264
FXUS66 KPDT 151645
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
945 AM PDT Thu May 15 2025

.UPDATED AVIATION...

.AVIATION...18Z TAFs...All sites will be VFR through today. However,
KRDM/KBDN may drop to MVFR due to showers from the frontal system
this evening (30-40% confidence). KDLS/KPDT/KALW could drop to MVFR
as well for tomorrow due to mainly low clouds as the system passes
(<30% confidence). KDLS/KALW are currently gusting at 20-30 kts with
sustained winds of 12-20 kts, though with KALW ending this afternoon
and KDLS continuing through tomorrow morning. KPDT/KRDM/KBDN will be
mainly breezy at 20-25 kts with sustained winds of 12-15kts until
this evening. Feaster/97

&&


.PREV DISCUSSION... /issued 415 AM PDT Thu May 15 2025/

.SHORT TERM...Today through Saturday...Satellite imagery this
early morning shows stratus decks have developed across portions
of the Cascades as well as across portions of the eastern
mountains and Grande Ronde valley, with cirrus developing on the
lee side of the Cascades. Otherwise, dry conditions have developed
across the forecast area with breezy conditions along the Blue
Mountain foothills and through the Cascade gaps.

A transient upper level ridge will move across the forecast area
this morning and continue to provide dry conditions at the
surface. By the afternoon, the ridge will weaken and seemingly be
swallowed into the northwest flow aloft, with a shortwave and
surface frontal system passing through the PacNW this afternoon
through tomorrow morning. The coupled shortwave/frontal system
will bring increasing rain shower activity mainly across the
Cascades and eastern mountains, with chances (25-45%) of very
light precipitation across central OR and the Blue Mountain
foothills. Any rain amounts in the mountains will mainly be around
0.1 inches, with a trace to 0.02 inches in the lower elevations
(confidence 60-80%). The frontal boundary approach and subsequent
passage through the forecast area will reinforce the cross-Cascade
pressure gradient this afternoon, resulting in breezy winds
20-30mph and gusts 30-40mph through the Cascade gaps and winds
15-20mph with gusts up to 30mph across portions of the Columbia
Basin, central OR, and Blues foothills (confidence 70-90%).

Light orographic showers will continue across the Cascades,
Blues, Wallowas into Friday afternoon under a northwest flow
aloft, but a brief dry period will develop later Friday evening as
the flow aloft turns more zonal, then to the southwest ahead of an
upper trough approach.

Late Friday night through Saturday, an upper level trough will
slide southeast across the PacNW with widespread shower chances
developing across the forecast area. Southwest flow developing
ahead of the upper trough passage early Saturday morning will help
to increase surface instability across the mountainous terrain of
the Ochoco-John Day Highlands and southern Blues, resulting in
the initiation of convective showers in these areas, though
confidence is very low (5-10%) in isolated thunderstorms
developing in the morning. Throughout the day, increasing surface
based instability(200-500 J/kg), modest low to mid level lapse
rates, and 0-6km shear increasing to around 30-40kts will also result
in isolated to widely scattered thunderstorm chances (15-25%)
mixing in with showers across most of the forecast area Saturday
afternoon through the late evening (confidence 40-60%). Through
Saturday night, there is a 60-80% chance for rain accumulations of
0.25 inches across the mountain zones, including the Grande Ronde
valley...50-80% chance for 0.1 inches in the lower elevations.
Cold air advection behind the trough axis will also result in a
tightening of the cross-Cascade surface pressure gradient
Saturday afternoon through the overnight hours, resulting in a
return of breezy conditions spilling through the Cascade gaps into
portions of the Columbia Basin and central OR. Lawhorn/82

.LONG TERM...Sunday through Thursday...

1. Mountain rain & dry conditions along the lower elevations

2. Diurnally driven breezy gap winds, breeziest day Sunday

3. Steadily warming temperatures

The long term will be characterized by an array of upper level
systems. Models show the start of the period to be under the
influence of an upper level low nearly overhead of the CWA. Models
as well as ensembles show the higher amount of precipitation to be
locked along the Cascades as well as the eastern mountains while the
lower elevations will see lower amounts. The Basin, and
Central/North Central OR will see a 40-70% probability of 0.05-0.10
inches of accumulated rainfall Sunday while the higher elevations
and the foothills of the Northern Blues of both WA & OR will 60-85%
probabilities of 0.10-0.20 inches of rainfall. By Monday, models
have the upper level low to the southeast of the region and the
leading edge of an upper level ridge moving in to the Cascades with
some lingering showers over the Cascades and with lower confidence
(<20%) that the upper level low will allow for some rap around
moisture being brought in Wallowa County. By Monday afternoon, the
upper level ridge weakens but moves over bringing dry conditions to
the region with only some linger showers over the ridges (<20%).
Tuesday onwards the models somewhat fall out of sync with the next
incoming upper level system. However, most of the models do show the
region to be mostly dry through the period.

Winds on Sunday will be driven by the upper level low and models
show the pressure gradients at the surface strengthen. With the
strong upper northwest flow and the strong pressure gradients at the
surface, the mountain gap winds will be strong through the I-90
corridor as well as the I-84 through the Gorge. The are forecasted
to funnel out of the Gorge and travel along the foothills of the
southern and northern Blue mountains as we as in Kittitas and Yakima
Valleys. 70-90% of the raw ensembles show the aforementioned areas
will see between 35-40 mph gusts and the foothills will see 60-80%
probabilities of 25-30 mph gusts. Winds thereafter will be diurnally
driven due to the temperatures differential between the west and
eastern Cascades. Raw ensembles show that the wind gusts will be
between 20-35 mph daily with 60+% probabilities.

Lastly, temperatures through the long term will steadily rise. The
upper level ridge followed by the near steady west flow aloft,
temperatures will steadily increase from the 70s (Basin) and 50-60s
elsewhere to upper 60s to low 70s most everywhere. Raw ensembles
show the Basin probabilities in the 80+%, central OR, foothills and
the Gorge and Yakima Valley show 60-70% probabilities of over 70,
and 30-50% probabilities for Ellensburg and the lower portions of
the Kittitas Valley will see upper 60s to low 70s. The higher
terrains will remain in the low 50s along the Cascade crest and low
to mid 50s for the Northern Blues and mid to upper 60s along the
southern Blues. Bennese/90

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  66  44  67  46 /   0  50  20  10
ALW  66  46  66  48 /   0  50  40  10
PSC  71  46  71  48 /   0  20  10  10
YKM  69  44  71  47 /  20  20  10  10
HRI  69  44  71  48 /   0  30  10  10
ELN  65  41  66  44 /  20  30  10  20
RDM  64  37  66  41 /  10  40   0  20
LGD  64  42  61  42 /   0  60  40  20
GCD  64  39  63  41 /   0  60  20  30
DLS  64  45  70  48 /  20  30  10  20

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
&&

$$

SHORT TERM...82
LONG TERM....90
AVIATION...97