Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
759 FXUS66 KPDT 220510 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 1010 PM PDT Mon Jul 21 2025 .UPDATED AVIATION...06Z TAFs...VFR conditions will persist through the forecast period. Prevailing winds will be from the northwest with some sites seeing diurnally driven winds below 10 kts and variable. CIGs will vary from SCT100-250 to BKN100 overnight. Bennese/90 && PREV DISCUSSION... /issued 131 PM PDT Mon Jul 21 2025/ DISCUSSION...Today through Tuesday...An anomalously deep, closed upper-level low centered over the OR/WA border will continue to drive active, cooler-than-normal weather through Tuesday. This afternoon and evening, isolated to scattered (areal coverage) showers and thunderstorms are likely (>90% confidence in occurrence) across much of our forecast area. Partial clearing across the Blue Mountains, coupled with daytime heating, steepening lapse rates beneath the upper low, and orographic lift has facilitated isolated showers and thunderstorms early this afternoon over the Blue Mountains and the east slopes of the Washington Cascades. Looking through the remainder of the afternoon and evening, 12Z HREF model guidance keeps the bulk of the convection co-located with the best surface-based CAPE (500 to locally 1000 J/kg) and orographic forcing; namely, this is over the Blue Mountains and the east slopes of the Washington Cascades where activity has already begun. Elsewhere, NBM probabilities paint lower chances of showers (20-50%) and thunderstorms (10-25%) for the Blue Mountain foothills, portions of central Oregon, and the lower Columbia Basin. The main hazards with showers and thunderstorms today will be locally heavy rainfall and small hail. Ensemble and deterministic NWP guidance are in excellent agreement that the closed low will transition to an open wave as it departs to the northeast overnight through Tuesday. While shower and thunderstorm chances will be lower Tuesday, NBM probabilities still place a 15-40% chance across the Blue Mountains and east slopes of the Washington Cascades. Wednesday through Sunday...Near- to above-normal temperatures are forecast, but the main weather concerns will revolve around convective potential and the return of breezy to windy conditions through the Cascade gaps. Clusters of ensemble NWP members all show a closed upper low developing off the California coast Wednesday. While there are small differences in timing and amplitude of the low among clusters of members, all clusters then show the low gradually tracking north-northeast along the coast through Friday, eventually tracking inland across northern California and the Great Basin over the weekend. Given modest synoptic-scale lift from the low, coupled with surface-based instability and mid- level moisture, a slight chance (10-15%) of showers and thunderstorms is forecast for the high terrain of central Oregon and the Blue Mountains east afternoon and evening Thursday through Saturday. Strong, gusty outflow winds and cloud-to-ground lightning currently appear the mostly likely hazards associated with convection. No Fire Weather Watches have been issued due to insufficient confidence in timing/location of convection and any threat of abundant lightning. Regarding wind and RH concerns, NWP guidance suggests a 7-10 mb pressure difference across the Cascades from PDX to GEG Thursday and Friday as a trough moves over southern Canada. Coupled with warm temperatures in the upper 80s to upper 90s and a dry air mass (ensemble-mean PWATs of 70-100% of normal), there is some potential for Red Flag conditions through the Cascade gaps. That said, confidence is too low (<50%) to issue any Fire Weather Watches at this time. Plunkett/86 && .PRELIMINARY POINT TEMPS/POPS... PDT 56 88 60 93 / 20 10 0 0 ALW 60 87 64 93 / 20 10 10 0 PSC 57 90 60 96 / 20 10 10 0 YKM 58 88 61 93 / 30 30 10 0 HRI 58 91 60 96 / 10 0 10 0 ELN 58 87 59 93 / 40 40 10 0 RDM 46 84 50 90 / 10 10 0 10 LGD 52 81 54 88 / 40 20 0 10 GCD 50 83 54 89 / 20 10 0 10 DLS 61 89 65 94 / 10 10 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...86 LONG TERM....86 AVIATION...90