Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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257 FXUS66 KPDT 271851 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 1151 AM PDT Mon May 27 2024 .MORNING UPDATE...Aside from increasing mid- and high-level clouds through the day, quiet weather is anticipated through mid- afternoon as the axis of an upper-level ridge continues to amplify as it tracks eastward over the northern Rockies. Later this afternoon, surface-based convection may develop (10-20% chance) over the mountains of central and east-central OR, though 12Z HREF members are not in good agreement on coverage due to a wide spread in forecast boundary layer moisture and CAPE. Best chances for showers and isolated thunderstorms will be over the high terrain, namely the Ochocos, Elkhorns, and other sub-ranges of the central and east-central OR Blue Mountains. East of the Cascade crest, isolated activity is forecast over the Newberry Crater area. As we transition to the evening and overnight hours, CAMs are in excellent agreement that surface-based instability from daytime heating will wane, though a spread of solutions is evident when examining mid-level moisture. Any subsequent instability as southwesterly flow aloft advects moisture and increasingly steep lapse rates and northward across central/north-central OR and limited portions of south-central WA and northeast OR. Should the moister solutions with near-saturated mid-levels materialize, showers and a few isolated thunderstorms may develop this evening and track northward overnight (10-20% chance PoPs). Global guidance is highlighting areas of near zero to negative Theta-E lapse rates coincident with 70-95% 700-500 mb RH, increasing confidence in at least some altocumulus and showers, though synoptic-scale lift is not robust until later Tuesday morning. The forecast was updated to reflect more aggressive CAM solutions and a slight chance of thunder was also expanded this evening and overnight. Plunkett/86 && .PREV DISCUSSION... /issued 905 AM PDT Mon May 27 2024/ SHORT TERM...Today through Wednesday night...An upper ridge will be located over the Pacific Northwest early today. This ridge will move eastward later in the day, allowing for southwesterly flow to establish itself ahead of an approaching trough and cold front. The trough and associated cold front will move onshore Tuesday afternoon and cross the region through Tuesday night. There could be lingering precipitation over the mountains into Wednesday afternoon, then dry conditions Wednesday night. However it will be cooler on Wednesday. In the developing southwesterly flow this afternoon/evening, CAPE will increase to over potentially over 1000 J/KG in spots, especially across the Blue Mountains and LI values will be in the range of -1 to -4 degrees Celsius. However, moisture is quite lacking. During the evening hours, there is at least some better moisture. So it looks as if there could be some isolated thunderstorm potential during the afternoon and evening hours, but overall potential is low (<20%). The Storm Prediction Center has placed a large portion of central and northeast Oregon and southeast Washington in general thunderstorms today, though any activity looks to be later in the afternoon into the evening. The ECMWF EFI is not all that excited about thunderstorm potential today. The better chance of showers in the mountains and thunderstorms is on Tuesday in increasing southwesterly flow ahead of the advancing trough. The Storm Prediction Center has placed much of Wallowa County and a very small portion of Union County in Marginal Risk of severe thunderstorms, with general thunderstorms expected west of this area from about Walla Walla to Pendleton, including Grant County and points eastward. CAPE values on Tuesday will range from 200 to over 1000 J/kg with LI values -1 to around -6 degrees Celsius. The highest values will be further east. As the cold front works its way eastward, conditions will stabilize, and based on current timing it appears that any thunderstorm activity should be winding down by early evening, with CAPE decreasing and LI values becoming positive behind the passage of the cold front. The ECMWF EFI does key in on thunderstorm potential on Tuesday mainly across the Blue Mountains and Wallowas on Tuesday, with a large area of 0.6 to 0.7 for CAPE and some embedded areas of 0.7 to 0.8. The other concern on Tuesday will be wind with the front/trough. It does look to be breezy, especially in the Oregon Basin, Columbia Gorge, Simcoe Highlands, Kittitas Valley. NBM probabilities of wind gusts >=39 mph are 80 to 90%. However, NBM wind gust probabilities of wind gusts >= 47 mph drop to 40% or less everywhere except in the Kittitas Valley, where they are about 50%. If a wind advisory is needed anywhere, the most