Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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825 FXUS66 KPDT 050354 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 854 PM PDT Sat May 4 2024 .EVENING UPDATE...A wet and windy 24 hours is in store for the forecast area as a deep low pressure system continues to move onshore right around the Oregon/California coastline. The main band of wraparound moisture has been late to materialize based on what models originally foresaw, but latest radar imagery this evening shows the band finally starting to develop, with much of the forecast area expected to be enveloped with rain by midnight tonight, gradually spreading northward as we head into tomorrow morning. Latest HREF has come in a bit wetter than previous runs, with the latest CAMs putting quite the bullseye over the Oregon Basin early tomorrow morning. NBM QPF is not quite as bullish, but this wrap around banding does have the potential to produce pretty respectful amounts of moisture potentially exceeding half an inch by tomorrow afternoon in the Oregon Basin. Whether that actually materializes is another question, as CAMs have been a bit off with regards to the start of rainfall, but did update QPF based on latest HREF as well as recent trends. Overall messaging with the winds and mountain snow remains the same. Webcams show accumulating snow starting to occur over the Oregon Cascade passes, with snow expected to spread into the John Day/Ochoco Highlands as this oncoming low pulls in colder air into the forecast area. Wind gusts also look fairly consistent compared to the latest HREF. Overall changes to the forecast package focused primarily on updating PoPs to reflect the drier-than- anticipated conditions this evening, but that is expected to change quickly as this band of wraparound moisture is expected to materialize overnight. Evans/74 && .PREV DISCUSSION... /issued 443 PM PDT Sat May 4 2024/ SHORT TERM...Tonight through Monday night...A deep low currently centered about 130 miles west of Coos Bay has brought widespread overcast skies and a band of light precipitation over south central WA and north central OR since early this morning. This weak deformation band is only a precursor to a stronger band from the Cascades westward where numerous rainfall amounts of 1-2 inches have been observed over the past 24 hours. This is all associated with an AR in the range of 500-600 kg m-1 s-1 aimed directly over the OR Cascades. Areas east of the Cascades will not observe as much precipitation as this band tracks to the east, but this is definitely one to bring above average rainfall to the area. For the next 24 hours, many areas will have 0.25-0.75" and locally around 1". The east slopes of the OR Cascades have already received 0.25-0.5" already, and the Mt. Wilson RAWS site in Wasco County measured 0.78". A Winter Weather Advisory is in effect for the Cascade east slopes of OR above 4000 feet (public zone OR509) through 11 AM Sunday. Based on current temperatures and dewpoints, the snow levels in OR509 are around 5000 feet--a little higher than previously expected. However, temperatures at Santiam Pass are in the mid 30s and precipitation will soon change over the snow as snow levels fall to around 3500 feet tonight. As the low travels to the ESE, colder air behind the system will cause snow levels to lower to around 3500-4500 feet over the southern Blues (public zone OR503) and the John Day- Ochoco Highlands (public zone OR506). A Winter Weather Advisory is also in effect for OR506 above 4000 feet tonight through 11 PM Sunday. OR503 will have snow accumulations around 3-6 inches, but mainly along the higher peaks. The only location along HWY395 with several inches of snow in the forecast is Long Creek Mountain. The northern Blue Mtns will also have around 3-6 inches of snow, but mainly above 5000 feet which includes Tollgate. There are no plans to issue any Winter Weather Advisories for the Blue Mtns. It`s important to note that the probability progs from the NBM and HREF use a 10:1 ratio which will indicate higher than the current forecast, as 10:1 is a little too high with our warm temperatures we have observed today. The upper low will weaken as it travels east across southern OR and NV and into southern ID tomorrow. This will be followed by a northwest flow aloft Sunday night. Precipitation will decrease during this time, but numerous snow and rain showers will continue over the mountains and valleys. Snow levels will be around 3500-4500 feet. A second system will follow as a shortwave trough on Monday, keeping snow levels down around 4000 feet along with light orographic showers. Even though QPF and snow amounts will be light, snow