261
FXUS66 KPDT 120548
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1048 PM PDT Fri Sep 11 2026

.Updated Aviation Discussion.


.KEY MESSAGES...
- Scattered rain showers and Isolated thunderstorms developing
  Saturday evening and rain showers becoming more numerous
  though Sunday morning.

- Widespread wetting rain Saturday Night and Sunday

- Breezy to windy across the eastern Columbia River Gorge
  Saturday Night and Sunday Morning.

&&

.UPDATE...
The incoming system continues to trend towards a wetter
scenario, which is advertised by a 20-30% increase in
precipitable water via the AI ECMWF ensembles over the last 24
hours. This is a direct result of increased resonance time near
a closed low pressure system further offshore over the Eastern
Pacific. The delay has also attributed to a stronger transient
ridge of high pressure building ahead of this system, which will
allow for thunderstorm development across the Blue Mountains and
leading to increased potential for heavy downpours in and around
fire scars of the Paradise and Hagen wildfires. The REFS
suggests convection beginning around 6 PM and extending until
shortly after midnight. As a result, a FLOOD WATCH has been
issued across both burn scars from Saturday afternoon through
Sunday evening as rain rates of around 0.5"/hour may be achieved
within a developed rain core. 75

&&

.DISCUSSION...
Regional radar was clear this afternoon with GOES satellite day
cloud phase showing thin high level clouds spreading east
across the Upper Columbia Basin. Eastern WA Eastern OR, while
most of central Oregon and the southern Blue Mts were
clear/sunny. The weak zonal flow aloft across the region Today
will be disrupted as an upper low spreads onto the Oregon coast
by late Saturday Afternoon and Evening, placing the Interior NW
under a broad area of PVA advection/diffluence aloft. The
showers that are already going on Today west of the Cascades
will be diurnal in nature, so no rain is anticipated overnight.
Increasingly broad scale lift and moisture transport will
promote instability on Saturday, enough so for isolated to
widely scattered thunderstorm and widespread wetting rains from
few tenths of an inch to potential for over an inch in the
Mountain zones though the day on Sunday, once the upper system
exits the region. Initially a few showers are possible along the
Cascade crest, but everywhere farther east will be dry through
about the end the afternoon Saturday. HREF (ensembles) show a
ramp up in the mean 1 hourly precipitation fields between about
5 and 9 pm, across the Eastern Slopes, the Columbia Basin and
foothills of the Blue Mts. There is nothing all that anomalous
with the PWATS for this system. About a 1 standard deviation in
precipitable water shows up across the Cascades and the Eastern
Mts Saturday afternoon/Evening and spreading north into eastern
WA though Sunday morning as the cold conveyor belt lifts across
the Palouse. Trends for thunder continue to be pushed back in
time as the 12z HREF now shows 4hr calibrated thunder (10 to
30%) around 6z (nearing midnight Saturday) or so for the
foothills of the Blues. The period of heaving rainfall will be
between midnight Saturday and about noon Sunday with model
spreads fairly large. Uncertainty at the Tri-Cities is a range
of (using 25th to 75th percentile) anywhere form at least about
a quarter of an inch to and inch and and quarter by late Sunday
evening. That uncertainty/spread is compressed a bit over the
mountain zones. For instance at La Grande where at least one
half inch can be expected with a higher outcome around an inch
an a quarter of rainfall. Rainfalls of around an inch are fairly
likely (above about 60%) for the Paradise/Hagan fire areas.
Westerly winds ramp up through the gaps on Saturday evening as
well with the stronger period running from around midnight
Saturday though late morning Sunday morning. The NBM mean shows
potential for sustained winds in excess of 30 mph and gust in
excess of 40 mph however these stronger winds are limited to the
higher ridge-tops along the Gorge. Most other areas look breezy
and falling under the general wind advisory criteria. The wet
regime will hamper temperatures Sunday, where mid 60s will be
the highest temps across the the Kittitas Valley, central Oregon
and parts of the Basin. Dry weather and pleasant temperatures,
with not a lot of wind will finish out the balance of the week
before NBM trends toward a warmup by the next weekend.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
VFR conditions currently across all sites, which will stay the
course through the period. Ceilings drop to around 10kft in the
afternoon, with breezy winds of 15-25kts and gusts of 20-30 kts
expected early evening. Light rain is anticipated to begin
toward the end of the period. 75

&&

.Preliminary Point Temps/PoPs...
PDT  47  76  45  61 /   0   0  90  80
ALW  52  79  49  60 /   0   0  90  90
PSC  50  80  47  63 /   0   0  90  90
YKM  54  76  50  63 /   0  10  90 100
HRI  49  79  46  63 /   0   0 100  80
ELN  48  72  44  59 /   0   0 100 100
RDM  40  71  36  60 /   0  10  80  10
LGD  47  81  44  59 /   0   0  90  80
GCD  46  81  43  59 /   0   0 100  40
DLS  54  77  51  66 /   0  20  90  60

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
WA...Flash Flood Watch from Saturday afternoon through Sunday
     evening for WAZ030.
OR...Flash Flood Watch from Saturday afternoon through Sunday
     evening for ORZ502.

&&

$$

UPDATE...75
DISCUSSION...71
AVIATION...75