likely location would be the Kittitas Valley. The ECMWF EFI ranges from 0.5 to 0.7 across the Columbia Basin on Tuesday. On Wednesday (which would mainly be Wednesday morning), it has values of 0.6 to 0.7 across the Simcoe Highlands and 0.7 to 0.8 across the Kittitas Valley and 0.6 to 0.8 across the Blue Mountains and portions of Wallowa County. High temperatures will be about 5 to 10 degrees above normal today and will generally be in the upper 70s to lower 80s. Highs will be cooler on Tuesday and close to normal, with values ranging from the low 70s to lower 80s. Wednesday, will be even cooler, with high temperatures ranging from the mid 60s to the lower 70s. These temperatures are 5 to 10 degrees below normal. LONG TERM...Thursday through Monday...Models start out in good agreement in having a trough moving off to the east with a ridge building over area from the eastern Pacific. On Friday, model clusters show general agreement in having the ridge building further with the ridge axis just off the coast. The trough moves further east in 80 percent of the model ensemble members while the remaining 19 percent keep the trough over Idaho and still having an influence over Wallowa County. By Saturday, models develop greater differences though each case keeps dry weather over our area. 48 percent have a flat ridge centered offshore or just at the coast with a westerly flow over head and the other 52 percent have the ridge (with varying amplitude) centered over the Rockies and a shallow trough just off the coast. On Sunday, 63 percent have a ridge centered over the Rockies with a southwest flow over our area, 23 percent have a ridge centered along the coast and 14 percent have the ridge centered several hundred miles offshore with a weak trough centered along the Idaho border. On Monday, models favor a strong ridge with significant differences as to the location of the ridge axis. 38 percent have the ridge axis at the coast while the other 62 percent have the ridge centered over the Rockies with a trough approaching the coast. However, the ECMWF and Canadian deterministic runs have a trough near the coast with a front moving through the area Sunday night into Monday. The extreme Forecast Index indicates little in the way of unusual weather aside from cool temperatures Thursday into Friday with values of around -0.75. The result of all this favors mainly dry weather Thursday through Sunday aside from a slight chance of rain showers along the Cascades and in the higher terrain of Wallowa County at times. There will also be a warming trend. Thursday will have a slight chance of light rain showers from the departing front over the Wallowa mountains and the higher terrain near Hells Canyon. Winds will be generally light, though the Kittitas Valley will have northwest winds around 20 mph in the afternoon. The Columbia Basin will have highs in the upper 60s to mid 70s with mid 50s to mid 60s elsewhere. Lows Thursday night will be in the upper 30s to mid 40s in the Columbia Basin and in the lower to mid 30s elsewhere. Friday and Saturday will remain dry with mainly light winds. Temperatures Friday will warm to the 70s in the lower elevations with upper 50s and 60s in the mountains. Saturday will rise to the upper 70s to mid 80s with upper 60s and 70s in the mountains. On Sunday, the NBM brings a chance of rain showers to the Cascades and a slight chance to the eastern mountains in the late afternoon and Sunday night reflecting the ECMWF and Canadian deterministic runs. The ECMWF brings in the cold front with the bulk of the precipitation on Monday, so have a chance of rain in the mountains and a slight chance in the lower elevations. Temperatures Sunday rise a couple of degrees from Saturday. Monday begins to show some cooling with temperatures dropping back to near Saturday`s values. Overnight low temperatures will show a similar warm up Friday night through Sunday night, ending up in the mid 50s to lower 60s in the Columbia Basin and in the mid 40s to lower 50s elsewhere. Perry/83 AVIATION...18Z TAFs...VFR conditions will prevail through the period. A weak disturbance will be moving through the area overnight. Clouds will increase, but only expecting mid to high level clouds at this time. A few showers may develop near KPDT and KALW after midnight, but should remain light. Winds will be 10 kts or less with some higher afternoon gusts. Earle/81 && .PRELIMINARY POINT TEMPS/POPS... PDT 78 53 73 45 / 0 20 20 10 ALW 81 56 78 49 / 0 20 20 20 PSC 82 58 81 50 / 0 20 20 0 YKM 80 50 76 40 / 0 0 10 0 HRI 82 56 78 48 / 0 20 20 0 ELN 76 52 70 42 / 0 10 10 0 RDM 78 46 69 40 / 0 10 10 0 LGD 78 50 73 45 / 0 10 50 50 GCD 81 49 77 43 / 10 10 60 40 DLS 79 55 70 48 / 0 0 10 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ MORNING UPDATE...86 SHORT TERM...77 LONG TERM....83 AVIATION...81