Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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237 FXUS66 KPDT 230324 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 824 PM PDT Wed May 22 2024 .UPDATE...An upper level low continues to pass over the region but now has 2 circulations. The most impactful one has been moving south through north central Oregon and is currently heading into Grant County. This had produced some thunderstorms earlier but is now just a cluster of convective showers. This will continue moving SSE and into SE Oregon after midnight. The other circulation is over central Idaho. This is pulling some moisture from Montana and Idaho back into SE Washington and NE Oregon producing some light precipitation that will persist into early Thursday morning mainly over the far eastern mountains. The rest of the forecast area will see some partial clearing overnight and winds will continue to decrease over the entire area. Update was mainly to POP, Weather and Sky conditions for tonight through Thursday morning. .SHORT TERM...Today through Friday... Key Messages: 1. Windy conditions this afternoon and evening. 2. Thunderstorms possible through this evening. 3. Warming conditions Thursday and Friday. Current visible satellite imagery shows the upper level low pressure system moving into northeast Oregon as it slowly transits the area from northwest to southeast through the day today. This, coupled with a passing cold front, has allowed a pressure gradient to develop along the Cascades, bringing elevated winds across the area. Wind gusts between 45-55 mph have already been observed at the Ellensburg, Pendleton, and Hermiston airports, with these winds expected to linger into the evening hours. Thus, a Wind Advisory has been issued for the Lower Columbia Basin of Oregon and the northern Blue Mountain foothills until 5 PM; as well as north central Oregon and the Kittitas Valley until 9 PM this evening. Guidance has had a tough time capturing the extent and strength of winds today, which lends only moderate confidence (50-60%) in these elevated wind gusts lingering through much of the evening. Current radar imagery is showing several discrete thunderstorm cells developing across the Lower Columbia Basin and northern Blue Mountains/foothills as increased cloud-to-ground lightning strikes have been occurring in the last hour. This activity is expected to continue through the afternoon and extend south into the John Day-Ochoco Basin before subsiding shortly after 8 PM. The highest surface CAPE values (100-200 J/kg) exist from Boardman east to the northern Blue Mountains, with the higher wind shear (30-40 knots) further south in the John Day-Ochoco Basin as advertised by the HREF. These marginal convective conditions lead to high confidence (80%) that cells will stay discrete and storms sub-severe. However, there is a chance of pea-sized hail (30-50%) and a slight chance of a brief cold-core funnel (5-15%). Showers will linger through the evening across the John Day-Ochoco Highlands, Elkhorns, and higher elevations of the northern Blue Mountains before drying into Thursday morning. Another upper level shortwave will be passing through the Pacific Northwest on Friday, returning shower chances to our mountain zones (40-60%) and the foothills (20-35%). A slight chance of thunderstorms also returns to areas of the northern and southern Blue Mountains Friday afternoon and evening. Surface CAPE looks marginal between 100-150 J/kg, so discrete cells and sub-severe conditions are likely (70-80% confidence). A transient upper level ridge will move into the area in the wake of the departing upper level on Thursday, providing drier and warmer conditions with the latter extending into Friday as an approaching and passing upper level trough will enhance southwest flow aloft. High temperatures will be bumping up 10-15 degrees on Thursday and another 5 degrees on Friday as highs reach into the mid-to upper 70s for lower elevations of the Basin, which is near normal values for this time of year. 75 .LONG TERM...Saturday through Wednesday...The long term will be characterized by active weather with a slight reprieve between systems. An upper level tough will begin the period off ahead of an upper level ridge followed yet another trough. Temperatures will begin to hover back towards near normal with near normal precipitation as well. Models are in relatively decent agreement with the upper level trough axis being over the forecast area on Saturday. Clusters show the main variance between the ensembles to be with the amplitude of the trough. All solutions however, do suggest there to be some light precipitation over the mountains with some rain/snow along the higher peaks with snow levels between 4500-6000ft. 20-40% probabilities of between 0.01-0.03 inches of rain along the Cascades and less than 15% along the northern Blues Mountains. Also, NBM ensembles are showing between 10-15% probability of thunderstorms along the far eastern mountains of Wallowa County late Saturday morning into early afternoon. Guidance is picking up on some light instability over the area and slightly warmer temperatures along the Snake River. Couple that with the westerly flow and orographic lift, one can not rule out the probability of a stray thunder clap. Temperatures over the forecast area will be near normal according to the EFI guidance with over 48% of the raw ensembles showing the Pendleton area in the upper 50s to 60s, 41% has the Basin in the upper 60s to low 70s, 67% having north and central OR in the 50s and lastly, the higher elevations in the mid 40s. Winds will be breezy as the trough passes with 60-80% probabilities of the Gorge, Blues, Columbia Basin and adjacent valleys seeing sustained winds of 25 mph or greater. By Saturday evening the models show the upper level tough move eastward while the system behind the trough flattens the flow to a zonal flow with westerly winds. With the zonal flow, surface pressure gradients will loosen allowing a reprieve from the windy conditions. Sunday through Tuesday afternoon the models continue to be in decent agreement with the zonal flow beginning to give way to an upper level ridge. This will bring dry and warming conditions back to the area. Clusters remain vigilant with the main variance being with the amplitude of the incoming ridge. With this shift in pattern, winds will be mostly light with some diurnal variances through the Gorge, lower Columbia Basin and adjacent Valleys with 30-50% probabilities of sustained winds of 20 mph. EFI remains steady with the temperatures being at or slightly above normal with 45-50% of the raw ensembles showing the Pendleton area in the low 70s, increasing to low 80s Monday and Tuesday. 50-60% show the Basin in the mid to upper 70s Sunday increasing to mid 80s Monday and Tuesday, over 50% showing norther and central OR in the upper 70s to low 80s and the higher elevations in the upper 50s increasing to the low 60s Monday and Tuesday. Tuesday night through Wednesday the models remain in decent agreement with the upper level trough begins to push back into the region bringing with it chances of precipitation and slightly cooler temperatures. With this upper level low, guidance is beginning to pick up on some elevated instability over the region with the bulk showing up primarily over the eastern mountains. NBM ensembles show a 10-20% 6 hour probability of thunderstorms over the eastern mountains Tuesday evening. Ensembles show the CAPE values increasing to over 100 J/kg during the same time frame with 40-60% probabilities. With this being day 7 and 8, confidence in the chances of thunderstorms is low/moderate at this time. With the incoming trough, NBM is showing temperatures to decrease with the passage as well as an increase in the surface winds nearing 20 mph through the Cascade Gaps and along the foothills. Bennese/90 && .AVIATION...00Z TAFS...An upper level low over the region will continue to provide showers through the evening mainly impacting PDT and ALW with showers and possible thunderstorms then ending and partial clearing overnight. Overall expecting VFR conditions but could see brief MVFR conditions at PST and ALW through 06Z. Winds are decreasing but could see some 10-20kts winds with gusts around 30kts through the early evening then becoming less than 10 kts overnight and Wednesday. && .PRELIMINARY POINT TEMPS/POPS... PDT 41 67 43 68 / 70 10 0 10 ALW 45 69 47 73 / 30 10 0 20 PSC 48 74 49 76 / 30 10 0 10 YKM 43 74 45 74 / 20 10 0 10 HRI 44 73 47 74 / 60 10 0 10 ELN 44 69 43 70 / 10 10 0 10 RDM 33 65 39 65 / 10 0 0 10 LGD 38 61 39 67 / 60 10 0 30 GCD 37 62 38 67 / 80 10 0 30 DLS 47 71 48 71 / 20 10 0 10 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...75 LONG TERM....90 AVIATION...97