FXUS66 KPDT 150444 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 844 PM PST Wed Feb 14 2024 .SHORT TERM UPDATE...A deep--almost vertically stacked--low is just west-southwest of Astoria with a baroclinic band across WA and northern OR. Central Oregon received moderate to heavy snow this morning and afternoon with a quick 3-4 inches of snow. The band of precipitation is lifting northward, and snow will decrease to very fine flakes later this evening. The current Winter Storm Warnings for the east slopes of the OR Cascades, north central OR, and the southern Blue Mtn Foothills will remain as radar and satellite show the large swath of moisture in these areas. The Winter Weather Advisory for central OR will also continue, although there may be amounts of around 5-6 inches over the west- southwest side of Bend by Thursday morning. One update to the forecast has been to upgrade the Winter Weather Advisories to Winter Storm Warnings for heavy snow in the eastern Columbia River Gorge (for elevations above 500 feet) and the lower slopes of the southern Washington Cascades. Strong low level upslope flow will be present with boundary layer winds from the ENE...at least until 18Z Thursday. Precipitation will decrease Thursday as the upper flow increases from the west, and the upcoming day shift may decide to cancel the warning if precipitation tapers off. However, there is another front that will become stationary along the WA/OR border. The GFS and ECMWF are a little farther north than the NAM which would indicate continue snow in the eastern CR Gorge and areas just south of Mt. Adams. A majority of the HREF members are in better agreement with the GFS and ECMWF. If the majority models verify, then there will be another round of snow for the CR Gorge and Cascade east slopes from Mt. Adams to Mt. Jefferson as well as north central OR. Dense fog has developed along the Blue Mtn Foothills near Weston as well as I-84 along Cabbage Hill. The forecast will be updated to add the dense fog, and an advisory may be in order for this evening. Wister/85 && .PREV DISCUSSION... /issued 337 PM PST Wed Feb 14 2024/ Updated Aviation Discussion. SHORT TERM...Tonight through Friday...As a winter system moves into the region, forecasting it has been difficult due to the interaction of warm and cold air keeping the region near freezing. Central Oregon has of note significantly resisted warming up likely thanks to colder air aloft plus cooling from falling precipitation. Areas such as Redmond briefly got to 34, but it was short lived and snow continues to fall. This colder air has enhanced the amount of snowfall that has been seen, but have kept this zone in an advisory rather than a warning as the bulk of the heaviest activity continues to the west closer to the mountains, but the delineation is, as always for this area, quite thin. Meanwhile, this cold may also allow for rise in snowy conditions across the Columbia Gorge and along the slopes of the Washington Cascades. Still expecting a wide swath of possible snowfall amounts for the Gorge, with areas closest to the mountains seeing nearly 6-8 inches, while areas further east may only see an inch or so. Much of the rest of the region shouldn't see drastic changes to snowfall amounts, but expect at least a small uptick compared the overnight forecast. The path of the low will take the upper low northwards along the coastline before it merges with stronger flow to the north and then moves eastwards. Overnight lows at or below freezing widespread tonight, with highs on Thursday maybe reaching near 40 degrees before colder air continues to overspread in the aftermath of this system, with overnight lows by Friday morning widespread below 30 degrees. Similar conditions possible Friday into Saturday as high pressure builds and shifts eastwards. Goatley/87 LONG TERM...Saturday through Wednesday...The extended period is characterized by a deep, upper level trough that slowly approaches the coast over the weekend, before pushing onshore along the California coast and weakening through the beginning part of the workweek. Persistent southwest flow aloft associated with this system will promote a warming trend through the period, as two shortwaves (Saturday and Monday) bring widespread precipitation chances to the area. These are the only two sensible weather concerns through the long-term, with minimal precipitation amounts expected as temperatures slowly approaching normal values by Tuesday. The transitory ridge that built over the region on Friday will continue to push east and weaken Saturday as an upper level shortwave approaches the northern California coast and moves into the Inland Northwest from our south. This feature will bring widespread showers across the area, impacting Central Oregon through the late afternoon before