FXUS66 KPDT 092035 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 135 PM PDT Thu May 9 2024 .SHORT TERM... Tonight through Saturday night... A large ridge of high pressure will build over the Pacific Northwest through Saturday. This ridge will flatten a bit through Saturday night. Dry and increasingly warm conditions are expected through the weekend. High temperatures will be 15 to 20 degrees above normal on Saturday, on average across the area. Probabilities of high temperatures >=90 degrees on Saturday range from about 60% in the lower Columbia Basin of Oregon to about 60 to 70% in the Yakima Valley and 70 to 80% in the Columbia Basin of Washington. None of the GFS ensemble members reach >=90 degrees on Saturday for Tri-Cities whereas approximately 50% of the ECMWF ensemble members >= 90 degrees. High temperatures will range from the upper 70s to mid 80s on Friday, then generally in the 80s to around 90 degrees on Saturday. Overnight lows will be in the 40s tonight and the 40s to lower 50s Friday night. .LONG TERM... Sunday through Thursday..A shortwave trough will approach the area on Sunday and move across the region Sunday night. The ensemble clusters favor a flat wave compared to the higher amplitude depicted in the deterministic models. Sunday will be the last very warm day with high temperatures in the mid 80s to lower 90s in the lower elevations. Probabilities of 90+ degree high temperatures Sunday are 30-70% across the Columbia Basin and Yakima Valley. A cold front will cross the area Sunday associated with the shortwave and it will become breezy through the Cascade gaps in the afternoon and evening. The winds will continue on Monday along with cooler temperatures (8-10 degrees cooler than Sunday.) The 12Z GFS is forecasting the surface pressure gradient between PDX and GEG to exceed 10 mb on Sunday night and Monday which supports windy west winds in the lower elevations. Probabilistic NBM for the 24 hours ending at 11 PM Monday shows a 50-70% chance of winds exceeding 45 mph in the usual windy locations under strong westerly flow regimes. On Tuesday into early Wednesday the GEFS and ECMWF ensemble build a ridge into the region from offshore with NWLY flow aloft over the area. This will result in dry and warm conditions on Tuesday and possibly Wednesday though the uncertainty in the forecast begins to increase on Wednesday and even more on Thursday. For Thursday the ensemble clusters are split with roughly half the ensemble members favoring a trough solution and the other half favoring a ridge. This is creating significant uncertainty in the temperature forecast for mid week. For example, on Thursday at Pendleton the NBM 25th percentile high temperature is 66 degrees and the 75th percentile is 85 degrees. The GFS and ECMWF ensemble temperatures are cooler than the official forecast so it is possible that temperature will have to be nudged downward in later forecasts. The chance for precipitation appears quite low through the period. Except for occasional POPS 15-20% along the Cascade crest, Northern Blues and Wallowas, the chance for measurable precipitation elsewhere is 10% or less. 78 && .AVIATION... 18Z TAFS...VFR conditions through the period with FEW-SCT cirrus at 25 kft AGL. Winds this afternoon mainly from the N-NE at 10-15 kt with a few gusts to 20 kt, becoming light terrain driven after sunset. 78 && .PRELIMINARY POINT TEMPS/POPS... PDT 47 79 51 84 / 0 0 0 0 ALW 50 82 54 86 / 0 0 0 0 PSC 52 86 55 91 / 0 0 0 0 YKM 48 85 51 90 / 0 0 0 0 HRI 49 84 53 90 / 0 0 0 0 ELN 49 83 53 87 / 0 0 0 0 RDM 40 78 46 83 / 0 0 0 0 LGD 44 76 49 81 / 0 0 0 0 GCD 43 77 47 81 / 0 0 0 0 DLS 53 86 57 89 / 0 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...77 LONG TERM....78 AVIATION...78