Monument, Oregon Weather Forecast Discussion

829
FXUS66 KPDT 291130
AFDPDT
Area Forecast Discussion
National Weather Service Pendleton OR
429 AM PDT Mon Sep 29 2025
.DISCUSSION...
Key Messages:
1. Showers and isolated storms possible through mid-week.
2. Breezy afternoon winds through the week.
3. Cooling and wetter trend begins today.
Current radar and infrared satellite imagery shows light returns
over Wallowa county and over the east slopes of the Cascades
under partly to mostly cloudy skies. This is in response to the
upper level trough and attendant cold front beginning to slowly
push onshore. The cold front is poised to cross into the Columbia
Basin late this afternoon before stalling over eastern Oregon and
southeast Washington tonight into Tuesday morning. As a result,
showers are expected to be mainly confined along the east slopes
of the Cascades, Central Oregon, southern Blue Mountains, and the
John Day Highlands before becoming more widespread after 5 PM
today. Rain will then taper off overnight into Tuesday morning
before a reinforcing cold front moves onshore to return widespread
showers chances (30-60%) to the region Tuesday afternoon through
Wednesday evening. Better rain chances will reside at elevation
over the east slopes of the Cascades and Blue Mountains.
Even with the moderate (45-65%) overnight humidity recoveries
this morning and afternoon humidities still dipping into the
35-45% range, today looks to provide the most efficient moisture
transport and highest rain chances as indicated by Precipitable
Water (PW) amounts around 1" and Integrated Water Vapor of 24-28
mm from the subtropics. Rain amounts of 0.10-0.20" are expected at
elevations above 4000 feet, with amounts of 0.05-0.10" along the
Blue Mountain foothills, and 0.05" or less for elevations of 1500
feet or less. Confidence in these rain amounts is moderate to
good (50-70%) as the NBM suggests a 60-70% chance of 0.10" over
the east slopes and Blue Mountains, a 50-60% of 0.05" or greater
over the Blue Mountain foothills and Central Oregon, and 40-60%
chance of 0.02" across lower elevations of the Columbia Basin
(Tri-Cities, Hermiston, Sunnyside, Arlington).
The incoming system will be eroding the backside of the exiting
ridge, which will provide enough instability to allow for some
isolated, discrete thunderstorms to develop through the afternoon.
There also looks to be some cloud breaks by late morning, which
would allow additional energy and lifting properties to be
present. This is indicated by 100-200 J/kg of surface CAPE via
the NAM and HRDPS. HREF ensembles also indicated isolated
thunderstorm potential initiating after 1 PM across Central
Oregon, moving into the Columbia Basin and Grande Ronde Valley
after 3 PM, then into Wallowa county after 5 PM and dissipating
after 8 PM. The main concerns with any developing thunderstorms
will be frequent lightning, brief downpours, small hail, and
gusty/erratic winds. Severe thunderstorms are not expected (

NWS PDT Office Area Forecast Discussion