305
FXUS66 KPDT 040530
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1030 PM PDT Thu Sep 3 2026

.KEY MESSAGES...
- Wet, unsettled weather with multiple rounds of light and
  moderate rain through Monday, heaviest in the mountains.

- Low repeated chances of thunder, mainly in the mountains, around 15-30%.

- Cooler below normal highs by 10-15 degrees through Saturday
  before returning to near average mid next week.

&&

.DISCUSSION...
Latest water vapor imagery shows a deep mid-
level closed low with its centered roughly offshore northwest OR
with broad troughing encompassing much of the West Coast with
another disturbance lifting north across Alberta. Day cloud
phase RGB imagery depicts multiple bands moving inland with
infrequent lightning noticed at times. Otherwise, moisture
continues to be transported northeastward into the region with
latest MIMIC-TPW showing best transport south across northern
into eastern OR with values over 1.5 inches. Latest radar
display shows light showers blossoming across central OR
mountains into the eastern mountains the last two hours with
clear skies over parts of south central WA and north central OR.

Overall guidance in good agreement in the large scale pattern
through Tuesday based on ensemble clustering that will be
conducive for cooler weather with chances for rain, highest in
the mountains through Saturday. Closed low offshore this morning
will make limited eastward progress the next 48-hrs. EPS 500 mb
mean shows a deep closed low initially with its forecast falling
outside model climo before moving into eastern OR/WA by Friday.
While this feature will evolve and fill in through Saturday, the
ensemble mean will still remain below the the %tile. As this
occurs, ample moisture will stream and penetrate more inland
into the eastern OR mountains the next 24-hrs with precipitable
water values exceeding 150% of normal per the EPS by early
tonight. The arrival of this increased moisture ahead of the
best synoptic forcing moving into southern OR now that will be
rotating north and east in time, combined with orographic
enhancement, will promote moderate to locally heavy rainfall
across the eastern mountains. NBM Guidance shows 24-hr amounts
ending 18Z Friday/11 AM in excess of 0.5 over 80% with best
chances for more rain, 1 or more, above 50% localized to the
Wallowa county where best transport and forcing will be
maximized. As the upper trough evolves, repeated chances for
rain persist Friday-Saturday. While the wettest period remains
the next 24-hrs this afternoon through early tomorrow, tomorrow
afternoon will hold better chances for thunder (30-50%, highest
in the mountains) with excellent agreement seen between REFS and
HREF with an additional window expected on Saturday.

Current model guidance is in good agreement on the upper level
pattern for Saturday night through Sunday with a closed upper-
level low centered over Northwestern California and Southwestern
Oregon (80% confidence). This upper level trough should remain
relatively stationary throughout the day. It is worth noting
that models indicate the presence of a jet streak downstream of
the trough which will cause the trough to start lifting towards
the end of the day Sunday. However, the weather locally will be
driven by the closed upper low.

Overnight temperatures Sat-Sun will be generally around normal
with lows in the upper 40`s at lower elevations like the
Columbia Basin and low to upper 30`s for higher elevations like
the Blue Mtns (60% confidence). Temperatures on Sunday will be
below average by about 5-10 degrees with highs in the mid to
upper 70`s for the lower elevations. The difference is driven by
models suggesting that there will be increasing cloud cover
throughout the day on Sunday. However, models are handling the
cloud coverage differently and thus increasing uncertainty in
the high temperatures. Models do also suggest there is a chance
of measurable rain (>0.01 in) increasing throughout the
afternoon with the highest probabilities in areas of the highest
terrain (50-60% chance). The Columbia Basin has the lowest
chance throughout the afternoon with probabilities between
10-20%. Additionally, with the upper low being close to
overhead, weak instability will develop which could lead to
isolated rumbles of thunder across central Oregon (10-20%).

Sunday evening to Monday, the overhead low should continue to
move eastward through the forecast area and bring light-moderate
rain chances to lower elevations in the columbia basin (confidence
50%) and more persistent rain towards the blue mts. There is a
low chance for isolated thunderstorms (confidence 10-15%)
focused in central oregon since there is upper-level
southwesterly flow bringing in weak instability and cold air
aloft from the low, but this threat should diminish as the
system pushes eastward late sunday night into early monday
morning.precipitation amounts across the cascades and blues will
about 0.1 to 0.5 inches of 24-hr precip totals.

Per deterministic guidance, the upper-level low should pivot
northeastward and bring slight chances of rain (25-30%) to
portions of central and northeast oregon monday morning-
afternoon from 12z-18z before moving east into western idaho. By
monday evening (Tuesday 00z), the unsettling weather should be
further to our east and we will see more zonally flow a lot
bringing mostly dry conditions,light winds and near normal
temperatures (confidence 60-70%).

Meanwhile, a very cool airmass will remain in place with the
grand ensemble of the EPS, GEFS, and CMC showing forecast 850 mb
mean temperatures below the 25th %tile area-wide aloft. The
combination of cloud cover, limited insolation, and precip will
keep highs well below normal. The EPS Extreme Forecast Index
increases overall confidence with EFI at -0.5 to -0.8 across
much of the area Friday persisting through Monday. The EPS EFI
signals the greatest departure from model climo in the EPS
member forecasts roughly across Central OR and the John Day
Highlands. Of note, Wallowa valley will be flirting with low
30s tonight, however, confidence in freezing conditions are
tampered between cloud cover and precipitation that is forecast.
Of note, NBM chances for freezing lows 32F or cooler are 30% or
less, which seem elevated given the lack of radiational cooling
conditions and limited advection tonight.

Tuesday and beyond, increased spread in amplitude and phase are
seen in the ensemble guidance with respect to the synoptic
pattern. The mean upper-level troughing pattern is anticipated
to be weakening along the coast by early next week with
increased potential for a building ridge becoming more likely offshore
the West Coast (40-50% confidence). With a more zonal flow
expected to be in place mid-week, the main outcomes of this
evolution is a drying and warming trend Tuesday-Thursday.
Current highs are forecast near average Tuesday to reading
slightly above average by Thursday. Meantime, precipitation
chances will be low 5-20% Tuesday-Thursday with highest chances
along the crest of the Cascades.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Mainly VFR conditions are expected through the period. However,
if there is a heavy shower or thunderstorm that affects any TAF
site than MVFR (or possibly lower) can briefly occur. IFR
conditions should be 20% or less.  Winds will generally be 10
kts or less.


&&

.Preliminary Point Temps/PoPs...
PDT  42  71  42  69 /  60  10  10  40
ALW  47  72  48  71 /  60  30  10  30
PSC  45  74  45  73 /  20  10  10  30
YKM  48  73  47  71 /  10  10  10  30
HRI  45  74  45  72 /  30  10  10  40
ELN  42  69  41  68 /  10  20  10  40
RDM  33  64  34  67 /  70  50  60  10
LGD  42  68  40  71 / 100  40  20  50
GCD  40  68  40  70 / 100  20  30  40
DLS  50  75  51  74 /  50  20  20  50

&&
An upper low will continue bringing rain showers and isolated
thunderstorms through the TAF period to the mountain zones.  Most of
the cloud ceilings will be in the VFR to MVFR range, with about a 10
to 20% chance for (2-3kft) cigs in rain showers in the Blue Mt
foothills (ALW/PDT) overnight, focused through around 9 am Friday.
Winds under 12 kts.
.PDT WATCHES/WARNINGS/ADVISORIES...
WA...None.
OR...None.

&&

$$

DISCUSSION...80
AVIATION...77