Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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679 FXUS66 KPDT 091017 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 317 AM PDT Sun Jun 9 2024 .SHORT TERM...Today through Tuesday... Key Messages: 1. Thunderstorm potential this morning and afternoon. 2. Breezy winds this afternoon through Monday morning, returning Tuesday. 3. Warm, above normal temperatures extend through Tuesday. Current radar and infrared satellite imagery showing pockets of light returns across the Lower Columbia Basin, Central Oregon, and through the John Day-Ochoco Basin under cloud skies. This is a result from the upper level trough and associated cold front moving onshore, before passing through the area later this afternoon. These synoptic features will allow for enhanced lift and moisture to occur, leading to the development of scattered showers (20-50%) and isolated thunderstorms (5-15%) across our Oregon zones this morning before extending north into south- central Washington this afternoon as the frontal system slowly crosses the area. The Storm Prediction Center does include our area in the General Thunderstorm (0 of 5) category of their Convective Outlook for today, as the ECMWF EFI focuses above normal CAPE and low level shear values over our southern zones, Simcoe Highlands, and the Yakima/Kittitas Valleys with 60-80% of ensembles in agreement. The best chances (20-30%) for developing thunderstorms will occur this afternoon across Southern Deschutes, Southern Crook, Grant, Southern Union, and Wallowa Counties, with slight chances (10-20%) across the Yakima and Kittitas Valleys. Convective parameters are much more conducive for storm development over our southern and eastern zones as the HREF advertises surface CAPE (SBCAPE) of 250-450 J/kg and low level shear of 40-55 knots, compared to SBCAPE of 400-550 J/kg and low level shear of 15-25 knots over the Yakima and Kittitas Valleys. The RAP suggests stout low level lapse rates of 8-9C/km over the Yakima/Kittitas Valley compared to 6-7.5 C/km over our southern and eastern zones. These parameters correlate to a better chance of longer duration storms across our southern and eastern zones, with more brief and discrete storm cells expected over the Yakima and Kittitas Valleys. Showers and thunderstorms will track east after 5 PM as chances (10-20%) linger across Grant, Wallowa, Union, and eastern Crook, eventually exiting to our east by 11 PM. The main concern for any developing cells will be gusty winds, frequent lightning, and moderate to heavy rainfall. However, precipitable water (PWAT) values will be dropping substantially from 1.0" to 0.6" (180% to 100% of normal) through the afternoon as drier air advects into the area, so the threat of heavy rainfall will diminish through the afternoon and evening. Rain amounts will stay between 0.01-0.05" for most of the region, with up to 0.15" over areas in and near developing storms. Lower elevations of the Basin are expected to stay dry, with potentially a trace along the northern Blue Mountain foothills. Today`s passing cold front will also allow for a pressure gradient to develop along the Cascades, enhancing winds this afternoon through Monday morning across the Gorge, Simcoe Highlands, and the Kittitas Valley. Sustained winds of 15-25 mph and gusts of 30-40 mph are expected in these areas, peaking between 8 PM today through 5 AM Monday. Confidence in these wind values is high (80-90%) as the NBM suggests a 60-80% chance of wind gusts of 40 mph or greater over the aforementioned areas, as the GFS, NAM, and SREF all highlight a pressure gradient between Portland and Spokane of 6-8 mb. Winds will slacken through the day on Monday as an upper level ridge builds over the region in the wake of the exiting system. This will be short-lived as a quickly moving upper level shortwave passes to our north and associated cold front passes through the area on Tuesday, tightening isobars to allow another pressure gradient to develop along the Cascades as it suppresses the incumbent ridge. The GFS and SREF both highlight a 8-11.5mb gradient between Portland and Spokane, with the GFS hinting at the potential of advisory-level winds (11 mb threshold) over the Simcoe Highlands, Lower Columbia Basin of Oregon, and the Kittitas Valley. Thus, gusts of 35-45 mph will be possible over these areas, with the NBM showcasing a 70-90% chance of 45 mph gusts or greater over these areas - which is advisory- level. Moderate to high confidence (60-80%) resides with wind advisories needing to be issued for Tuesday afternoon and/or evening. High temperatures peaked at 90 degrees yesterday in the Tri-Cities and Hermiston, but after a short reprieve will again approach these values by Tuesday. After a mild night with morning lows only dropping into the upper 50s to low 60s across the Basin and foothills due to cloud cover, afternoon temperatures