textproduct: Pendleton

KEY MESSAGES

- Widespread precipitation chances return Tuesday into Wednesday, with persistent mountain precip chances continuing into the weekend.

- Winter Weather Advisory in effect above 4500ft for the OR Cascade Crest Wednesday.

- Potential for breezy to gusty winds (gusts 35-50 mph) developing Wednesday with cold front passage.

DISCUSSION

Through Tuesday Afternoon: A transient upper ridge is currently moving onshore the PacNW today, resulting in a return of area- wide dry conditions. Some warm air advection with the upper ridge will bring temperatures closer to the lower to mid 60s across much of the Columbia Basin in the afternoons Today and tomorrow. Otherwise, sky cover will begin to increase late tonight into tomorrow morning from the west, as the next upper level trough approaches the PacNW. This may limit viewing potential for the lunar eclipse early tomorrow morning, with cloud cover expected to cover the western third of the forecast area by the time of the eclipse totality (3am-4am PST). The only saving grace with the incoming cloud cover is that it may start off as a thinner or scattered cirrus deck (confidence 50-70%).

By tomorrow afternoon, a weak frontal boundary will reach the Cascade crest, with light rain/snow showers developing into the evening. As precipitation starts, snow levels will be above 5kft to 5.5kft, resulting in mostly rain at pass level, with snow or a rain/snow mix above these elevations.

Tuesday night through Thursday morning: There is great agreement amongst ensemble and deterministic guidance that an upper trough passage Tuesday night through Wednesday night will bring widespread precipitation chances to the PacNW. Snow levels will start off just above 5kft with the approach of the trough, but drop to 4kft to 5kft as the trough moves inland Tuesday night into Wednesday morning. The heaviest of the precipitation will align with snow levels just falling to or below the surface along the Cascade crest Tuesday night, resulting in wet snow accumulations of 2 to 5 inches by Wednesday afternoon(confidence 70-85%). Through the rest of Wednesday, snow levels will continue to lower to 3kft to 4kft, resulting in more widespread snow in the mountains. Scattered snow showers will develop as the trough axis moves overhead and a cold front boundary to the east. As the trough axis pushes further east, snow rates will become very light across the forecast area. Through Thursday morning, Santiam Pass will see a 65-85% chance for snow accumulations greater than 6 inches. As a result, a winter weather advisory has been issued for the Oregon Cascade East Slopes above 4,500ft for Wednesday. At Snoqualmie pass, snow levels will generally be above pass level through Wednesday, resulting in only a 35-50% chance of an inch of snow accumulating through Thursday morning. In the interior northern Blues and in White Pass, there is a 55-85% chance of 2 inches of snow through Thursday morning; there is a 65-85% chance of 4 inches of snow through Thursday morning along the Elkhorn crest.

Below the mountains, most lower elevation locations will see a 45-80% chance of rain accumulations greater than 0.05 inches develop as the trough axis and an attendant cold front sweep through the area. Heaviest rain amounts will be along the northern Blue mountain foothills, where there is a 40-80% chance of accumulations greater than 0.2 inches through Thursday morning. Otherwise, ensemble guidance is depicting breezy winds (gusts 35-50mph) developing with the aforementioned cold front passage Wednesday (confidence 65-75%), with strongest winds along the Cascade peaks/ridges and ridges in the Columbia Basin.

Thursday through the weekend: There is good agreement in the latest ensemble cluster guidance that northwest flow aloft will develop initially, leaving light rain/snow chances along the Cascade crest and Northern Blue mountains Thursday. Weak northerly flow will then develop over the PacNW Thursday night into Friday as a ridge amplifies offshore and a trough digs equatorward in the inland western CONUS. Weak mountain showers along the Cascade crest will continue under this flow regime throughout Thursday, with dry conditions elsewhere. Ensembles remain in good agreement Friday through the weekend that northerly flow will shift to the northwest as the ridge flattens offshore, resulting in light showers across the Cascades and Northern Blues (confidence 55-75%). Snow levels will also rise Friday night into Saturday, limiting snow to mountain peaks, with rain or rain/snow mix mainly below 6kft (confidence 45-65%). Otherwise, locally breezy west winds (25-40mph gusts) will redevelop in the daylight hours each day through Sunday (confidence 50-65%). Lawhorn/82

AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/

VFR conditions will prevail through the forecast period. CIGs are mostly 25kft with the exception of some lower CIGs in BDN/RDM of 3-5kft.

Winds are light and variable less than 6kts across all TAF sites.

Preliminary Point Temps/PoPs

PDT 35 60 43 53 / 0 0 40 90 ALW 40 61 45 53 / 0 0 60 90 PSC 35 63 44 59 / 0 0 20 60 YKM 34 58 40 56 / 0 10 30 50 HRI 35 62 43 57 / 0 0 30 80 ELN 33 52 37 50 / 0 10 40 60 RDM 30 58 37 47 / 0 0 10 90 LGD 33 59 41 49 / 0 0 50 100 GCD 32 58 39 47 / 10 0 20 100 DLS 40 57 45 55 / 0 20 70 90

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...Winter Weather Advisory from 4 AM to 10 PM PST Wednesday for ORZ509.