FXUS66 KPDT 141725 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 1025 AM PDT Mon Oct 14 2024 Updated Aviation Discussion .AVIATION...18z TAFs...VFR conditions expected. High pressure will continue to make for clear skies and light, terrain-driven winds generally less than 12 kts, except for DLS which could see some breezy gap winds this afternoon. High clouds will start to move into the area later today into the overnight hours as a weather system starts to move into the region, expected to arrive in the forecast area by the latter half of Tuesday. Evans/74 && .PREV DISCUSSION... /issued 341 AM PDT Mon Oct 14 2024/ SHORT TERM...Today through Wednesday... Key Messages: 1. Breezy afternoon winds each day, increasing through the period. 2. Widespread rain chances Tuesday evening through Wednesday. 3. Cooling trend through midweek. Current radar and infrared satellite imagery showing dry conditions as a light line of high level cirrus pass through the Basin. This is in response to the upper level ridge and surface high pressure still holding onto their influence across the region. However, this will be changing as the upper level ridge axis shifts east this morning and a slow cold front begins to move onshore along the Washington coast. Rain chances (25-55%) will begin to materialize along the Washington Cascades this afternoon as a weak pressure gradient sets up to provide breezy winds over the Simcoe Highlands and the Gorge. Sustained west winds of 15-20 mph will be possible with gusts up to 25 mph across the Simcoe Highlands and sustained winds of 10-16 mph with gusts up to 20 mph expected through the Gorge west of Arlington. Confidence in these winds is high (80%) as the GFS, NAM, RAP, and SREF all advertise a pressure gradient of 3-5 mb across the Basin (KPDX-KGEG), and the HREF and NBM both suggest a 50-80% chance of sustained winds of 20 mph or greater and a 55-85% chance of gusts reaching 30 mph or greater over portions of the Simcoe Highlands. The GFS, NAM, and SREF highlight an even greater pressure gradient of 4-7 mb across the Basin (KPDX-KGEG) Tuesday, peaking on Wednesday at 7.5-10.5 mb. The slow moving cold front will pass through the area from northwest to southeast Tuesday afternoon/evening ahead of a reinforcing cold front that will pass northwest to southeast on Wednesday. Winds Tuesday will be similar to those experienced on Monday, but will increase on Wednesday to sustained winds of 15 to 25 mph with gusts of up to 40 mph over portions of the Simcoe Highlands and up to 35 mph through the Kittitas Valley and areas of the Oregon Basin west of Hermiston. Confidence in these values is moderate to high (60-80%) as the NBM advertises a 40-70% chance of 35 mph gusts or greater on Tuesday and a 70-90% chance of 40 mph gusts or greater on Wednesday over the Simcoe Highlands and portions of the Oregon Basin west of Hermiston. The NBM also suggests a 50-80% chance of advisory-level winds (30 mph or greater) and gusts (45 mph or greater) over portions of the Oregon Basin and Simcoe Highlands. These areas are rather isolated, so confidence in an advisory being issued on Wednesday is low (20-40%), but will still need to be considering as the event nears. The approach of the initial cold front this afternoon will lead to rain chances (25-55%) developing along the Washington Cascades, slowly extending south to include the Oregon Cascades overnight into Tuesday morning. The front finally transects the region Tuesday afternoon as the second, reinforcing cold front approaches the Washington coast. This will lead to widespread rain chances (35-75%) across the area Tuesday evening, with chances increasing to a 50-90% chance Wednesday morning and afternoon. Tuesday's rain amounts look to stay 0.10" or less through the Basin and Central Oregon, with 0.10-0.20" along the Blue Mountain foothills, and 0.15-0.30" at elevation over the Cascade and Blue Mountains. Confidence in these rain amounts is high (80%) as the NBM shows a 35-50% chance of a wetting rain (0.10" or greater) over lower elevations of the Basin and Central Oregon, a 60-70% chance of a wetting rain along the Blue Mountain foothills, and a 70-85% chance of a wetting rain at elevation over the Cascades and Blue Mountains. Due to the orientation of the