textproduct: Pendleton

KEY MESSAGES

- Light rain and high mountain snow across central and eastern portions of Oregon through Monday morning.

- Widespread precipitation chances return Tuesday into Wednesday, with persistent light to moderate snow in the mountains through the end of the week.

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

DISCUSSION

Water vapor satellite imagery this afternoon shows a well defined upper level low approaching the northern CA coast, while a plume of moisture is directed into southern OR. Radar and surface observations show light rain showers have made it into central OR, with showers developing across southern OR moving northward.

Tonight through Monday Morning: Shower chances will continue to increase across portions of central OR, John Day Basin, and the Ochoco-John Day Highlands through through this afternoon as the upper low offshore pushes closer to northern CA. By late Monday morning, precipitation chances will taper off across the affected areas as the low transitions into a trough over the CA/NV border.

Snow levels will remain above 6kft to 7kft during a majority of the lows transit through the region, owing to the low remaining to the south of as well as directing a mild southerly flow into the forecast area. This will result in very low snow totals across mountain areas (Or Cascade peaks, Mt Bachelor, Paulina peak, Strawberry Range), with snow amounts by Monday morning totaling at best 1 to 3 inches (confidence 65-80%). Otherwise, rain showers across the remainder of the southern portions of the forecast area will result in rain amounts of around 0.15 to 0.3 inches through Monday morning.

Monday through Tuesday Morning: A transient upper ridge will move over the PacNW during this period, 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 Monday and Tuesday afternoons. Otherwise, sky cover will begin to increase late Monday night into Tuesday morning from the west, as the next upper level trough approaches the PacNW. This may limit viewing potential for the lunar eclipse Tuesday 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 30-60%).

Tuesday afternoon through Thursday morning: There is great agreement amongst ensemble and deterministic guidance that an upper trough passage Tuesday evening through Wednesday night will bring widespread precipitation chances to the PacNW. Snow levels will start off 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 4 inches by the end of Wednesday morning(confidence 60-75%). Through the rest of Wednesday, snow levels will continue to lower to 3kft to 4kft, resulting in more widespread snow in the mountains, though snow rates will become very light as the trough axis passes east of the forecast area. Through Thursday morning, there is only a 35-45% chance that White Pass will see 6 inches of snow, while Santiam Pass will see a 75-85% chance. 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, 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 50-80% 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 60-70%), with strongest winds along the Cascade peaks/ridges and ridges in the Columbia Basin.

Thursday through Saturday: There remains good agreement amongst ensemble guidance that an active northwest flow aloft will result in persistent light snow and rain/snow chances along the Cascade crest/east slopes, Northern Blue mountains, and Eagle Caps through Friday afternoon. The flow aloft will become more zonal late Friday through Saturday (confidence 55-65%), resulting in snow levels rising above pass level again by Saturday (confidence 50-60%). This will result in snow or a rain/snow mix transitioning to mostly light rain across the Cascade crest passes, with a rain/snow mix in the interior portions of the northern Blues (confidence 50-65%). Otherwise, locally breezy west winds (25-40mph) will redevelop in the daylight hours each day through Saturday (confidence 40-65%). Lawhorn/82

AVIATION /00Z TAFS THROUGH 00Z TUESDAY/

Mostly VFR conditions to prevail through the period. An upper low pressure system will produce light rain showers at sites RDM/BDN through tomorrow morning. CIGs/vsby are expected to remain at VFR conditions at sites RDM/BDN through this evening, but then CIGs are expected to lower to between 1kft to 3kft, with vsby lowering to 3SM to 5SM. Winds will be light, 12kts or less, through the period. Lawhorn/82

Preliminary Point Temps/PoPs

PDT 36 56 36 60 / 20 10 0 0 ALW 36 55 39 60 / 10 10 0 0 PSC 33 60 37 62 / 0 0 0 0 YKM 31 59 34 57 / 0 0 0 0 HRI 35 59 37 62 / 10 0 0 0 ELN 30 55 34 51 / 0 0 0 10 RDM 35 54 30 56 / 70 30 0 0 LGD 40 57 33 59 / 40 30 0 0 GCD 41 52 34 57 / 80 70 0 0 DLS 38 61 41 56 / 10 0 0 30

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...None.