will drop 4-8 degrees from yesterday. Highs across the Basin and foothills will reach into the low to mid-80s, as a response to cloud cover and a brief stint of west-northwest flow aloft. These features will not last long as an upper level ridge begins to build over the area Monday morning, clearing skies and bringing southwest flow aloft back as highs increase 1-4 degrees on Monday with values in the mid-to upper 80s for lower elevations of the Basin. Highs bump up another 1-3 degrees Tuesday as the approaching shortwave enhances southwest flow aloft to see low-90s again return to the Tri-Cities area. Confidence in the return to low 90s over portions of the Basin is moderate (50-60%) as the NBM suggests a 58% chance for the Tri-Cities and a 21% chance for Hermiston reaching high temperatures of 90 degrees or greater on Tuesday. 75 .LONG TERM...Wednesday through Sunday... Key Messages: 1) Conditions remain largely similar Wednesday and Thursday under zonal flow. 2) Low pressure approaches the PacNW on Friday bringing breezy winds, cooler than normal temperatures, and gusty winds. The long term will begin fairly benign but by next weekend we should see the return of active weather including spring-like temperatures helping to stave off summer heat. Model ensembles remain largely in good agreement on the first few days of the forecast period, indicating we`ll remain in a zonal flow pattern through at least early Thursday before flow turns more southwesterly due to an approaching system from the Gulf of Alaska. Under this pattern, Wednesday and Thursday look largely uneventful for sensible weather concerns, with highs Wednesday in the upper 70`s to mid 80`s in a post-frontal environment, and temperatures rising 2-6 degrees on Thursday due to a weak, transient ridge passing through. There is high confidence (70-80%) in the forecast conditions for these days. Over the weekend is when we will see more significant weather as a deep low from the Gulf of Alaska drops down into the region. Ensembles begin to lose consistency during this timeframe leading to a bit more uncertainty overall. Ensemble clusters show the low will largely be just offshore to the northwest Friday morning, then expected to move inland by Saturday but with around 13% of members expecting this system to weaken notably at that time. Then on Sunday, clusters are largely in disagreement due to issues resolving the speed and strength of the system. There is moderate confidence in the forecast on Friday and Saturday (50-60%), with low confidence on Sunday (30%). The NBM expects temperatures to already begin dropping on Friday for everywhere outside of the Columbia Basin and Foothills of the Blues as the front associated with this Alaskan low progresses quickly across the region. Then by Saturday we should see highs notably below normal for this time of year by 5 to 10 degrees, continuing into Sunday, ranging in the upper 60`s to mid 70`s. Overnight lows will similarly decline, dropping into the upper 30`s for Central Oregon and up to the mid to upper 40`s elsewhere for our population centers. Meanwhile our usual breezy winds make their return on Friday as the pressure gradients tighten with this system. The NBM has a widespread 40-100% chance of gusts of 40+ mph, with a 40-80% chance of gusts 45+ mph for the lower Columbia Basin/Foothills of the Blues in Oregon, Simcoe Highlands, and Kittitas Valley. Probabilities go down around 10-30% on Saturday, but breezy winds should persist for the day as well, decaying much further by Sunday. Finally, although the Cascades may receive mountain precipitation through the duration of this event, low PWAT values and rain shadowing are anticipated, preventing notable rainfall for the majority of our region. Overall look for a cool and breezy spring-like weekend as we move into the middle of June. Goatley/87 && .AVIATION...12Z TAFs...VFR conditions expected over the next 24 hours. Widespread scattered showers possible with VCSH all sites this morning into the early afternoon except for PSC/DLS - these sites have probabilities too low (< 15%) to include, but a rogue shower moving nearby cannot be ruled out. Breezy winds with gusts 20-25 knots on average possible for DLS/RDM/BDN this afternoon - winds generally 10 knots or less for all other sites. Low to mid- level cloud decks this morning through the beginning of the afternoon (around 5-10k feet), rising and then clearing thereafter as the system that brought the precipitation and breezy winds exits. Goatley/87 && .PRELIMINARY POINT TEMPS/POPS... PDT 80 54 83 55 / 20 10 0 0 ALW 83 57 85 59 / 30 10 0 0 PSC 87 59 89 59 / 20 10 0 0 YKM 86 53 87 55 / 30 10 0 0 HRI 87 58 88 58 / 20 0 0 0 ELN 81 54 80 56 / 30 10 0 0 RDM 80 47 82 51 / 30 0 0 0 LGD 76 50 79 52 / 30 10 0 0 GCD 78 49 82 52 / 40 10 0 0 DLS 83 56 84 58 / 10 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...75 LONG TERM....87 AVIATION...87