passing cold front and the incoming upper level trough, rain amounts are expected to be greater on Wednesday over our mountain zones, similar along the foothills, and slightly less over lower elevations of the Basin. Anticipated rain accumulations Wednesday reach 0.20-0.50" over the Cascade and Blue Mountains, 0.10-0.20" along the Blue Mountain foothills, and 0.08" or less over lower elevations of the Basin and Central Oregon. Confidence in these rain amounts are moderate to high (60-80%) as the NBM advertises a 90-100% chance of a wetting rain over the Cascade and Blue Mountains, a 70-85% chance along the Blue Mountain foothills, and a 30-50% over lower elevations of the Basin and Central Oregon. High temperatures will peak today before a cooling trend commences through the midweek. Highs will break into the mid-to upper 70s across lower elevations of the Basin, Central Oregon, and Blue Mountain foothills today, which is 8-12 degrees above normal. The initial cold front on Tuesday will drop highs 2-6 degrees from today as the stronger, secondary front drops high temperatures another 6-10 degrees on Wednesday to stay in the mid-to upper 60s across lower elevations of the Basin and Blue Mountain foothills. The reason behind the more substantial drop on Wednesday is a result from flow aloft shifting from the southwest to the west/northwest, advecting a cooler airmass into the region. Overnight temperatures will also be cooling through the period as Wednesday night temperatures could drop into the upper 30s over lower elevations of the Basin, and near to slightly below freezing over Central Oregon. 75 LONG TERM...Thursday through Monday...Models are in pretty good agreement through Saturday, but do differ on the evolution of a possible cutoff low over the Great Basin and how the upstream pattern responds. Cluster analysis shows the upper level trough pushing east with widespread rain decreasing from west to east. As the system moves east and digs deeper, 63% of members (two clusters) show a cutoff low developing over the Great Basin Friday into Saturday. In response, a more amplified upper level ridge develops over the Pacific Northwest. The other cluster members show a less amplified pattern leaning more towards zonal flow. By Sunday, two clusters (57% of members) continue to show a cutoff low over the Great Basin with the other members continuing to lean towards a more zonal flow pattern over the region. By Monday, 87% of members (three clusters) show general zonal flow over the Pacific Northwest. The remaining cluster member, depicts a deep low over the eastern Pacific with southwest flow over the area. The GFS and ECMWF ensembles agree with the clusters showing a cutoff low developing over the Great Basin with the GFS a little stronger and more amplified with the upper level ridge. They also agree with zonal flow developing Monday as a broad upper level trough begins to dig southward from BC. At this time the best chance to see a widespread wetting rain (0.10 of inch) will be Thursday. There's a moderate chance (30-55%) over portions of the Basin and Foothills. There's a likely chance or better (60-90%) over the mountains of seeing a 0.25 inch or more. NBM may be a little aggressive with the chance of widespread precipitation on Sunday as zonal flow generally is best for west facing slopes with a distinct rainshadow over portions of the Columbia Basin. The deterministic models are not as aggressive as the NBM either. At this time, will not make any changes, but would not be surprised if NBM trends downward. If the zonal flow does develop, breezy conditions can be expected over portions of the Columbia Basin and Cascade gaps. There's a moderate (40-55%) chance of seeing breezy conditions Thursday. Daytime highs are expected to be below normal Thursday and Friday, above normal Saturday and Sunday, and near normal Monday. Earle/81 && .PRELIMINARY POINT TEMPS/POPS... PDT 75 45 70 47 / 0 0 0 70 ALW 77 50 73 50 / 0 0 0 70 PSC 77 51 73 52 / 0 0 0 70 YKM 76 48 72 42 / 0 10 10 50 HRI 77 50 74 50 / 0 0 0 70 ELN 74 48 69 43 / 0 10 20 60 RDM 75 40 69 42 / 0 0 0 60 LGD 79 43 76 47 / 0 0 0 50 GCD 80 44 78 46 / 0 0 0 60 DLS 76 51 72 49 / 0 10 20 70 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...75 LONG TERM....81 AVIATION...74