Union, Oregon Weather Forecast Discussion

457
FXUS66 KPDT 112017
AFDPDT
Area Forecast Discussion
National Weather Service Pendleton OR
117 PM PDT Thu Sep 11 2025
.DISCUSSION...Current satellite and radar imagery show some
isolated storms beginning to develop off portions of the southern
Blues and the Wallowas this afternoon, with scattered cloud cover
across northeast OR, and portions of southeast WA.
Today through Saturday: An upper low will continue to push across
the eastern half of the PacNW today and slowly push east into the
northern Rockies through Friday night. High-res members of the
HREF show another round of scattered thunderstorms and showers
developing through this evening from central OR up to southeast
WA, with best chances (20-40%) of thunderstorm activity across the
eastern mountains. By late tonight, shower/thunderstorm activity
will retreat to far northeast OR and southeast WA, then ill
diminish by late Friday evening as the low pushes further east of
the region. Plenty of moisture support from the upper low, modest
CAPE (250-750J/kg), and modest lower to mid level lapse rates
allow thunderstorms to develop fairly quickly through the
afternoon, however storms will be slow moving as steering flow is
fairly weak (10-15kts) in the lower and mid levels. These
atmospheric ingredients will allow storms to produce some gusty
outflow winds, small hail, heavy rain, and abundant lightning.
The slow moving cells and/or training cells with heavy rain will
bring a risk of flash flooding (at least 5%) to recent burn scars
across the eastern mountains. By Saturday, drier conditions will
settle into the region as transient ridging slides across the
PacNW.
Saturday Night through Monday: Ensemble guidance is in good
agreement of an upper trough and surface cold front swinging
across the PacNW through Monday, though ensemble members (ECMWF,
GFS, and Canadian) are split 50/50 on whether the upper trough
exits by late Monday morning or by late Monday afternoon. The
upper trough/cold front passage will produce a round of
showers with mountain thunderstorms starting across the Cascade
crest early Sunday morning, then spreading east throughout Sunday.
Best precipitation chances (60-80% mountains and 25-50% elsewhere)
will be associated with the cold front passage Sunday morning and
afternoon, with drying conditions in the lower elevations and
chances decreasing in the mountains (20-45%). Ensemble guidance
also depicts the trough taking on a negative tilt as it comes
ashore, which will lend enough instability for slight chances
(15-20%) of isolated thunderstorms across the Cascade crest and
eastern mountains. Breezy west winds (15-25mph and gusts
25-40mph) will also develop Sunday afternoon behind the cold front
passage, but winds will be light to locally breezy
Monday(confidence 50-80%).
Tuesday through Thursday: Ensemble guidance continues to be in
good agreement that upper level ridging will once again slide back
over the PacNW Tuesday, resulting in drying conditions area wide.
Behind the upper ridge, upper level troughing ejecting from a
larger trough over the Aleutian Island chain will approach the
PacNW late Tuesday, however, ensemble guidance comes into
disagreement with the location, strength, and timing of the
trough passage. About 60% of ensemble members favor the upper
trough inland by Wednesday afternoon, while the remaining 40% keep
the trough offshore. Of the aforementioned 60% of members, only
20% have the trough clipping the PacNW with shower activity across
the mountains while the other 40% keep the trough and
precipitation further north in Canada. As for late Wednesday into
Thursday, ensemble guidance does agree that shortwave troughing
will develop over the PacNW, though conditions look to remain
mostly dry, except for the low potential (15-20%) of a shower or
two over the WA Cascade crest. Lawhorn/82
&&
.AVIATION...Previous Discussion...18Z TAFs...Mostly VFR
conditions to prevail through the period. An upper weather system
moving through the area will bring another round of showers and
thunderstorms to the mountains. Currently, showers moving over
sites RDM/BDN are expected to produce MVFR or lower conditions,
with showers moving out by 19Z. Otherwise, thunderstorm chances
were too low (10-20%) at sites PDT/ALW/RDM to include mention,
however, have introduced a prob30 for this afternoon/evening at
site BDN. Winds will be mostly light, 12kts or less, through the
period...exception being site DLS where winds will increase to
12-20kts with gusts 20-30kts this afternoon and evening.
Lawhorn/82
&&
.PRELIMINARY POINT TEMPS/POPS...
PDT  56  82  56  85 /  20  10   0   0
ALW  61  83  61  85 /  30  10   0   0
PSC  57  86  55  87 /  10   0   0   0
YKM  56  86  56  86 /  10   0   0   0
HRI  58  85  56  87 /  10   0   0   0
ELN  53  84  52  85 /  10   0   0   0
RDM  44  79  46  83 /  30   0   0   0
LGD  51  79  51  83 /  50  40  10   0
GCD  50  78  51  82 /  40  30  10   0
DLS  58  83  57  87 /  10   0   0   0
&&
.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Flash Flood Watch until 11 PM PDT this evening for ORZ503-505-
     506.
WA...None.
&&
$$
SHORT TERM...82
LONG TERM....82
AVIATION...82

NWS PDT Office Area Forecast Discussion