Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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160 FXUS66 KPDT 220521 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 1021 PM PDT Tue May 21 2024 Updated aviation discussion .UPDATE...A closed upper level low currently over Vancouver Island will continue to drop southeast and be centered rough over Pendleton by Wednesday afternoon. Ahead of the low we are seeing a moisture field pushing across the forecast area associated with some widespread rain. Some drier air aloft on the southern fringe of the low is currently moving into NW Washington which is creating a distinct back edge to the precipitation field. This dry slot will begin pushing over the Washington Cascades shortly bringing and end to the precipitation along the east slopes of the Cascades and the western portion of the Lower Columbia Basin. The eastern portion of the Basin and eastern mountains will continue to see a steady light rain with the eastern Basin turning to showers late tonight. Central Oregon will continue seeing some light rain up through midnight then transition to showers. Forecast has been updated to reflect this timing and coverage. .SHORT TERM...Tonight through Thursday....Active mid-week weather pattern is occurring as a Canadian low is currently sweeping a warm front across the region, with stratiform rain overspreading today followed by breezy winds and a chance at showers and occasional thunderstorm tomorrow before clearing out in Friday. The aforementioned precipitation overspreading the region is expected to continue through the evening and overnight hours, heaviest across the Blue Mountains. Probability of 24 hour rainfall amounts shows high confidence (70-100%) that locations across the foothills and into the mountains themselves will receive 0.5 inch; increasing this threshold to 1.0 inch, there remains moderate confidence (40-60%). Into Wednesday, the focus shifts to then showers, thunderstorms, and breezy winds. Showers will be a bit more scattered in nature than compared to the widespread stratiform rain ongoing today, but with moderate to high confidence still (60-90%). Instability has seen a noteworthy jump, with a bit more widespread coverage of low amounts of CAPE from the Ochoco-John Day Highlands through the Blues and Wallowas. Still, with how weak parameters are, there`s low confidence (10-30%) in thunderstorms, and any that do occur should just have occasional lightning and perhaps some locally breezy winds, though with cooler temperatures aloft the chance of graupel with an isolated storm cannot be ruled out. On the subject of winds, confidence is moderate to high (60-80%) in widespread wind gusts of 30-50 mph, strongest across the lower Columbia Basin of Oregon. A wind advisory for this region has been issued in response with the expectation of gusts 45+ mph. Finally, as this system cools our temperatures, snow levels descend to between 4.5-5k feet. This should promote late season mountain snow, heaviest across the Strawberry Mountains into the Wallowas, and even a noteworthy amount falling in the Blue Mountains. Heavy accumulations should be limited to the highest peaks and crests of this region, but there is moderate confidence (60%) in accumulations of 1-3 inches for some of the passes including locations such as Santiam Pass and Tollgate. Late Thursday into early Friday the low progresses eastwards out of our region, with a chance of some light wrap around moisture possible in the Wallowas but by Friday morning we should be dry and cleared out with another system expected into the weekend. Goatley/87 Key Messages: 1. Breezy winds Friday and Saturday. 2. Rain showers and isolated thunderstorms return late in the week. 3. Above normal high temperatures Sunday onward. The extended period is characterized by an upper level low pressure system dropping along the British Columbia coast before driving through the Pacific Northwest early in the holiday weekend. This synoptic feature, coupled with a passing cold front, will allow for a pressure gradient to develop along the Cascades that will cause breezy conditions to occur over the east slopes of the Cascades, Simcoe Highlands, Lower Columbia Basin, foothills of the northern Blue Mountains, and the Kittitas Valley Friday and Saturday - peaking during the afternoon hours. Confidence in elevated winds is high (90%), especially on Saturday as the ECMWF EFI highlights unclimatologically high sustained winds and gusts across the aforementioned