FXUS66 KPDT 202115 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 215 PM PDT Sun Sep 20 2026 .KEY MESSAGES... - Warming and drying trend through Monday. - Mountain showers Tuesday, widespread rain late in the week. - Breezy winds Tuesday and Friday. && .DISCUSSION... Current radar and visible satellite imagery shows dry conditions and clear skies as an upper level ridge of high pressure slowly moves to our east. This has allowed high temperatures to increase 4 to 6 degrees from yesterday as highs peak in the mid-80s across the Columbia Basin, Gorge, and Central Oregon. High temperatures will continue to climb a couple more degrees on Monday as flow aloft becomes more from the southwest and brings warmer air across the region ahead of the next system on Tuesday. Low afternoon humidities are also present through Monday with values dropping into the mid-to upper teens across Central Oregon and the John Day Basin. Humidities will be slowly improving through the week as multiple systems eye the Pacific Northwest. The first of these systems, an upper level shortwave, will move onshore over Central British Columbia on Tuesday. The best chance (15%) for light showers and isolated thunderstorms will occur over the John Day Highlands. 60% of ensemble members showcase CAPE values of 250-350 J/kg, which extends across southern Union and Wallowa counties on Tuesday afternoon. However, the limiting factor may be a moisture trigger as only 51% of ensembles suggest measurable precipitation (0.01" or greater) over these areas, with best chances across the John Day Highlands. The lack of shear may also attribute to minimal thunderstorm development as only 50% of ensemble members show shear of 20 knots or more, with only 21% of members showcasing both elevated CAPE values of around 300 J/kg and shear of 20 knots or greater. As a result of the incoming system and departing ridge, breezy winds are expected through the Columbia Gorge and Kittitas Valley with west-northwest winds of 20-25 mph with gusts of 30-35 mph possible. A ridge of high pressure will build across northern California and into Idaho Wednesday and early Thursday ahead of the next, more robust system Thursday afternoon. Guidance is coming into slightly better agreement with the timing and strength of this system, as 64% of ensemble members advertise showers arriving along the Cascades Thursday afternoon, with 92% of members suggesting widespread rainfall by Friday afternoon. The AI ECMWF and GFS ensembles highlight precipitable water of 0.7-0.9" across the Columbia Basin, which is 120-140% of normal and aligns with more of a wetter scenario than what guidance was suggesting just 24 hours ago. In response to this recent shift in guidance, confidence is still quite low related to specific rain amounts. The highest rain amounts are expected to occur across the Blue Mountains and Wallowa/Union counties Thursday through Friday. Due to the building ridge of high pressure to our south, breezy winds through the Columbia Gorge and Kittitas Valley are likely. Current guidance hints at slightly lower winds than on Tuesday, but this will have to be analyzed further as the event nears. 75 && .AVIATION /18Z TAFS THROUGH 18Z MONDAY/... Light wind and no ceilings are a high confidence forecast over the next 24 hours (95-100%) as a weak high pressure system remains in place over the region. && .Preliminary Point Temps/PoPs... PDT 84 52 84 52 / 0 0 0 0 ALW 84 57 85 57 / 0 0 0 0 PSC 85 53 86 54 / 0 0 0 0 YKM 83 54 86 54 / 0 0 0 0 HRI 85 54 86 55 / 0 0 0 0 ELN 80 52 81 51 / 0 0 0 0 RDM 84 46 84 45 / 0 0 0 0 LGD 80 51 84 52 / 0 0 0 0 GCD 83 49 85 52 / 0 0 0 0 DLS 88 60 86 56 / 0 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... WA...None. OR...None. && $$ DISCUSSION...75 AVIATION...71