FXUS66 KPDT 151737 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 937 AM PST Thu Feb 15 2024 .UPDATED AVIATION DISCUSSION... && .PREV DISCUSSION... /issued 911 AM PST Thu Feb 15 2024/ SHORT TERM...Today through Saturday...Latest satellite imagery shows an area of low pressure centered right around the Oregon/Washington border near Astoria. Guidance has this low pushing eastward today as it is picked up by a NW-oriented trough situated over the PacNW. The forecast area will then be enveloped by precip through the early morning hours, before letting up a bit during the day Thursday. A band of snow will then develop late Thursday into early Friday morning as warmer air surges in ahead of a building ridge, leading to frontogenesis. Friday looks mostly dry before another shot of moisture from the SW brings a chance of light precip on Saturday. No significant changes to ongoing winter weather headlines as the bulk of the moisture expected with this ongoing low pressure system is expected to occur through the early morning hours today, before its absorption into the trough provides for a bit of a respite for the remainder of the day. Only real change was to extend the Winter Weather Advisory for central Oregon out until early morning Friday. While conditions are dry there for now, the snow band expected to develop overnight Thursday into Friday could dump another 1-3 inches of snow for the area, so went ahead and extended the ongoing product rather than create a new one altogether. Decent snowfall accumulations have already occurred across north central Oregon as a result of the initial lifting of the low, with the bulk of the precip for the foothills of the Cascades from Oregon through south- central Washington expected to occur as a result of the low's inland push. Some precip is expected to spill over into the Basin through the morning hours, however latest CAMs are very meek on QPF, and borderline temps with dewpoints in the low 30s suggest that even if snow does materialize, amounts will be light and of little impact. Warming during the daylight hours will then preclude any wintry precip for the rest of the day. As a result of the light precip received yesterday, along with light winds across the Basin, fog and low clouds have formed, with dense fog having been observed along the foothills of the Blues and into the mountain passes, where Dense Fog Advisories are in effect through noon. Some of this colder air may settle into the Basin over the next few days even as warmer air advects in aloft, so low clouds may persist heading into the weekend. The last of the precip associated with this system looks to taper off by daybreak Friday based on latest CAMs. Transitory ridging then builds in, before SW flow pumps in another round of moisture into the forecast area heading into Saturday. QPF forecast thus far, however, suggests very light precip with this next round. Evans/74 LONG TERM...Sunday through Thursday...Sensible weather concerns through the long term forecast will revolve around an upper low offshore producing periods of mountain snow and lower elevation light rain into the middle of next week. Ensemble guidance in good agreement that a shortwave trough will continue east across the forecast area as it ejects from the upper low offshore OR and undercuts an upper ridge centered just east of the forecast area Sunday. There is moderate-high confidence (60-75%) that the heaviest mixed precipitation and mountain snow will occur as the shortwave axis passes overhead in the morning Sunday. Behind the shortwave exit Sunday afternoon, precipitation chances will diminish area-wide, except for light snow flurries that will be possible along the Cascade crest and interior Blues/Wallowas. Overall, ensemble guidance is leaning towards light snow accumulations in the mountains, with 90th percentile snow amounts in the NBM only at 4 to 5 inches across the Cascade crest, and the interior northern Blues; lower elevation 90th percentile snow accumulations only top 1 inch. Monday into Tuesday, the ensemble cluster guidance shows the upper low offshore weakening and becoming an open trough by early Tuesday morning, then pushing onshore through Tuesday night. Ensemble members of the ECMWF, GFS, and Canadian suites do show uncertainty in the track of the shortwave as it pushes east. While nearly all the members making up the cluster solutions do show light rain and mountain snow chances across the forecast area, a majority of the ECMWF and Canadian solutions indicate a more northerly track resulting in better precipitation chances in the mountains but drying out still in the lower elevations by Tuesday afternoon; meanwhile, the GFS shows the shortwave trough further south resulting in precipitation chances diminishing by Tuesday afternoon area-wide. At this time, clusters are nearly split in either solution, resulting in low-mod (25-40%) confidence in extent of precipitation chances. Wednesday into Thursday, ensemble guidance are in good agreement that upper level ridging will build over the PacNW behind the shortwave trough exit to the east. Moderate confidence (40%) that light upslope mountain snow will continue along the Cascade crests, northern Blue, and Wallowas through Wednesday as the ridge places the forecast area under a west northwest flow aloft, but as the ridge axis moves into the intermountain PacNW, light precipitation will come mostly to an end. Lawhorn/82 AVIATION...18Z TAFs...TAFS will be a bit tricky this forecast period with the snow showers and fog across TAF sites PDT/ALW & PSC/DLS. The aforementioned four sites are fluxing between LIFR & MVRF/IFR respectively. BDN/RDM/YKM are currently VFR but RDM/BDN will drop to IFR/LIFR due to the next incoming system bringing some snow and fog across central OR while PSC will see a lift in CIGs and VIS this afternoon. VIS will flux across most sites between 1-4SM and CIGs will drop to below 2000ft for all sites except YKM/PSC. The next system expected later in the period will primarily affect all other sites except YKM/PSC. Bennese/90 && .PRELIMINARY POINT TEMPS/POPS... PDT 25 35 23 36 / 60 10 0 20 ALW 26 37 26 39 / 40 10 0 20 PSC 27 41 27 41 / 20 10 0 20 YKM 24 40 26 41 / 20 10 10 20 HRI 27 39 26 38 / 40 10 10 20 ELN 22 38 24 40 / 10 10 0 20 RDM 24 34 24 38 / 100 10 20 30 LGD 27 40 24 44 / 80 10 0 10 GCD 30 44 29 46 / 100 10 10 30 DLS 30 39 31 39 / 90 20 20 50 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Winter Storm Warning until 4 AM PST Friday for ORZ041-508>510. Dense Fog Advisory until noon PST today for ORZ502-507. Winter Weather Advisory until 4 AM PST Friday for ORZ502-511. WA...Winter Storm Warning until 4 AM PST Friday for WAZ024. Dense Fog Advisory until noon PST today for WAZ029. Winter Weather Advisory until 4 AM PST Friday for WAZ030-521. && $$ SHORT TERM...74 LONG TERM....82 AVIATION...90