areas. Thus, wind gusts of 25 to 35 mph on Friday and 30 to 40 mph on Saturday will be possible out of the west northwest. Confidence in these wind gusts is moderate to high (60-80%) as the NBM a 70-90% chance of 39 mph gusts across the Simcoe Highlands on Friday before bumping up to a 75-95% chance of wind gusts of 47 mph on Saturday. Saturday does look to be the windier day, which is also advertised by the GFS and NAM as incurring a pressure gradient of 9- 10 mb between Portland and Spokane - just shy of the normal advisory threshold of 12 mb. However, these forecast gradients may continue to trend upward as the NBM showcases a 70-80% chance of 47 mph winds on Saturday across the northern Blue Mountain foothills and Kittitas Valley. Thus, there is a moderate (70%) chance of a Wind Advisory being issued for the earlier mentioned areas on Saturday. The upper level low and associated cold front arrival will also bring showers along the Cascades Friday afternoon before extending across the Blue Mountains and foothills Friday evening through Saturday afternoon. Minimal rain amounts are expected along the foothills (less than 0.05 of an inch), with only 0.10-0.20 of an inch expected at elevation over the Cascades and Blues. The only exception will be over Wallowa County, which should get 0.15-0.25 of an inch. This additional moisture will also allow for the potential for afternoon thunderstorms both Friday and Saturday, with a much better chance on Saturday. Friday`s storm chances (10-15%) will primarily stay confined over Wallowa county, extending south and west over the Blue Mountains and John Day-Ochoco Basin on Saturday. Storm probabilities will also bump up to a 15-25% chance as SBCAPE increases from 50 J/kg to 150 J/kg and shear increasing from 20-30 knots Friday to 30-45 knots on Saturday. One thing to note, is that these higher SBCAPE and shear values are primarily associated with the GFS solution, which does drive the upper low further south across the area - correlating to a better potential for thunderstorms. At this time, 52% of ensembles on Friday and 60% on Saturday align more with a shallower path of the upper level low as advertised by the ECMWF. As such, it is expected that isolated, discrete storm cells will have the potential for development as the low tracks over mountainous terrain over the eastern mountains, but will not exhibit severe characteristics in the form of large hail, damaging winds, or tornadic activity. The upper low pressure continues to depart to our east over the latter half of the holiday weekend (Saturday evening onward), drying conditions and clearing skies into the beginning of the workweek. This is a result of an upper level ridge that builds across the Pacific Northwest as an upper level low pressure lingers over the Gulf of Alaska and slowly drops south. Guidance is in good agreement of this feature as 86% of ensemble members show an upper level ridge over the Pacific Northwest and southwest flow aloft on Monday. This flow will advect warmer and drier air into the region to allow above normal high temperatures to arrive Monday onward as highs will break into the low to mid-80s across lower elevations of the Basin. Confidence in reaching high temperatures of 80 degrees or more on Monday is as followed: 72% for Pasco, 6t4% for Hermiston, 69% for The Dalles, 55% for Yakima, 45% for Pendleton and Redmond, 44% for Bend, and 42% for Walla Walla via the NBM. These probabilities ramp up to between 80-90% on Tuesday as southwest flow aloft continues to enhance. 75 && .AVIATION...06Z TAFS...General VFR conditions are expected over the next 24 hours. There will be some rain and showers overnight impacting PDT and ALW with some occasional MVFR conditions through 09-12Z. Conditions improve a little overnight through Wednesday morning before a return of showers midday. Winds will begin to increase overnight becoming 15-30kts on Wednesday with gusts around 35kts. Winds will begin decreasing late afternoon and evening. && .PRELIMINARY POINT TEMPS/POPS... PDT 44 56 42 67 / 100 70 50 0 ALW 47 59 45 68 / 100 70 40 10 PSC 50 64 48 74 / 90 50 20 0 YKM 40 64 41 74 / 40 30 10 0 HRI 47 62 45 72 / 100 50 30 0 ELN 40 59 43 70 / 40 30 10 0 RDM 42 54 31 66 / 70 30 10 0 LGD 43 52 39 61 / 100 80 60 10 GCD 43 53 37 61 / 100 80 80 0 DLS 47 61 46 70 / 70 40 10 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Wind Advisory from 5 AM to 5 PM PDT Wednesday for ORZ044-508. WA...None. && $$ SHORT TERM...87 LONG TERM....75 AVIATION...97