Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
650 FXUS66 KPDT 172042 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 142 PM PDT Sun Aug 17 2025 .DISCUSSION...Satellite imagery this afternoon shows fairly sparse cumulus fields developing over the higher terrain of the OR Cascades and the eastern mountains. Otherwise, skies are mostly clear with a band of cirrus passing over central WA. Today through Monday: The PacNW will continue to sit under the influence of a broad upper low in the northeast Pacific, keeping temperatures much cooler than normal. Shortwave impulses swinging around the bottom of this low will move across the forecast area later this afternoon and overnight, initiating shower and thunderstorm activity mainly across the eastern mountains, with an isolated cell or two developing in central OR. Some CAMs members and the deterministic NAM and ECMWF indicate another impulse passage late tonight into early tomorrow morning, resulting in shower chances (15-20%) developing across portions of the eastern mountains into the Columbia Basin and eastern portions of the Yakima valley. Upper level ridging will begin to build northwest into the PacNW Monday, greatly decreasing chances of shower activity across the forecast area. That said, a slight chance (~10-15%) of isolated thunderstorms will be possible across portions of Wallowa county in the afternoon. Otherwise, the area will start to see some warming, though temperatures will be near to below normal. Tuesday Through Saturday: High confidence (75-90%) in upper level ridging continuing to build into the region Tuesday with a warming and drying trend that will persist throughout the week. Wednesday, ensemble cluster solutions are in fairly good agreement that a dry shortwave will flatten the ridge briefly, with locally breezy winds developing through the Cascade gaps as the main threat. However, 12Z deterministic guidance and the ECMWF ensemble mean indicate a slight chance (~15%) for isolated thunderstorms to develop across the Blues and Wallowas as the shortwave reaches this area. Confidence is currently low (10-20%) in occurrence, so have left out of the afternoon forecast package at this time. After Wednesday, ensemble cluster members stay in great agreement that upper level ridging over the Desert Southwest will continue to build into the PacNW and into western Canada. During this period, confidence is mod-high (60-85%) that by the end of the workweek a warming trend will bring temperatures back into the lower to mid 90s in the lower elevations, with temperatures warming into the mid to upper 90s over the weekend. A drying trend during this period will also result in widespread RHs into the mid teens to lower 20s by Friday, while winds will remain light through most of the period. Lawhorn/82 && .AVIATION...Previous Discussion...18Z TAFs...VFR conditions to prevail through the period. A weak upper weather system passing over the region tonight will bring a chance (15-30%) of scattered showers along the Blue Mountain foothills and portions of the Columbia Basin, resulting in a prob30 grouping for sites PDT/ALW. Probability/confidence in precipitation impacts for site PSC are too low (15-20%) to include at this time. CIGS will mostly be few- sct and above 8kft AGL at all sites through the period. Winds will increase to 12-15kts with gusts up to 25kts at site DLS this afternoon, becoming light overnight. Otherwise, winds will be 12kts or less at all other sites through the period. Lawhorn/82 && .PRELIMINARY POINT TEMPS/POPS... PDT 59 86 57 88 / 20 10 0 0 ALW 63 87 61 88 / 20 20 0 0 PSC 60 90 58 90 / 20 20 0 0 YKM 59 86 56 89 / 10 10 0 0 HRI 61 88 59 90 / 20 10 0 0 ELN 57 83 55 85 / 10 10 0 0 RDM 50 84 46 86 / 20 0 0 0 LGD 55 87 54 90 / 20 10 0 0 GCD 55 89 51 93 / 20 0 0 0 DLS 62 82 59 86 / 10 10 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ DISCUSSION....82 AVIATION...82