Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
898 FXUS66 KPDT 151829 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 1029 AM PST Mon Dec 15 2025 .AVIATION...18Z TAFs...Predominantly VFR conditions are anticipated for the next 24 hours. Breezy winds of 10-20 kts with gusts of 25-35 kts are forecast in the wake of this morning`s weather system at most sites. Aside from some lingering showers, mostly dry conditions are forecast. Did include mention of low- level wind shear at YKM/PSC today due to strengthening winds aloft (40-55 kts at 2kft AGL); will also note that if forecast winds fail to materialize at the surface at PDT/RDM/BDN/ALW, these sites will also see borderline conditions for a mention of LLWS today. && .PREV DISCUSSION... /issued 346 AM PST Mon Dec 15 2025/ SYNOPSIS... Another wet and windy week will take shape as yet another atmospheric river threatens the Pacific Northwest. Rivers and streams along the Washington Cascades will recede through the first half of the work week, before many are expected to rise back into flood stage by Wednesday into Thursday. Colder temperatures will allow snow to fall in our mountain zones during the latter half of the week as well. In addition, this atmospheric river will also invite gusty winds for much of the week, but especially late Tuesday into Wednesday. DISCUSSION... Our weekend respite will come to an end as high pressure shifts eastward in favor of yet another atmospheric river stemming from the central Pacific. The fronts embedded within this pattern will lead to gusty conditions today, but especially on Tuesday night into Wednesday as the first well-defined cold front of this ongoing wet and warm pattern materializes, leading to 1) strong wind gusts across the vast majority of the forecast area, and 2) a dose of mountain snow across primarily the Cascades. Focusing on the wind threat today, mid-level winds will ramp up as the atmospheric river initially noses its way into the PacNW. Models are admittedly divided - while they agree that winds will be howling across the ridgetops, they are a bit more reluctant to mix the winds down into the valley and Basin given how stable the boundary layer is due to this past weekend`s high pressure ridge. But with how strong these ridgetop winds are expected to be (upwards of 60 mph across the Simcoe Highlands), CAMs are hinting that winds may gust down into the Yakima and Kittitas valleys, as well as the lower Columbia Basin in Washington, primarily around the 2-6 pm timeframe today. HREF hourly probabilistic guidance inches up into the 50-70% range for wind gusts to exceed 45 mph then. Opted to issue a Wind Advisory for these areas as a result, but should note that errant wind gusts from downsloping are more of the bigger threat, rather than a prolonged period of sustained gusty winds. Also included the southern Blues of Oregon, as the winds will primarily be southwesterly, which generally favors strong downslope winds at the base of the southern Blues. The more defined cold front and attendant surface low then arrives late Tuesday, causing winds to really ramp up during the overnight hours heading into Wednesday. With 850-mb NAM winds reaching up to (and even over) 70 kts across the Basin, and this surface low bottoming out down to around 983 mb as it crosses British Columbia, the High Wind Watch currently in effect looks more than suitable. With this kind of synoptic set-up, this period could be quite messy as even NBM probabilistic guidance depicts high probabilities (60-80%) of wind gusts eclipsing 60 mph across the Basin. Would not be surprised if the vast majority of the forecast area ends up represented by some kind of wind product by the issuance of Tuesday`s forecast. This same system will also allow a colder air mass to intrude upon the PacNW, reversing this ongoing trend of abnormally warm temperatures we`ve (somewhat) enjoyed over the last week, had it not been for the wind and rain. The good news with this shift in temperature trends? Runoff from precipitation won`t be quite as pronounced as it was last week as snow levels across the WA Cascades collapse down toward 2000 ft by Wednesday afternoon. The bad news? There`s still plenty of moisture falling to allow rivers and streams (especially the Naches, Yakima, and Klickitat) to climb back towards flood stage after bottoming out today into Tuesday. River forecasts thus far are, again, not as severe as last week, but we`re certainly not out of the woods yet as far as hydro concerns go. Issued a Winter Storm Watch for the WA Cascade upper slopes for the snow expected to fall primarily on Wednesday. Should note that messaging is tricky, as the atmospheric river really locks into place by the latter half of the week, meaning precip essentially becomes nonstop Wednesday onward for the high mountains. The snow will really pile up as a result, and Snoqualmie Pass could easily end up with more than a foot of snow by the end of the week. Is there any light at the end of the tunnel to guide us out of this mess? Well, global model ensembles suggest a shift to a more SW flow pattern heading into Christmas week, which would perhaps steer us clear of a widespread wintry weather event as travel ramps up, but would ultimately shift our wet weather from the Cascades toward the eastern mountains. Time will tell as model solutions evolve, but unfortunately, this week looks to be a continuation of what`s already been a wet and wild December. 74 && .PRELIMINARY POINT TEMPS/POPS... PDT 65 42 61 40 / 70 40 10 90 ALW 63 45 60 42 / 90 60 30 90 PSC 64 42 60 43 / 60 20 10 90 YKM 63 37 56 38 / 70 20 50 90 HRI 64 42 61 42 / 50 30 10 90 ELN 55 34 48 32 / 80 40 70 100 RDM 64 37 58 33 / 50 30 10 90 LGD 51 40 54 37 / 80 60 20 90 GCD 58 42 56 36 / 70 40 10 90 DLS 60 45 58 42 / 90 50 60 100 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...High Wind Watch from Tuesday evening through Wednesday afternoon for ORZ041-044-507-508-510. Wind Advisory from 1 PM this afternoon to 10 PM PST this evening for ORZ508. WA...High Wind Watch from Tuesday evening through Wednesday afternoon for WAZ024-026>029-521. Wind Advisory from 1 PM this afternoon to 10 PM PST this evening for WAZ026>028. Flood Watch through Wednesday morning for WAZ522-523. Winter Storm Watch from Tuesday evening through Wednesday evening for WAZ522. && $$ SYNOPSIS...74 DISCUSSION...74 AVIATION...86