Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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138 FXUS66 KPDT 170946 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 246 AM PDT Tue Sep 17 2024 .SHORT TERM...Today through Thursday...Water vapor satellite imagery tonight shows an upper low swinging into southern ID at this time, with radar showing light showers developing along the northwest side of the low over Grant, Union, and far southern Umatilla county. Meanwhile, another upper low is diving south along the BC coastline, and is approximately 400 miles off the coast of Vancouver Island. Today will the tale of two lows, as two upper level low pressure systems bring precipitation and wind impacts to the forecast area. First, the upper low that is currently entering southern ID will continue to swing northeast throughout today and exit into the northern Rockies late this evening. This low will be the main driver of shower activity across far eastern OR, with rain shower chances (25-45%) mainly over the Strawberrys, Blues, and across Wallowa county. The upper low currently west of Vancouver Island will continue to dive south and glance the PacNW coastline by this afternoon, after which it will continue to push south of the OR/CA border. This low will briefly provide shower chances across the Cascade crest and down into portions of central OR late this afternoon and early evening, when it is at it`s closest approach to the forecast area. The rest of the area will see little to no precipitation throughout today. Secondly, surface pressure gradients across the Cascades and east will tighten in response to the second upper low approaching the region. This will result in west to northwest winds of 20-30mph and gusts around 40mph developing through the Cascade gaps and into portions of the OR Columbia Basin this morning through this evening. Elsewhere, winds will be westerly (southwest along the Blue mountain foothills) 15-25mph with gusts 30-35mph. Wednesday through Thursday, the PacNW will see a northwest flow develop aloft as upper level ridging amplifies over the northeast Pacific. The flow aloft will be fairly dry, and with the lack of the shortwaves/troughs clipping the region, precipitation chances will be less than 10% area-wide. The cross-Cascade surface pressure gradient will only weaken slightly during this period. While light winds will be anticipated across much of the forecast area each day, locally breezy afternoon winds (around 15mph with occasional gusts to 25mph) will develop in the Columbia Basin, while breezy afternoon winds (around 20-25mph with gusts to 35mph and locally 40mph) will develop through the Eastern Columbia River Gorge and the Kittitas valley. Lawhorn/82 .LONG TERM...Friday through Tuesday...Models are coming into better agreement on the synoptic pattern in the long term, depicting continuous NW flow over the PacNW, before differences arise at the start of next work week, lowering forecast confidence significantly by the tail end of the period. Until then, expect mild temps and persistent chances for showers, especially in the mountains. The pattern being depicted as is, it should be noted, would yield only light precip outside of our high mountain zones. This continuous NW flow pattern looks to be the result of an upper- level low cutting off and diving down toward the southwestern CONUS, which will then be slow to move out over the weekend. Looking at ensemble clusters, models still differ a bit as to the extent and magnitude of the NW flow, with some members even suggesting weak zonal flow, but either way, such a pattern supports slight chance to chance PoPs across our high mountain zones. Confidence is moderate to high (50-60%) in our mountains seeing wetting rains over the weekend, but much lower (10-20%) on the Basin and central Oregon seeing any meaningful rainfall at the same time. Ensembles fall out of phase Monday onward, with solutions varying tremendously across guidance. Everything from the onset of ridging to another NW trough moving in is being depicted across the GEFS and EPS, therefore forecast confidence is very low (<20%) after the upcoming weekend. Further hindering confidence is the deterministic GFS and ECMWF falling completely out of phase starting Monday, with ridging and troughing being depicted around the same time, respectively. Evans/74 && .AVIATION...12z TAFs...VFR conditions expected. Windy conditions are expected to impact most sites, with W to NW wind gusts potentially reaching up to and over 30 kts at times in the afternoon, particularly at DLS and PDT. Wind gusts closer to 20 kts expected at all other sites. Sites outside of PDT, ALW, and PSC generally expected to see clear skies to light mid-level clouds, while bkn-ovc cigs around 10 kft are expected elsewhere, with similar cigs building at sites along the Cascades closer to the evening. Light rain may impact the central Oregon sites at the tail-end of the period as well, but impacts at this time are expected to be minimal. Evans/74 && .PRELIMINARY POINT TEMPS/POPS... PDT 71 48 73 45 / 20 10 0 0 ALW 74 52 76 50 / 30 10 0 0 PSC 77 56 79 53 / 10 10 0 0 YKM 79 49 79 46 / 0 10 0 0 HRI 75 53 78 50 / 10 10 0 0 ELN 74 51 76 48 / 0 10 0 0 RDM 67 43 70 40 / 10 30 0 0 LGD 67 45 71 44 / 40 10 0 0 GCD 67 43 71 42 / 40 20 0 0 DLS 72 55 77 51 / 10 30 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...82 LONG TERM....74 AVIATION...74