FXUS66 KPDT 251748 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 1048 AM PDT Sun Aug 25 2024 .AVIATION...18Z TAFs...VFR conditions are forecast through the period for all sites, accompanied by sustained winds of 10 kts or less. With regard to cloud cover, 12Z HREF guidance has generally under-forecast the FEW-SCT stratocumulus between 035-050 that has developed along the higher terrain of central, north-central, and parts of northeast Oregon, likely due to issues capturing boundary layer moisture profiles. While no mention was made in the 18Z TAFs, confidence is medium-high (40-70% chance) that a few of these stratocumulus will drift over BDN/PDT/ALW this morning. Any significant change in cloud cover will be handled with amendments. This afternoon, forecast soundings indicate this stratocumulus will likely (>70% chance) be over the higher terrain, away from terminals, with just FEW-SCT mid/high cloud drifting over the region. Plunkett/86 && .PREV DISCUSSION... /issued 742 AM PDT Sun Aug 25 2024/ UPDATE...A morning forecast update was done to add some patchy fog to central Oregon south of Bend. SHORT TERM...Today through Tuesday night...The main forecast challenge in the short term will be a strong cold front sweeping across the region Monday night and Tuesday. Until then, the weather will be relatively quiet. Today will be a great day for outdoor activities with dry conditions and slightly cooler than average temperatures. A departing upper level trough is followed by a drier northwest flow. Skies are mostly cloudy east of the Columbia Basin, and the clouds are keeping temperatures mild compared to this time Saturday morning. The low level upslope will keep the stratocumulus clouds along the northern Blues and Wallowa this afternoon while the rest of the area will be partly to mostly sunny. A shortwave ridge will bring dry and warmer temperatures on Monday. An inverted surface thermal trough on Monday will push temperatures into the upper 70s to upper 80s -- an 8-12 degree warmup compared to today. Models have consistently shown a shortwave trough with strong upper level support traveling across BC and northern WA Monday night and Tuesday. There are some differences in the strength and precipitation associated with the trough, as the deterministic ECMWF and at least half of the EC ENS members are deeper with more QPF along the WA Cascades, and these solutions are also spreading precipitation across the Upper Columbia Basin down to Yakima and the Tri-Cities. Due to the right entrance region of the jet positioned over the Columbia Basin and signs of elevated instability, a slight chance (15-20%) of showers will be added to the forecast Monday night for this area. Any precipitation will be quick as there is dry air behind the front. The main concern with this front will be the winds...a classic pattern for strong winds through the Cascade gaps, particularly the Kittitas Valley and the eastern CR Gorge near Maryhill and Rufus. Out of the 17 members of the NBM, 50% show wind gusts as strong as 35 mph, 25% over 45 mph, and 10% over 50 mph at Ellensburg. The probabilities are not as strong for Maryhill, but a majority show gusts at least 30 mph. It will be breezy for other areas of the Lower Columbia Basin, central and north central Oregon, and the Grande Ronde Valley. Patchy blowing dust was also included for some areas of the Basin. However, the relative humidity will be in the upper teens and 20s...not critical for red flag. The Hot Dry Windy Index shows conditions predominantly in the 75th percentile and a few ensembles above the 90th percentile. At this time, the highlights that may be needed for Tuesday will be Wind Advisories but no red flag concerns at this time. Wister/85 LONG TERM...Wednesday through Sunday...As the shortwave trough exits eastward, a upper ridge takes over the PacNW through the weekend. This will bring back warmer temps and light winds across the forecast area. High temperatures will be in the 70s Wednesday and then increase by 6-12 degrees Thursday onwards. The Extreme Forecast Index (EFI) even shows as a climatologically unusual event for temperatures Wednesday and Thursday evening. Winds will be around 12 mph or less. However, winds could return slight breezy at the Cascade Gaps and the Blue Mountain Foothills Thursday night and Friday night around 10-20 mph (20-40% chance). RH values will remain decreasing to the teens and low 20s with dry conditions returning. With low RH values and elevated winds, this could raise a potential threat to fire weather conditions. Over the weekend, the ridge will begin breaking down as the closed low moves over nearly to the shoreline of the OR/CA border. This could increase chances of thunderstorms along with precip for Sunday night over central OR and Ochoco-John Day Highlands. However, the GFS model is showing little to no instability with low MUCAPE values and lapse rates above 6.5 C/Km. Due to low confidence, will hold off mentioning thunderstorms in the forecast for now. Feaster/97 && .PRELIMINARY POINT TEMPS/POPS... PDT 75 48 84 55 / 0 0 0 0 ALW 78 54 87 60 / 0 0 0 0 PSC 81 56 88 63 / 0 0 0 10 YKM 78 51 84 53 / 0 0 0 10 HRI 81 52 88 61 / 0 0 0 0 ELN 77 51 82 55 / 0 0 0 20 RDM 75 44 85 48 / 0 0 0 0 LGD 74 46 84 50 / 0 0 0 0 GCD 72 45 86 50 / 0 0 0 0 DLS 81 56 87 61 / 0 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ UPDATE...91 SHORT TERM...85 LONG TERM....97 AVIATION...86