FXUS66 KPDT 040530 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 930 PM PST Tue Dec 3 2024 Updated aviation discussion .Update...Forecast was updated to include the John Day Valley in the Freezing Fog Advisory. .SHORT TERM...Today through Thursday...The Freezing Fog Advisory remains in effect through 11 AM PST tomorrow for portions of the Foothills of the Southern Blues, north central and central OR due to visibility falling to one mile or less from the freezing fog. Based on current radar and satellite imagery, the low stratus layer remains over the Columbia Basin, valley zones extending down to central OR. Freezing fog will continue over north central and central OR with patchy fog at portions of the Foothills of the Southern Blues. Visibilities may still drop to one mile or less for the aforementioned areas. Thursday morning, an approaching shortwave trough will bring showers at the WA/OR Cascades and continue lingering over mainly WA Cascades Thursday night (<25% chance). And with the upper ridge returning, patchy fog will then redevelop into Friday morning. High temperatures will range between mid 30s and 40s at the Columbia Basin and 40s and 50s across the Blues. The low stratus layer will keep the temps cooler for the Columbia Basin, but not much for the Blues with cloud coverage decreasing thus allowing more radiation to warm the temps. Feaster/97 .LONG TERM...Friday through Tuesday...Models are still trying to refine some details in the long term forecast, but the overall picture looks to be that of widespread rain and high mountain snow over the weekend, followed by a colder pattern moving in for next week. Ensembles diverge a bit Monday and beyond, but confidence in the forecast this weekend is starting to become relatively high (60- 70%). On Friday, we get some transitory ridging overhead after the passage of a weak shortwave. This ridge may be a bit dirty in nature, as models depict PoPs in the 30-40% range across the central WA Cascades as this ridge picks up on some SW moisture. Not expecting any meaningful moisture on Friday, however, as a more organized system in the form of a broad low pressure system sweeps through the forecast area over the weekend, with its associated moisture occurring primarily between late Saturday morning and early Sunday morning. This system is broad enough in scope to produce rain across much of the forecast area, with snow occurring mainly in elevations above 4000-5000 feet. Snow levels will drop steadily over the weekend, decreasing down around the 3000-4000 feet range by Sunday afternoon, but by then, cold, drier air will start to filter in, and only light orographic precip looks to occur after the passage of the low. One complicating factor in the forecast is the ongoing stratus deck across the lower elevations. Am wondering if models are being a bit overzealous with snow levels Friday into the weekend across the lower elevations, as Thursday's system has trended weaker and weaker, so am not expecting widespread mixing out of the stratus deck until the weekend, when winds pick up with the passage of the low. How much this will impact precip type over the weekend is uncertain, but will hold off on messaging these complications too much until higher-res guidance comes in. Still, something to keep in mind in the coming days. Guidance seems to trend toward that of dry, northerly flow Monday onward next week, which may bring about a cold air outbreak, however ensemble clustering does show model discrepancies that come in the form of where the flow aloft is oriented after the weekend low passes through, and whether or not it is more due north, northwesterly, or if ridging moves in over the PacNW. Either way, the biggest precip concerns look to occur over the weekend, but with snow levels expected to climb across the mountains, expectation is that the mountain passes will see only light snowfall at best over the period. Evans/74 && .AVIATION...06z TAFs...Low stratus and fog continue to plague all TAF sites, with little change in the forecast expected over the next 24 hours with MVFR conditions mainly due to low ceilings. The only sites expected to see variable conditions are BDN and RDM. Both sites are expected to see LIFR conditions through Wednesday morning then some improvement during the day. && .PRELIMINARY POINT TEMPS/POPS... PDT 28 32 27 37 / 0 0 0 0 ALW 30 33 29 38 / 0 0 0 0 PSC 33 35 32 35 / 0 0 0 0 YKM 28 32 26 36 / 0 0 0 0 HRI 33 37 30 36 / 0 0 0 0 ELN 25 32 25 38 / 0 0 0 0 RDM 16 40 19 43 / 0 0 0 0 LGD 27 44 25 47 / 0 0 0 0 GCD 29 47 26 50 / 0 0 0 0 DLS 33 35 31 42 / 0 0 10 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Freezing Fog Advisory until 11 AM PST Wednesday for ORZ508-510- 511. WA...None. && $$ SHORT TERM...97 LONG TERM....74 AVIATION...74