344 FXUS66 KPDT 132132 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 232 PM PDT Mon Sep 13 2021 .SHORT TERM...Tonight through Wednesday night...A cool northwest flow over the region today will transition into a ridging pattern on Tuesday. This will result in warming temperatures on Tuesday compared to today. Afternoon highs will rise to the upper 70s to lower 80s across the lower elevations and 60s to 70s in the mountains. After that a mostly dry cold front will move across the area on Wednesday with an increasing westerly flow at the base of a very broad upper trough over western Canada. This will result in increasing westerly winds, especially through the Cascade gaps, passes and the eastern Columbia Gorge with breezy to windy conditions by Wednesday late afternoon and evening. It will also cool down again on Wednesday with highs only in the lower to mid 70s in the lower elevations and 50s to 60s in the mountains. There will be some light rain showers possible in the WA southern and central Cascades/east slopes with this front. Elsewhere will remain dry. The winds will diminish later Wednesday evening and overnight. Temperatures Wednesday night will also be substantially cooler with lows in the 40s lower elevations and upper 20s to 30s in the mountains. Many areas will see patchy to areas of frost in the higher elevations, especially in open meadows and fields. There will not be any fire weather concerns in the short term period due to any low RH's and windy conditions not coinciding to create critical fire conditions, and no thunderstorms expected. 88 .LONG TERM...Thursday through Monday...Dry conditions to start the period as a weak ridge traverses the PacNW Thursday, though highs will be slightly below seasonal. Sensible weather concerns arrive Friday as a deep, anomalously strong upper-level low slides down from the Gulf of Alaska, weakening to an open wave by the time it reaches the PacNW Saturday through Sunday. The event looks to have a weak-moderate atmospheric river associated with it, albeit some uncertainty still remains regarding the exact location of the moisture plume. That said, our entire forecast area could see significant precipitation, especially across the higher terrain where moderate-locally heavy rainfall may occur. Otherwise, the ECMWF EFI tool hints that Friday and Saturday will be breezy to locally windy, especially across central Oregon. ECMWF EPS, GEFS, and Canadian GEPS ensemble means are all consistent in bringing an anomalously deep upper-level low down from the Gulf of Alaska into the PacNW Friday through Sunday. However, timing and amplitude differences exist. Further examining ensemble means, the 12Z GEPS and 12Z EPS are somewhat slower than the more progressive solution of the 12Z GEFS. Moreover, flow aloft appears more zonal in the EPS and GEPS, whereas the GEFS has the region entrenched in southwesterly flow aloft by Friday afternoon. By Saturday afternoon, all three ensembles have southwesterly flow over the PacNW and into the Inland NW, though the GEFS remains more progressive with a deeper trough than the EPS and GEPS. Solutions start to diverge considerably come Sunday afternoon with the GEPS and EPS keeping the upper low in place offshore of British Columbia or northwest Washington, while the GEFS is far more progressive with the trough sliding across our CWA by that time. By Monday afternoon, the EPS and GEFS start to align as the upper low continues to slide east with a ridge taking its place over the PacNW. The GEPS does show this trend as well, though it is about 12 hours slower and not as aggressive with the ridge's amplitude. To gauge ensemble spread, we can examine clusters of like-patterned members. The total ensemble mean is zonal for the Inland NW for Friday afternoon, with the primary source of variance being the amplitude of 500 mb heights over the PacNW. A secondary source of variance, though smaller, is heights across the Northern Rockies. Ensemble mean 500 mb heights lower for Saturday afternoon and flow turns more southwesterly aloft. The primary source of variance is still the amplitude of the heights across the PacNW, though now we see a secondary source of variance creep in with the timing of the pattern. Cluster phase space shows the EPS favoring higher-than-mean heights across the PacNW compared with lower-than-mean heights for the GEFS, while the GEPS is somewhere in the middle with solutions more evenly spread across clusters. Perhaps as expected, cluster mean QPF indicates a wetter-than-mean solution for the GEFS, and a drier-than-mean solution for the EPS for Saturday for our CWA. The upper trough across the PacNW persists in the ensemble mean through Sunday afternoon, with QPF remaining in the ensemble mean for our forecast area. However, there is considerable uncertainty in the progression of the pattern. Timing of the disturbance remains the primary source of variance Sunday through Monday, while the amplitude of the feature remains a secondary source of variance. Precipitation looks to taper off Sunday night through Monday. Deterministic and ensemble guidance both suggest a ridge building back over the PacNW Monday night through Tuesday as the upper low exits to the east. Plunkett/86 && .AVIATION...18Z TAFs...VFR conditions expected for all TAF sites through the period as skies continue to clear into the afternoon. Current gustiness at KALW should subside through this morning. Winds of 5-15 kts then expected for the afternoon and evening at all TAF sites, with gusts of 15-25 kts for the Gorge and Kittitas Valley. Light winds expected overnight, though KYKM and the Kittitas Valley may gust around 15 kts through midnight. Plunkett/86 && .PRELIMINARY POINT TEMPS/POPS... PDT 43 79 56 72 / 0 0 0 0 ALW 48 81 58 75 / 0 0 0 0 PSC 50 81 61 77 / 0 0 0 0 YKM 44 80 52 73 / 0 0 10 0 HRI 48 82 59 76 / 0 0 0 0 ELN 46 79 54 67 / 0 0 10 0 RDM 41 79 46 72 / 0 0 0 0 LGD 41 79 54 72 / 0 0 0 0 GCD 41 82 48 77 / 0 0 0 0 DLS 52 84 61 74 / 0 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...88 LONG TERM....86 AVIATION...86