944
FXUS66 KPDT 132335
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
435 PM PDT Sun Sep 13 2026

.KEY MESSAGES...

- Additional mountain rainfall with amounts in of 1 to 1.5
  inches over the next 18-hrs promoting a flash flood and debris
  flow risk for burn scars in the northern Blue mountains.

- General drying and modest warming trend this upcoming work
  week.

- Next chances for rain confined mainly to the mountains mid-
  week, late Tuesday/Tuesday night and late Thursday/Thursday
  night.

&&

.DISCUSSION...
Latest water vapor imagery shows the upper-level
low over eastern WA and complex troughing over the West Coast.
Meantime, ample moisture is seen on the western side of this
low that is steadily progressing eastward into the interior with
the main shortwave trough rotating east into southeast OR.
Latest radar display shows a swath of light rain starting to
cross the western WA Lower Basin towards the eastern mountains.
1-hr rainfall totals have been light, a few hundredths to a
tenth of an inch at lower elevations, while 6-hr totals have
been reaching 0.1-0.2 inches. Elsewhere, a quarter inch to a
half inch has been seen along the eastern slopes of the WA
Cascades as of 1 PM PDT.

The aforementioned area of light precip will continue eastward
this afternoon across the rest of the Lower Basin with
orographic enhancement anticipated across the northern Blue
mountains. Appreciable moisture is being tapped into this system
for this time of year with IVT forecasts showing 200-300 kg/ms
penetrating into the Basin and the eastern mountains; EPS mean
precipitable water forecasts approaching/reaching 150% of
normal across the area. This increases confidence in the next
round of moderate to locally heavy rain in the northern Blues
and Wallowas. Latest 24-hr LPMM by HREF show a reasonable 1-1.5
inches ending 12 Z tomorrow morning/Monday, elsewhere a tenth
to a quarter inch in the lower Basin and foothills with less
than a tenth across north central and central OR. Confidence is
bolstered in this outcome with NBM probs showing 50-70% chance
for over 1 inch across the northern Blues. Otherwise, the
combination of residual moisture and recent rainfall could
promote a minor risk for patch fog, mainly along valleys and
drainages of the Lower Basin and in areas of the eastern
mountains. Confidence in the fog threat isnt high (30-50%) at
this time when it comes to occurrence.

After this upper-level system departs east across the Northern
Plains and Canadian prairies, large scale guidance is consistent
in the development of another upper-level trough midweek
overhead with additional amplification seen upstream of the
upper-level ridge across the Gulf of Alaska into AK. While
ensemble guidance shows some spread, the upper trough and its
closed low digs farther south into CA by mid-week with
clustering guidance tilting moderately towards this outcome by
Thursday (~60%) with then increasing odds in general mean
troughing persisting to Friday (~50%). Over this time period, a
short drying trend will be seen after this current system exits
the area before precip chances increase late Tuesday-Tuesday
night (20-50%, highest in the mountains) followed by an uptick
again in precipitation chances over the eastern mountains
Thursday/Thursday night. Meanwhile, a warming trend is expected
with highs near average by Thursday with readings slightly above
average Friday.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Light to moderate showers continue for the local area early
this evening. For the terminals, most of this activity will
taper out through mid to late evening. Most TAF sites will
remain at VFR for the duration of the period, while some
guidance indicates that ALW at least may drop to MVFR tonight
due to lower CIGs. However, VFR conditions will return around
10Z Monday morning area-wide. Sustained southwesterly winds this
evening look to increase to around 10-15 knots around 6Z, with
gusts mainly between 15-25 knots. West/southwesterly winds will
then gradually decrease into and through the day on Monday, with
gusts up to around 15 knots for some locations.

&&

.Preliminary Point Temps/PoPs...
PDT  45  70  44  68 /  60   0   0   0
ALW  50  71  50  70 /  80   0   0   0
PSC  46  74  45  72 /  40   0   0   0
YKM  44  69  47  67 /  20   0   0  10
HRI  46  73  44  71 /  50   0   0   0
ELN  41  65  42  62 /  30   0   0  30
RDM  33  71  36  71 /  10   0   0   0
LGD  43  70  42  72 /  90   0   0   0
GCD  41  70  42  74 /  20   0   0   0
DLS  51  74  50  74 /  10   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
WA...Flash Flood Watch until 8 PM PDT this evening for WAZ030.
OR...Flash Flood Watch until 8 PM PDT this evening for ORZ502.

&&

$$

DISCUSSION...80
AVIATION...99/CGAA