Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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363 FXUS66 KPDT 301119 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 419 AM PDT Thu May 30 2024 .Updated Aviation Discussion. .SHORT TERM...Today through Saturday... Bottom Line Up Front: (BLUF) 1. Mountain rain dissipates early today. 2. Dry and warm conditions to return tonight. 3. Mountain showers return with breezy conditions Saturday. Models are in excellent agreement with the upper level trough shifting out of the region today leaving only the Puget Sound Convergence slightly in place over the Cascades. This will allow for lingering mountain showers which will dissipate early this morning with less than 10% probabilities of 0.01 inches of rain. Temperatures today will remain mild with over 65% of the raw ensembles showing the foothills, central and north central OR to be in the upper 60s to low 70s and the Basin and adjacent valleys in the low to mid 70s. Winds will loosen up across the majority of the region with the exception of the the Kittitas Valley seeing sustained winds to 25% with 80% of the ensembles in agreement. Models remain in excellent agreement with the leading edge of the upper level ridge slipping in behind the trough. This will put the area under dry and warming conditions through Friday night. EFI shows temperatures at or slightly above climatological normal with over 50% of the raw ensembles showing the majority of the area to be in the mid to upper 70s with isolated locations in the Basin seeing temperatures in the 80s. With the upper level high in place, northwesterly flow will dominate through Friday which will in turn settle the winds across the region with high confidence (80%) winds will remain below 10 mph. Lastly, the models remain in excellent agreement in showing another upper level shortwave making its way into the PacNW Friday night. Day 3 ensembles clusters show little variances with the system. Main difference with the models are with the timing of the expected precipitation. Clusters are more in favor of the GFS solution which will bring the precipitation to the Cascades with raw ensembles showing a 20-40% chance of 0.01 inches of rain. Guidance is also showing yet another increase in the winds with surface pressure gradients tightening along the Cascades. However, winds will be around 25 mph with 60-80% of the ensembles in agreement. Ensembles also shows the majority of the winds to be along the Simcoe Highlands and through the Kittitas Valley. EFI shows temperatures to remain at or slightly above normal with over 58% of the raw ensembles showing the majority of the area to remian in the upper 70s to low 80s. Bennese/90 .LONG TERM...Sunday through Thursday... Key Messages: 1. Breezy afternoon winds, peaking on Monday. 2. Widespread rainfall expected Sunday and Monday. 3. Afternoon thunderstorms possible Sunday and Monday. The extended period is characterized by an upper level trough and associated frontal system that approaches the coast Sunday before driving through the Pacific Northwest on Monday. An upper level ridge then builds on Tuesday in the wake of the departing trough, allowing for drying and warming Tuesday onward. The main weather concerns in the period reside with the passing system Sunday and Monday, which will bring breezy winds, afternoon thunderstorms, and ample moisture to the area. Also of note, high temperatures will be reaching above normal values beginning Wednesday, and approaching low 90s for lower elevations of the Basin late in the week. A transient upper level ridge will quickly pass through the area on Sunday ahead of the approaching trough, which will tighten isobars and produce increasing winds into Monday. The cold front, associated with the upper level system, will allow a surface pressure gradient to develop along the Cascades on Monday. This gradient is forecast to hover around 10-13 mb between Portland and Spokane as advertised by the GFS, which does meet the normal advisory criteria of 12 mb. Elevated wind gusts of between 25-35 mph will be rather widespread, but will be closer to between 35-50 mph over the Simcoe Highlands, Kittitas Valley, northern Blue Mountain foothills, and along the Lower Columbia Basin of Oregon. Further confidence in these winds is gleamed from the NBM, which showcases an 80-90% chance over the Simcoe Highlands, a 75-85% chance across the Kittitas Valley, a 60- 75% chance along the Lower Columbia Basin of Oregon, and a 40-50% chance through the northern Blue Mountain foothills of experiencing gusts of 47 mph or greater. Thus, there is moderate to high (70-90%) confidence in advisory-level winds Monday afternoon over portions of the area. Ensembles are also in agreement with rather tight isobars on Monday, with members hinting at the gradient tightening even further than the current forecast (34% versus 23%). Also, the timing of arrival may also continue to trend slightly later as indicated by 33% of members. Rain will begin along the Cascades Sunday afternoon as a warm front nudges into the area ahead of the cold front, which will lead to showers spilling into the Basin and over the Blues Sunday evening into early Monday morning. Rain will continue through Monday morning before slowly tapering off through the afternoon and early evening. Showers will still linger across the Cascades and Blue Mountains into Tuesday before only slight chances (15-25%) remain over the Washington Cascades on Wednesday associated with upslope flow. Rain amounts still look to peak between 5PM Sunday and 5PM Monday, but may begin to nudge earlier as indicated by ensemble members mentioned in the previous section. Rain amounts of 0.05-0.15 of an inch are anticipated over lower elevations of the Basin and over Central Oregon, 0.10-0.20 of an inch across the lower Columbia Basin of Oregon, 0.25-0.50 of an inch along the northern Blue Mountain foothills and the John Day-Ochoco Basin, and 0.40-0.80 of an inch at elevation over the northern Blue Mountains and Elkhorns. Confidence in these rain amounts are moderate to high (55-75%) as the NBM suggests a 35-45% chance over lower elevations of the Basin, Lower Columbia Basin of Oregon, and Central Oregon, a 40-50% chance over the Yakima/Kittitas Valleys, and a 65-85% chance across the northern Blue Mountains/foothills and Elkhorns of 0.25 of an inch of rain or more. Confidence is further enhanced as a Atmospheric River (AR) looks to be the culprit of moisture supply to this system as the probability of Integrated Vapor Transport (IVT) greater than 250 kg/m*s is 90-100% Monday for inland regions east of the Cascades as advertised by the GEFS. The ECMWF and GFS are also in agreement with the amount of IVT between 600-700 kg/m*s on Monday, further enhancing confidence in widespread rainfall and readily available moisture Sunday evening through Monday evening. The final concern associated with this passing system will be of the potential for thunderstorm development Monday afternoon and evening across the Blue Mountains, John Day-Ochoco Basin, and Wallowa County. The passing cold front and terrain will help to enhance lift, as surface based CAPE of 100-200 J/kg is forecast to be present via the GFS and ECMWF. Low level shear of 35-45kts is also expected, with the highest amounts located over eastern Wallowa County. Low level lapse rates also look to peak later in the afternoon/evening at between 7-8C/km. Thus, developing storms are expected to stay discrete and sub-severe, but this will need to be monitored as the event nears. 75 && .AVIATION...12Z TAFS...VFR conditions currently at all sites, which will stay the course through the period. Infrared satellite imagery showing areas of smoke near KBDN and KRDM, which may lead to reduced visibility of 6SM and ceilings of FEW060 into the morning hours on Thursday. Ceilings will lift through the morning across all sites as temperature warm, with winds staying light and below 10 kts for most sites. KDLS, KBDN, and KRDM will be the only exception as gusts of 15-25kts will be possible through the afternoon. 75 && .PRELIMINARY POINT TEMPS/POPS... PDT 67 41 75 50 / 0 0 0 0 ALW 70 45 78 53 / 0 0 0 0 PSC 74 47 80 56 / 0 0 0 0 YKM 73 40 78 51 / 0 0 0 0 HRI 73 44 80 53 / 0 0 0 0 ELN 68 41 76 53 / 0 0 0 0 RDM 69 38 78 46 / 0 0 0 0 LGD 64 38 73 47 / 0 0 0 0 GCD 67 39 77 48 / 0 0 0 0 DLS 73 47 81 57 / 0 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...90 LONG TERM....75 AVIATION...75