290 FXUS66 KPDT 110447 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 847 PM PST Sun Jan 10 2021 .UPDATE... Overall, it is a quiet night. Areas of fog have developed once again, especially along the Oregon Blue Mountain Foothills, which has prompted the issuance of a dense fog advisory. Otherwise, just made some adjustments to temps where needed. Winds are light. && .AVIATION... 06Z TAFS...Once again, conditions are variable over the region. RDM and BDN are VFR and should stay that way. DLS and PSC are VFR but is expected to decrease to MVFR overnight. IFR is not out of the realm of possibility in the morning. YKM has been MVFR wand will likely stay that way for most if not all the period. ALW is currently VFR. It is expected to decrease to MVFR overnight and could go as low as IFR or even LIFR by morning before improving to MVFR. PDT has been LIFR with both very low CIGS and VSBYS. Once again, it looks like they may stay that way for a good portion of the period. There may be some improvement during the late afternoon, early evening before a return to low conditions once again. Winds will be light through the period, mainly 5 kts or less. && .PREV DISCUSSION... /issued 306 PM PST Sun Jan 10 2021/ SHORT TERM...Tonight through Tuesday...Upper level ridge over the region has kept conditions fairly dry this afternoon, while low stratus has been clearing and allowing some sunshine across the Columbia Basin. Stratus will continue to clear through the afternoon, but will redevelop along with patchy fog and freezing fog across the Basin starting this evening, and persisting into tomorrow morning. Starting Monday, a series of weather systems will move through the region while a deep moisture plume currently visible over the Pacific will enhance precipitation amounts, mainly over the Cascades. Early Monday, a weak cold front will swing across the forecast area with increasing mid to high level clouds and mainly light snow in the WA Cascades. Surface gradients will start to tighten with this frontal passage, leading to breezy conditions along the Blue Mountain foothills and in the southern Grande Ronde valley. West southwest flow aloft behind this weak system will help to advect a warmer, moist airmass over the region Monday while raising snow levels to 5-7k feet by Monday night for areas west of the Blue mountains. Late Monday, a surface low and upper level shortwave swinging around a parent low in the Gulf of Alaska of will approach the forecast area. Arrival of the moisture plume in the PNW will coincide with this system, and westerly upslope flow will aid in heavier precipitation amounts across the Cascades. While snow levels will be 5-7k feet Monday night into Tuesday, expect snowfall to be confined to the crests of the Cascades, while moderate to heavy rainfall will be possible below the crests of the Cascade east slopes. Frontal boundary associated with surface low will move across the forecast area early Tuesday with light to moderate rainfall expected. Westerly upslope flow will keep mostly rain showers going in the mountains through Tuesday while the lower elevations will see a brief break in the precipitation. Late Tuesday, another upper level shortwave and surface low pressure system will approach the forecast area with another round of moderate to heavy rainfall below the Cascade crests, and chance of rain everywhere else. Overall, models are indicating that portions of the Cascade east slopes will receive anywhere from 2-4 inches of liquid precip while portions of the Blue mountains can expect 1-2 inches of liquid precip. This amount of rainfall and possible snow melt will likely result in rising rivers and streams Tuesday through Thursday. Breezy to windy conditions will develop across the lower elevations with each passing system Monday and Tuesday, with peak winds expected both Tuesday morning and Tuesday night in the lower elevations. Lawhorn/82 LONG TERM...Wednesday through Sunday...Atmospheric river event will subside by the end of Wednesday, models starting to converge a bit more as far as the extent and quantity of precip produced by this event. Both the GEFS and Euro ensembles are producing more aggressive values for IVT transport, with the ECMWF QPF ensembles responding accordingly with more precip in the Cascades, with several inches possible, compared to previous days' runs. Now the Euro is wetter than the GEFS as far as rain in the Basin and central OR, with members producing several tenths of an inch while GEFS shows most areas barely receiving a tenth of an inch. Canadian ensembles appear to be approaching the Euro solutions as well. Wintry precip not expected to be an issue, with NBM snow levels forecast to be above 4500 ft for most areas, but the amount of rain falling on snow in the mountains could create hydro concerns, with rivers and streams along the eastern slopes needing to be monitored during the duration of this event. Beyond Wednesday, high pressure ridge takes over and leaves the area dry on Thursday. Both deterministic GFS and Euro then flatten the ridge a bit and usher in SW flow over the PacNW, leaving a slight chance for precip mainly in the Cascades on Friday. Next best chance for area-wide precip looks to be Saturday as an embedded shortwave within NW flow passes through the area. QPF ensembles are a bit divided on this feature, however, with the Euro much wetter than the GEFS. Snow levels will fluctuate a bit after the AR event as the flow aloft shifts, ranging from 3000-4000 ft in the Cascades during Friday's precip, to 2000-3000 ft on Saturday as upper-level winds shift more northerly. High temps will start at around the upper 50s in the lower elevation midweek, leveling off to the mid to upper 40s by next weekend. AVIATION...00z TAFs...Conditions have improved to VFR for most sites, however fog and low cigs expected to redevelop this evening as high pressure continues to dominate the area. Expecting most severe fog to develop in PDT/ALW/PSC, with YKM/DLS seeing low stratus, however brief periods of LIFR fog possible for these sites. BND/RDM expected to be VFR through the period with sct-bkn cigs of 10 kft and higher. Winds will be light and variable, with cigs seeing improvement Monday afternoon. && .PRELIMINARY POINT TEMPS/POPS... PDT 31 46 36 51 / 0 10 30 60 ALW 33 47 38 50 / 0 10 30 70 PSC 33 45 37 51 / 0 10 40 50 YKM 34 42 34 47 / 10 30 60 80 HRI 31 44 37 52 / 0 10 30 50 ELN 35 40 33 43 / 10 30 70 80 RDM 33 47 38 52 / 0 10 30 60 LGD 30 37 29 39 / 0 10 20 70 GCD 29 43 32 45 / 0 0 20 60 DLS 37 44 37 50 / 0 40 70 80 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Dense Fog Advisory until 10 AM PST Monday for ORZ507. WA...None. && $$ SHORT TERM...77 LONG TERM....74 AVIATION...77