156 FXUS66 KPDT 181725 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 1025 AM PDT Fri Sep 18 2020 .UPDATE...Timing for arrival of low pressure system has been delayed over last few forecast cycles, likely due to increased strength of upstream ridging over Rocky Mtn crest slowing trough down. As system is finally showing some signs of forward progress, don't see need to delay precipitation onset more. Storms today remain a tricky subject as modeled sfc instability in CAMs remains questionable with unparameterized smoke in the region. At face value, CAMs would suggest several strong, likely sub-severe storms through Cascade region and eastern mtns of Oregon, but this guidance seems a little suspect given issues with smoke, radiation budgeting, etc over past several days. Taking this into account, and looking at the likely triggers for afternoon storms just on the coast, still expecting a few storms this afternoon with primary risk being possible brief gusty winds and modest lightning activity. Storms occurring now to over next several hours over Cascade region also have the potential to drop a quick quarter inch or more of rain, leading to slight concern for flooding or debris flows over burn scars in short term, but this risk is greater on the west slopes where burned area is greater. Timing for storms will likely be in afternoon period with ventral oregon and east Cascades to southern Blue Mtn region seemingly holding best storm chances. Surface front pushing through Gorge region this evening to bring some breeziness to the Basin. Rain showers continue through the evening for northern Blue Mtns and commence in far NE Oregon. QPF continues to range from near nothing in lower elevations of mid- Columbia to a tenth inch for large swath of mid to higher elevations to quarter inch along Cascade Crest and in higher portions of Wallowa region (by end of Saturday). && .PREV DISCUSSION... /issued 500 AM PDT Fri Sep 18 2020/ Updated Aviation Discussion... SHORT TERM...Today through Sunday...An upper level low pressure system is currently off the southwest coast of Oregon at this time. It is generating some showers and thunderstorms west of the Cascades. This low will continue to push east today, passing over the forecast area overnight and exiting to the east on Saturday. We will see an increase of showers along the Cascades this morning and then these will push east across the forecast area today and overnight. There will be a slight chance of thunderstorms associated with the passing of this upper level low as it transitions across the forecast area. We will also see some increasing westerly winds today and through the evening before decreasing overnight. The combination of these winds and better atmosphere mixing due to the unstable conditions will begin to disperse the smoke across the forecast area and improve air quality. However, would not be surprised if not all smoke is dispersed or that smoke generation from fires continue to impact some portions of the forecast area, especially central Oregon. The low will exit to the east by Saturday leaving us under a westerly flow with just some lingering showers along the crest of the Cascades and over the eastern mountains. After that, the westerly flow will continue but be dry with no precipitation on Sunday. LONG TERM...Monday through Friday...Quieter weather will settle in behind the upper-level low by the end of Sunday. Flow aloft becomes more zonal for the work week, with temperatures slowly warming throughout the area: mid to upper 70's starting Monday rising to upper 70's to low 80's by Thursday. Only highlight early in the week is a weak shortwave passing over the Pacific NW during the day Monday into Tuesday, bringing a slight chance of showers along the central Washington Cascade crests. Global guidance differs on producing precip with this feature, however, with ECMWF ensemble producing only a few rain-making members, and GEFS dry all around. Scaled back some marginal PoPs up around NW Kittitas compared to the NBM as a result. Rest of the CWA expected to be dry through Wednesday morning, with some locally breezy conditions within the Basin and Columbia Gorge Tuesday into Wednesday as the shortwave passes through. By Wednesday into Thursday, a broad trough over the Gulf of Alaska is expected to pass over the Pacific NW, producing another chance of showers over our area. Global guidance beginning to converge on arrival time after some disagreement over the last few days, so looking at a Wednesday evening / early Thursday morning arrival time for the Cascades , but still need to wait for further guidance before narrowing the time of arrival any further. GFS brings in trough several hours earlier than the ECMWF, so will stick with NBM guidance of a Wednesday evening arrival for now. Cooler temps back down into the mid to lower 70's will prevail behind this trough as winds aloft shift back more northwesterly. AVIATION...12z TAFs...Smoke will linger into the morning, limiting visibilities to 1-3 SM and keeping conditions at IFR. Improving conditions later in the day as a low moving into the area increases winds and brings a slight chance of showers and isolated storms, especially along the eastern Cascades and the Blue mountains. Higher winds and rain should work to clear some of the smoke in our area by the afternoon into the evening. Expect gusts up to and over 20 kts through the early evening. && .PRELIMINARY POINT TEMPS/POPS... PDT 77 51 73 47 / 10 30 10 0 ALW 79 54 75 52 / 10 40 10 10 PSC 81 56 78 52 / 10 20 10 0 YKM 79 51 77 46 / 30 20 10 0 HRI 80 53 78 49 / 10 20 0 0 ELN 75 52 72 48 / 30 20 10 10 RDM 70 42 70 36 / 70 30 10 0 LGD 79 49 70 43 / 20 60 30 10 GCD 77 46 70 43 / 20 60 20 0 DLS 74 57 76 52 / 50 20 10 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...76 LONG TERM....82 AVIATION...82