extending through South Central Washington overnight and into Sunday morning. Moisture associated with this shortwave will be lacking by the time it reaches our area, as the Integrated Water Vapor Transport (IVT) incurs a southwest trajectory near the San Francisco area before becoming more from the south into Southern Oregon. This translates to decreasing precipitation amounts south-to-north, with lower elevation rain amounts staying below 0.10". Snow levels will be quite variable with ranges from 2500-4500 feet across North Central Oregon to 500-2500 feet through South Central Washington with a north-to-south gradient. Snow levels will be increasing through the event, eventually reaching between 2500- 3500 feet across South Central Washington and 3500-5000 feet for North Central Washington by Sunday afternoon. Snow amounts across our mountain zones will also be minimal, with only 1-3 inches expected above 4000 feet along the Cascade and Blue Mountains through Sunday afternoon. Another transitory ridge builds in the wake of the quickly departing shortwave on Sunday, confining precipitation at elevation over our mountain zones by the afternoon as the offshore trough deepens and approaches the Northern California coast. This will shift flow aloft to more from the south, further inhibiting moisture transport associated with a second shortwave on Monday. This will again provide widespread precipitation chances (15-40%) across the area with the majority of lower elevations staying dry. Snow levels will continue to trend upward and into the 3500-5500 foot range north-to- south as around an inch of snowfall is expected above 4000 feet. The trough moves onshore along the California coast Tuesday and quickly weakens as it continues east into Nevada and Utah on Wednesday, allowing for mainly zonal flow to occur across the Pacific Northwest. This will keep most of the area dry as light upslope showers will be possible along higher elevations of the Cascade and Blue Mountains. High temperatures will break into the low 50s for lower elevations of the Basin and Central Oregon Tuesday and Wednesday, with low temperatures in the low to mid 30s Wednesday morning. Guidance is in good agreement with the overall pattern of a deep trough approaching the coast and pushing onshore through the early part of the workweek, coupled with two shortwaves that will allow for increased rain chances Saturday and Monday. However, minor differences in shortwave strength and trough location arise, as the 500 mb EOF patterns highlight a 70% ensemble variance regarding the Saturday shortwave and a 35-45% ensemble variance associated with the trough strength and location. The GFS suggests weaker shortwaves with a later, stronger, and further south trough location than the ECMWF, which correlates to a slightly warmer and drier outcome with the GFS. Ensemble clusters provide additional confidence in the ECMWF scenario as ensembles are more in-line with one another and more representative of the deterministic solution. Thus, there is moderate confidence (60-70%) in stronger shortwaves Saturday and Monday and an earlier onshore push of the parent upper level trough, resulting in a slow increase in temperatures over the period and widespread precipitation chances both Saturday and Monday. 75 AVIATION...00Z TAFS...VFR conditions currently across most sites, with KDLS recently dropping to MVFR conditions due to reduced ceilings of OVC028 and LIFR conditions at KRDM/KBDN due to reduced visibilities and ceilings of 1/2SM and OVC003-005 respectively. These conditions are a result of a weather system impacting the region from the south, allowing for widespread precipitation to occur and transition to snow late tonight into Thursday morning. All sites are expected to degrade to at least IFR conditions due to rain transitioning to snow, with LIFR likely for KRDM/KBDN/KALW/KPDT with visibilities around 1SM and ceilings of OVC002-004. Conditions should improve by late morning for all sites except KRDM/KBDN, as fog develops to reduce visibilities back down to 1/4SM. 75 && .PRELIMINARY POINT TEMPS/POPS... PDT 30 34 25 35 / 90 90 60 10 ALW 31 36 26 37 / 80 80 60 20 PSC 33 38 29 41 / 90 80 30 10 YKM 30 37 24 39 / 80 60 20 10 HRI 32 38 27 39 / 90 80 40 10 ELN 28 34 23 37 / 70 60 10 10 RDM 25 42 25 35 / 70 60 70 10 LGD 30 42 27 41 / 100 80 80 20 GCD 32 42 30 45 / 90 80 70 10 DLS 32 37 30 37 / 100 80 60 20 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Winter Storm Warning until 4 AM PST Friday for ORZ041-508>510. Winter Weather Advisory until 4 AM PST Friday for ORZ502. Winter Weather Advisory until 10 AM PST Thursday for ORZ511. WA...Winter Storm Warning until 4 AM PST Friday for WAZ024-523. Winter Weather Advisory until 4 AM PST Friday for WAZ030-521-522. && $$ SHORT TERM...87 LONG TERM....75 AVIATION...75