Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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849 FXUS66 KPDT 070524 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 1024 PM PDT Mon May 6 2024 Updated Aviation Discussion .EVENING UPDATE...Radar and satellite imagery this evening show convective type showers have diminished area wide, with some light returns noted along the Cascade crest, Blues, and Strawberry mountains. Through tonight, a shortwave trough is expected to continue sliding east across the PacNW, with increasing shower activity along the OR Cascade crest and the higher terrain across the Blues. The most recent 00Z run of the HREF shows short-term high res members are also in agreement with the uptick in snow shower potential through early tomorrow morning. Expect an additional 2 to 5 inches in portions of the northern Blues and the OR Cascade crest above 4.5kft through 5AM tomorrow. As a result, have extended the winter weather advisory for the OR Cascade east slopes to 5AM as well. Otherwise, a strong surface pressure gradient will develop along the Cascade crest, resulting in another day of breezy winds in through the Cascade gaps and lower elevation zones. The strongest winds are expected through the Kittitas valley tomorrow, where sustained northwest winds of 25 to 35mph and gusts up to 50mph are forecast (confidence 70-80%). Updated winds through tomorrow morning, as well as snow amounts in the mountains, with rest of the afternoon forecast package still on track. Lawhorn/82 .AVIATION...06z TAFs...VFR conditions to prevail through the period. Light showers will still be possible across the mountainous terrain tonight, especially east of sites PDT/ALW and west of site DLS. Otherwise, precipitation not expected to impact sites through the remainder of the period. Breezy west winds of around 20kts with gusts 30-35kts possible will persist at site PDT through the period. Site ALW will see south/southwest winds of 12-17kts with gusts up to around 25kts through tomorrow evening. Winds will be 12kts or less at sites DLS/RDM/BDN/YKM/PSC through tonight, then increasing after 15Z-16Z to 12-20kts with gusts up to 30kts possible mostly from the west/northwest, except at site PSC where winds will be west/southwest. Lawhorn/82 PREV DISCUSSION... /issued 216 PM PDT Mon May 6 2024/ .SHORT TERM... Tonight through Wednesday night... The short term period will start off with several items of interest then end on the quiet side. First, another upper low will affect the Pacific Northwest through Tuesday morning, with the majority of the impacts later this afternoon into tonight, and mainly over the mountains. This upper low will move down from British Columbia and across Washington, before moving east into Idaho on Tuesday. Snow levels which are initially 4000-5000 feet, except for below 4000 feet in portions of the Washington Cascades will decrease to below 3000 feet by Tuesday morning. Snow levels will rise again to above 4000 feet by Tuesday afternoon, except for portions of the Washington Cascades. Winter Weather Advisories remain in effect for the Oregon Cascades and Northern Blue Mountains and these still look good. QPF values have come up a bit with the morning model runs, but the advisory still looks good and again with the time of year any precipitation that falls during the day will have a much harder time accumulating. There are thunderstorm chances (20-40%) across much of the region today, with the best chances across the Blue Mountains, Wallowas and John Day Highlands. CAPE values are forecast to be in the 300-400 J/kg range with LI values -3 to -4 degrees C. Guidance also suggests activity will decrease after 07/00Z. Current satellite imagery shows abundant cloud cover across much of the area, with more breaks in Washington and portions of the Columbia Basin. Will the clouds prevent thunderstorm development? Next topic is breezy wind gusts which should mainly be in the 25 to 35 mph range with gusts as high as 40 mph through tonight and into Tuesday before decreasing Tuesday evening. The Columbia Basin, Foothills of the Blue Mountains, Columbia Gorge, Simcoe Highlands, Kittitas Valley and Yakima Valley are the most likely areas to see the strongest winds. The NBM probabilities of winds >=39 mph across much of the aforementioned areas are 80% to 100%. The NBM probabilities of winds >=47 mph drops to between between 30% and 60% on average. The ECMWF EFI is keying in on wind gusts across the Basin, but more so across the Blue Mountains and Wallowa COunty, with values of 0.8 to 0.9. Also, another area across the Yakima and Kittitas Valley. By later Tuesday, any precipitation will be confined to the eastern oregon mountains and by Wednesday morning, dry north to northwesterly flow will build in followed by a ridge. This will bring increasingly warm and dry weather for the rest of the period. Finally, high temperatures will warm each day, from the 50s and 60s on Tuesday to the 60s and 70s on Wednesday. The ECMWF EFI is focusing on high temperatures on Tuesday, with much of Oregon below normal, in the -0.6 to -0.7 range and a large swath of eastern Oregon in the -0.7 to -0.8 range with some areas of the southern Blue mountains in -0.8 to -0.9. .LONG TERM... Thursday through Monday...Models in good agreement through the period. However, they do diverge slightly on how fast the upper level ridge weakens and moves east. EOFs show very little model differences through Saturday, but diverge on how quickly the ridge weakens and moves east thereafter. Looking at WPC cluster analysis, there is a good chance (50-70%) the ridge will hold on (but weakens) through the period and a low chance (15-20%) of a weak trough clipping Washington by Monday. The ensemble members paint a similar picture with the ECMWF weakening the ridge quicker allowing a weak trough to move through the area late in the period while the GFS is slower and develops general zonal upper level flow Sunday and Monday. The deterministic models on pretty good alignment through the period, but differ mainly in timing of a weak shortwave moving through Washington Monday. NBM is a bit drier for Monday which is likely a good bet given the cluster analysis, but it may start trending a little "wetter" Monday over Washington if the disturbance pans out. Daytime highs will be above normal through the period, but expected to peak Saturday 15-20 degrees above normal. There will be some hydrological concerns over the weekend as the strong upper level ridge develops off the coast. This will provide ideal conditions for melting some of our winter snowpack. All rivers will see rises, but portions of the Naches River may approach (or exceed) bankfull by Saturday or Sunday. && .PRELIMINARY POINT TEMPS/POPS... PDT 36 56 34 62 / 20 10 0 0 ALW 40 59 39 64 / 30 20 0 0 PSC 43 65 41 71 / 20 0 0 0 YKM 35 63 34 72 / 10 0 0 0 HRI 40 63 38 68 / 10 10 0 0 ELN 36 57 37 69 / 10 0 0 0 RDM 29 50 26 59 / 30 0 0 0 LGD 34 50 32 57 / 60 40 0 0 GCD 31 47 31 57 / 70 40 10 0 DLS 42 60 40 70 / 20 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Winter Weather Advisory until 5 AM PDT Tuesday for ORZ502-509. WA...Winter Weather Advisory until 5 AM PDT Tuesday for WAZ030. Wind Advisory from 8 AM to 11 PM PDT Tuesday for WAZ026. && $$ EVENING UPDATE...82 SHORT TERM...77 LONG TERM....81 AVIATION...82