covered roads in early May will catch some unprepared travelers by surprise and this will be addressed the next couple of days. The air mass will be marginally unstable, and an isolated thunderstorm cannot be ruled out, mainly over southeast OR. Not only will forecasters focus on precipitation amounts and snow, but wind is another concern. A Wind Advisory is in effect Monday for many areas across south central WA and north central OR. Surface gradients have been tight today, and the increasing winds aloft and tighter gradients on Sunday will result in very windy conditions gusting to 45-50 mph. Confidence in the wind advisory is around 80% but warning criteria (i.e. 58 mph or stronger gusts) is low (20%). Wister/85 LONG TERM...Tuesday through Saturday...Cool, breezy, and showery weather is forecast on Tuesday with a significant warming and drying trend through the remainder of the period, especially Thursday through Saturday. Synoptic Overview: The period will start out on Tuesday with a broad upper-level closed low downstream over the Great Plains and an upper- level ridge building in upstream offshore in the Pacific. This will place the forecast area under a cool northwest flow with light rain and snow showers forecast for the mountains. Wednesday, flow aloft turns north to northeast, and any lingering shower activity will likely be confined to the northeast mountains, mainly in Wallowa County. Thursday, ensemble guidance is in excellent agreement that the low downstream over the Great Plains will eject eastward with a piece of shortwave energy likely (>80% chance) facilitating the formation of another closed low over the Great Basin into California. Ensemble clusters show some spread in solutions regarding the development and evolution of the closed low over California, primarily with regard to the magnitude of the 500-mb heights over northern California and southern Oregon. Pattern details aside, all clusters are advertising some flavor of a Rex block pattern setting up Thursday and persisting through Friday (>80% chance) or Saturday (75-80% chance). This will favor a warming trend as the upper-level ridge/high sets up over the PacNW to the north of the closed low. There is a low chance (22% of members) that the closed low will migrate far enough north over NW CA/SE OR that showers will develop on Saturday over the mountains (mainly the Blues. Regarding the chances of any watches, warnings, or advisories, the NBM is suggesting a 60-95% chance of advisory-level winds Tuesday for the Kittitas Valley, Simcoe Highlands, Columbia Gorge, foothills of the Blue Mountains of Oregon, and portions of the lower Columbia Basin of Oregon. The probability of maximum temperatures exceeding 80 degrees increases Thursday, Friday, and Saturday (peaking Saturday at 60-90% chance for the Columbia Basin and adjacent valleys and foothills). Moreover, the Columbia Basin and Yakima Valley have a 45-65% chance of highs exceeding 85 degrees on Saturday, and there is a very low chance of exceeding 90 degrees (<15% chance). While NWS HeatRisk values increase Thursday, Friday, and Saturday, they are currently forecast to stay at a category 1 (minor). This level of heat primarily affects individuals extremely sensitive to heat, especially when outdoors without effective cooling and/or adequate hydration. The chance of any heat-related highlights is extremely low (<5% chance). Plunkett/86 AVIATION...00Z TAFs...MVFR to VFR conditions will prevail through the period, with locally IFR conditions possible in the overnight and morning hours at site BDN. CIGS AOA 4kft to 9kft AGL will continue through the evening, then drop to between 1kft to 3kft AGL at all sites. Site BDN will see CIGS drop to as low as 500ft after 13Z. An upper level low pressure system will continue to bring rain impacts to all sites through tomorrow afternoon. Westerly winds will be increasing at all sites to 12-20kts with gusts up to 30kts starting this evening, and continuing through the rest of the period. Lawhorn/82 && .PRELIMINARY POINT TEMPS/POPS... PDT 39 50 40 56 / 100 90 70 60 ALW 44 52 44 58 / 90 80 80 60 PSC 47 59 47 65 / 80 90 40 20 YKM 44 61 39 63 / 40 60 10 20 HRI 42 56 43 62 / 100 90 40 40 ELN 45 58 40 60 / 40 40 20 20 RDM 33 49 34 52 / 70 70 10 60 LGD 39 48 38 52 / 100 80 90 80 GCD 34 44 35 50 / 90 100 70 90 DLS 45 57 46 57 / 90 70 30 50 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Wind Advisory from 2 AM to 11 PM PDT Sunday for ORZ041-044-507- 508-510. Winter Weather Advisory until 11 PM PDT Sunday for ORZ506. Winter Weather Advisory until 11 AM PDT Sunday for ORZ509. WA...Wind Advisory from 2 AM to 11 PM PDT Sunday for WAZ024-521. && $$ SHORT TERM...74 LONG TERM....86 AVIATION...82