FXUS66 KPDT 250255 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 655 PM PST Sun Nov 24 2024 .UPDATE...Latest satellite imagery shows an upper low offshore of the PACNW with a shortwave trough moving NE along the Oregon coast. A cloud shield and associated precipitation is over western and southern Oregon. Took a look at the latest NBM QPF and amounts look light and spotty through the day on Monday (mostly less than .10 inch liquid). A bit more robust and organized precipitation/QPF is expected Monday night and Tuesday as the parent upper low finally swings inland across Oregon. Its a bit breezy this evening along the foothills of northern Oregon near the base of the Blue Mountains. The KMEH to KPDT pressure gradient peaks at around 3.7 mb at 06Z so winds could approach advisory levels for a short time. The current thinking is that a wind advisory is not needed. However will monitor wind trends during the remainder of the evening. The current forecast is on track for tonight with only a few minor changes made to POPS and low temperatures. 78 && .PREV DISCUSSION... /issued 253 PM PST Sun Nov 24 2024/ SHORT TERM...Tonight through Tuesday night...A large low pressure trough off the eastern Pacific has circulated a series of embedded shortwaves across the PacNW with periods of snow and rain. This pattern will continue through Monday night, then the trough and its parent low will move inland Monday night then across WA/OR Tuesday. By Tuesday evening, the trough will exit the region into ID followed by a drier northwest flow aloft. One of the forecast challenges in the short term is the timing and location of individual shortwaves. Models have been in agreement with the overall synoptic pattern, but the devil has been in the details. The good news is that rain and snow amounts will be relatively light. 850-700 mb winds are from the south-southwest and not favorable for significant upslope over the mountains, and upper level support is relatively weak with no indications of an atmospheric river. The likelihood of any winter highlights through Tuesday night is low (20%) but one area that will need to be monitored for snow accumulations will be along the John Day-Ochoco Highlands Monday night and Tuesday. Currently, a shortwave is off the central OR coast, as evident on water vapor loops. There has been steady light precipitation along the southern Washington Cascades, and occasional rain/snow mix has been observed in the Kittitas and Yakima Valleys. Snoqualmie Pass at 3000 feet has received 3 inches since last night. The shortwave will lift north across the northern half of WA tonight with precipitation tapering off. Another wave will follow right at the heels and over western Oregon and the Cascades on Monday. The heaviest snow will be on the west facing slopes due to the southwest flow aloft. The chance of precipitation will increase over eastern Oregon Monday night and Tuesday, especially from the Blue Mtns eastward and south of Condon. As previously mentioned, the area that will need to be monitored for snow accumulations meeting advisory criteria will be in the John Day-Ochoco Highlands. Precipitation will be steady Monday night and Tuesday, as the main upper low tracks across the southern half of Oregon. Snow levels will lower from near 5000 feet to 4000 feet,and there is around a 40% chance that this area will observe 3-6 inches above 4500 feet. If confidence increases and/or snow levels are forecast to be lower, a winter weather advisory may be needed. Otherwise, snow accumulations in the mountains/valleys will be light Monday night and Tuesday...generally 3 inches or less (confidence 50-70%). Wister/85 LONG TERM...WEDNESDAY THROUGH SUNDAY...Ensemble guidance is in very good agreement that the remnants of the closed low currently offshore will morph into an open wave trough as it tracks onshore, exiting the region by early Wednesday. Operation guidance hints at a weak shortwave trough embedded in the northwesterly flow aloft. This may facilitate some orographic showers across the Cascade crest and northern Blue Mountains through the day. Uncertainty in the longitudinal placement of an upper-level ridge axis appears to be the main driver of variance among ensemble NWP Thursday through Saturday. Ensemble clusters suggest the most likely scenario (65-75% of members) is dry conditions with the ridge axis centered just offshore or over the PacNW. This solution would also favor basin/valley fog coupled with cool to cold temperatures for the entire forecast area. The alternate scenario (25-35% of members) places the ridge axis a bit farther offshore with a shortwave trough digging southeastward amidst northwesterly flow aloft. This solution would keep persistent PoP chances across area mountains, with temperatures cool enough for snow. Both scenarios favor mostly dry conditions for the lower elevations. Sunday, the most likely scenario provided by ensemble clusters is persistent upper-level ridging over the PacNW, albeit with some variance in strength (57% chance of a strong ridge, 20% chance of a weaker offshore ridge axis). The remaining members (13%) advertise a return of precipitation with an atmospheric river directed into the PacNW. Due to the aforementioned uncertainty, the NBM places slight chance to chance PoPs across the Cascades and northern Blues through much of the forecast period. Plunkett/86 AVIATION...00Z TAFs...Predominantly VFR conditions are forecast for all TAF sites through the remainder of the day, though YKM has a chance of MVFR or IFR CIGs (40-50% confidence). Precipitation this afternoon is forecast to remain in south-central WA, with a brief period of light precipitation overnight as a decaying surface low and upper-level shortwave move into the PacNW. CIGs are expected to drop overnight into Monday morning, with the best chances of sub-VFR conditions at YKM/DLS/PSC due to low CIGS and/or VSBYs. Confidence in fog for basin sites is just low/medium (30-50%) due to the passage of the aforementioned weather system. Plunkett/86 && .PRELIMINARY POINT TEMPS/POPS... PDT 35 49 33 42 / 20 30 60 40 ALW 38 50 35 44 / 20 40 60 60 PSC 34 45 37 46 / 20 20 40 10 YKM 31 43 27 45 / 50 30 20 10 HRI 33 47 35 45 / 20 30 50 20 ELN 32 43 28 44 / 70 40 30 10 RDM 32 47 27 42 / 30 40 50 20 LGD 34 42 31 41 / 10 40 60 70 GCD 34 46 30 42 / 20 50 80 70 DLS 36 47 36 49 / 60 50 40 30 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...85 LONG TERM....86 AVIATION...86