout today. Precipitation will briefly diminish across the
forecast later tonight as the last shortwave passage will
effectively cutoff the AR into the region. Through tonight, there
is moderate high confidence (60-80%) of an additional 1 to 2
inches of rainfall across the Cascade crest and the northern
Blues. In the lower elevations, there is a 40-80% chance of an
additional 0.25 inches of rainfall through tonight. With the
arrival of the shortwave trough this evening, snow levels will
drop to between 5.5kft and 6.5kft across the Cascade crest,
resulting in snow developing along the higher ridges and peaks.
The shortwave trough and frontal boundary passages will also keep
surface pressure gradients tightened across the forecast area
through this evening, with the current breezy conditions expected
to persist throughout the day. The arrival of the frontal boundary
by this afternoon will also produce breezy to windy (40-50mph)
south winds across central and north central OR this afternoon
through the late evening.

Tonight, the exit of the shortwave trough will diminish
precipitation chances and weaken winds across the forecast area.
This break from precip/winds will be short lived however, as a
warm/cold front and upper low passage will bring another round of
precipitation chances and breezy to windy conditions across the
forecast area. Snow levels across most of the forecast area will
remain above 5.5kft, limiting snow chances to the higher
ridges/peaks along the OR Cascade crest. Along the WA cascade
crest, snow levels are expected to drop to between 4kft and 5kft,
resulting in light snow at or just above pass level, with moderate
to heavy snow accumulations above 5kft. Through Monday night,
there is a 70-90 chance of 0.5 inches of rainfall below the rain/snow
transition zone along the Cascade east slopes, and a 60-80% chance
across the northern Blue Mountains. In the lower elevations and
across the eastern mountains, there is a 40-80% chance for another
0.1 to 0.2 inches of rain accumulations. A strong low to mid
level jet (45-60kts) will also accompany the frontal passages in
the afternoon Monday. This has resulted in a convective signal
amongst short-term CAM guidance, with the best potential for
convective showers to develop across the Columbia Basin and into
the Blue Mountain foothills, which could result in graupel
developing as showers upslope into the northern Blues (confidence
30-50%). The warm/cold front passages will also help to mix these
winds down to the surface throughout the day, with a 70-90%
confidence of gusts between 45-55mph across much of the lower
elevations, and 50-80% confidence in winds 55-65mph in portions of
central OR, north central OR, and the OR Columbia Basin. The
upper trough/wave passage Monday night will reinforce winds at
the surface, though winds are expected to slightly weaken
overnight into Tuesday afternoon. That said, a High Wind Watch has
been issued across these areas starting Monday morning through
Tuesday afternoon, including the WA Columbia Basin and Blue
Mountain foothills.

Monday night the upper low will transition into an upper wave as
it moves across the PacNW, then exit into the Northern Rockies by
the late afternoon. By Tuesday morning, light rain chances will
diminish across the lower elevations, with the mountain snow/rain
diminishing by the early evening. As mentioned earlier, breezy to
locally wind conditions will persist into Tuesday morning as the
upper shortwave continues to pass over the forecast area. Winds
will gradually weaken through the remainder of the morning hours
into the afternoon. By Tuesday evening, an upper ridge is expected
to build into the PacNW, with dry conditions and light winds
developing area-wide.

Lastly, hydrological concerns will be increasing through today and
Monday as increased snowmelt and rain runoff will result in rises
in area rivers and streams. Current river forecasts show portions
of the Klickitat, Naches, Umatilla, and Walla Walla rivers
reaching bankfull Monday, with most other rivers remaining below
bankfull through this event. That said, there is moderate
confidence (40-60%) that some streams along the WA Cascade east
slopes will also reach bankfull, and those that have not been
cleared of debris could result in localized flooding along those
streams. After Monday/Tuesday, most rivers and streams will see
water levels gradually decrease as drier conditions settle back
into the region. Lawhorn/82

LONG TERM...Wednesday through Sunday...While some Ensemble NWP
spread exists through the forecast period, ensemble clusters
indicate at least some flavor of an upper-level ridge of high
pressure over the PacNW through Friday, with an upper-level trough
possible (40% of members) by Saturday and likely (all four cluster
scenarios, though there is significant spread in location/magnitude)
by Sunday. With the exception of 20-40% PoPs for the Cascade crest
on Thursday, the forecast area will likely remain dry through Friday
(85-95% chance) with temperatures running 5-10 degrees above normal.

Breezy southerly winds are then forecast Wednesday into Thursday
through the Grande Ronde Valley and along the base of the Blue
Mountains. Confidence in advisory-level winds Wednesday/Thursday is
low-medium (20-60%).

Latest forecasts from the NWRFC still suggest streamflow will be
steady or dropping for area rivers through the period. Plunkett/86

AVIATION...12Z TAFs...Periods of rain, gusty winds, and sub-VFR
conditions will persist through the morning. Confidence in sub-VFR
conditions after this morning was too low to include in TAFs, though
periods of rain and gusty winds will remain through the valid
period. Additionally, strong winds aloft will also support LLWS.
Plunkett/86

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  61  41  60  41 /  90  80  50  70
ALW  57  42  56  41 /  90  90  50  80
PSC  61  39  58  42 /  90  70  50  60
YKM  55  33  49  38 /  90  70  80  70
HRI  62  39  61  41 /  90  70  60  60
ELN  51  32  45  34 /  90  80  80  70
RDM  62  40  60  35 /  70  60  50  60
LGD  48  36  51  34 / 100  90  50  70
GCD  53  38  55  34 /  90  70  50  50
DLS  58  41  55  42 / 100  80  70  80

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Watch from Monday morning through Tuesday afternoon
     for ORZ044-507-508-510-511.

     Wind Advisory until 10 PM PST this evening for ORZ049-507-508.

     Wind Advisory until 10 PM PST this evening for ORZ510-511.

WA...High Wind Watch from Monday morning through Tuesday afternoon
     for WAZ028-029-521.

&&

$$

SHORT TERM...74
LONG TERM....86
AVIATION...86