Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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595 FXUS66 KPDT 202119 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 219 PM PDT Mon May 20 2024 .SHORT TERM...Tonight through Wednesday night...Instability showers and a rogue thunderstorm have been observed this afternoon in Wallowa County where SPC`s Mesoscale Analysis indicates 500 J/kg of MUCAPE and mid-level lapse rates around 7C between H7-H5. The rest of the forecast area is under partly cloudy skies, and the extent of fair weather cumulus on satellite is quite interesting. Skies will clear tonight in a drier NNW flow aloft. It will be another chilly night with lows in the 30s and 40s, but the southwest surface winds for most of the evening will keep temperatures from falling below freezing in the lowlands currently in the growing season. The dry conditions will not last long, as the next system spreads precipitation over the forecast area Tuesday afternoon and evening with numerous showers on Wednesday. This system is coming in as an upper level trough over BC moving south and developing into a closed low over WA and northern OR. This will be aided by a 100kt jet at 250mb with the left exit region of the jet aimed over northeast OR. Although rainfall amounts will not be substantial, there will be a steady light to moderate rain over the Blue Mtns, Grande Ronde Valley, and foothills Tuesday night through Wednesday. The most likely rainfall amounts during this time will be 0.25-0.5", and the NBM has a 40% chance that 0.75" will be observed over the northern Blue Mtns (20% for 1" or more). The ECMWF EFI that always does a good job indicating climatologically high QPF has this area at 0.8 out of 1.0 category. Snow levels will begin around 6500-7000 feet then will lower to around 5000 feet on Wednesday. By the time snow levels lower to 5000 feet, precipitation will decrease, therefore snow accumulations will not impact any of the populated areas. Only the Eagle Caps and the Elkhorns will have any noteworthy snowfall of around 3-6" at the highest peaks. Breezy to windy conditions will return on Wednesday with gusts 25-40 mph, and there are only few areas that have a chance (30%) of gusts stronger than 45 mph-- the Kittitas Valley, Maryhill area, and portions of north central Oregon. The deep upper low will travel into central ID Wednesday night with precipitation tapering off or ending from the west. Lapse rates will steepen on Wednesday as H5 temperatures lower to -28C, therefore there will likely be graupel within the showers and there is a slight chance (20%) of thunderstorms. Patterns like this often result in cold core funnel clouds, so I cannot rule that out as well. Scattered wrap around showers will continue in Wallowa County Wednesday night. Wister/85 .LONG TERM...Thursday through Monday... Key Messages: 1. Breezy winds Friday and Saturday. 2. Rain showers return late in the week. 3. Above normal high temperatures late in the holiday weekend. The extended period is characterized by an upper level trough dropping into the Pacific Northwest from the British Columbia coast late Friday into Saturday. This feature, coupled with a passing cold front late Friday, will allow for a pressure gradient to develop along the Cascades to promote elevated winds across the east slopes of the Cascades, Simcoe Highlands, and the Columbia Basin as gusts of 25 to 35 mph will be possible both Friday and Saturday. Confidence in these wind gusts is moderate (50-60%) as model guidance is not in full agreement in regards to the upper level trough`s location. The ECMWF keeps the trough slightly more north than the GFS, which would further tighten the pressure gradient and produce wind gusts of 30 to 40 mph versus 20 to 30 mph with the GFS solution. The NBM suggests a 50% to 80% chance of wind gusts of 39 mph or greater on Friday and a 55% to 85% chance of wind gusts of 39 mph or greater on Saturday over the aforementioned areas, with the highest chances residing across the Simcoe Highlands and the Kittitas Valley. Winds are expected to slacken Sunday and Monday as an upper level ridge builds over the region. A departing system early in the period will keep lingering precipitation occurring over Wallowa County through Thursday afternoon before drying into the early evening hours. Elsewhere, a transient upper level ridge will keep the Basin and Central Oregon under mostly sunny skies and dry conditions. However, the approaching trough that will bring elevated winds Friday and Saturday will also bring rain showers along the Cascades late Thursday before extending across much of the area overnight and through Saturday morning. Guidance is in slightly better agreement with the overall amount of rain, which is potentially a trace along the Blue Mountain foothills and 0.10 to 0.20 of an inch at elevation over the Cascades and Blues Friday and early Saturday. There is a slight difference in rain arrival Friday evening, as the GFS begins rain about 3 to 6 hour earlier than the ECWMF as its track is further south. Confidence in these rain amounts is moderate (40-50%) as the NBM pinpoints the best chances for rain will be 5 PM Friday through 5 PM Saturday, with the northern Blue Mountains incurring a 20% to 30% chance of 0.25 of an inch or more rainfall and the foothills only receiving a 5% to 15% chance. When dropping rain amounts to 0.10 of an inch or more, chances bump up to 40% to 50% across the northern Blue Mountains and 20% to 30% along the foothills. The trough exits to our east Saturday evening to open the door to a developing upper level ridge, bringing with it dry and warm conditions through the remainder of the holiday weekend. High temperatures will stay below normal through the week as predominately northwest flow aloft keeps cooler air advecting into the region. Over the latter half of the weekend, the departing system gives way to a building upper level ridge that sets up across the Pacific Northwest as an upper level low pressure stalls in the Gulf of Alaska. This low pressure slowly drops south into early next week, enhancing southwest flow aloft and increasing high temperatures to above normal values by Sunday as the trend continues into midweek as the system slowly approaches the coast. Currently, high temperatures Tuesday and Wednesday are forecast to be around 10 degrees above normal, but low confidence (20-30%) resides with these values as model variability exists and primarily associated with the location and strength of this upper low pressure system. Current confidence resides more with the ECWMF outcome, as 56% of ensemble clusters on Friday and 68% of ensemble members on Saturday align with more of a northern track of the trough. This hints at the expectation of higher wind gusts Friday and Saturday and a later onset of showers late Friday. Thunderstorm chances over northern eastern Wallowa County Friday and Saturday also are less likely with the more ECMWF solution, which is indicated by only a 10% to 20% chance of Thunderstorms via the NBM. The clusters are in more agreement when it comes to warming late in the period as a result from enhanced southwest flow aloft, as 82% of clusters show this flow initiating by Monday. 75 && .AVIATION...18Z TAFS (previous discussion)...VFR conditions currently at all sites, which will stay the course through the period. Breezy winds are occurring at KDLS, with high confidence (90%) of gusts between 25 and 30 kts will extend through midnight. Winds will also increase for KBDN/KRDM/KYKM late this afternoon and extend through the evening with gusts of 20 to 25 kts likely. Winds of 10 kts or less will occur at all other sites. Increasing cloud cover across all sites and lowered ceilings for KDLS toward the end of the period as a system approaches the region. 75 && .PRELIMINARY POINT TEMPS/POPS... PDT 41 66 45 57 / 0 40 90 60 ALW 46 69 49 60 / 0 40 90 80 PSC 48 71 51 65 / 0 30 70 50 YKM 42 66 42 62 / 0 40 30 20 HRI 46 69 48 64 / 0 40 70 40 ELN 45 63 42 59 / 0 40 40 30 RDM 33 65 42 54 / 0 30 60 20 LGD 36 66 44 53 / 0 20 90 80 GCD 35 66 44 54 / 0 20 90 70 DLS 46 63 49 61 / 0 50 60 20 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...85 LONG TERM....75 AVIATION...75