textproduct: Pendleton

KEY MESSAGES

- Walla Walla River flooding through Monday morning

- Warming trend through the week

- Rain in the WA Cascades will lead to increased snowmelt

DISCUSSION

Upper level ridging will begin to build over the PacNW tonight, bringing a prolonged period of warming temperatures, WA Cascade precip, and dry conditions elsewhere.

Tonight, an upper level ridge offshore will begin its transit towards western CONUS, where it will gradually push east through Thursday. Accompanying the upper ridge will be a weak to moderate atmospheric river, that will clip northern and central WA throughout the week. Ahead of the ridge, a surface warm front will lift across the PacNW, with the frontal boundary ushering in snow levels between 8kft to 10kft. Most locations across the forecast area will see the warm airmass mix to the surface with little trouble, however warm air will remain aloft across the sheltered areas of the WA Cascade crest and east slopes. As the AR impacts the WA Cascades, precipitation will start off as snow, but will transition into a wintry mix if freezing rain and snow late this evening as falling precip interacts with the warm air aloft. By Monday morning, the wintry mix/freezing rain is expected to transition to rain as warm air begins to mix down to the surface. A winter weather advisory is in effect for the Upper east slopes of the WA Cascades between 8PM tonight and 5AM Monday morning.

Snow levels will remain above 7-8kft through at least Thursday as the upper ridge continues to move across western CONUS (confidence 75-90%). This will mean that precipitation along the WA Cascades will mainly fall as rain through this period. Combined with impending warming temperatures under the upper ridge, the increased snowmelt will result in river levels rising across rivers and streams originating from the WA Cascades. At this time, none of the rivers/streams are forecast to reach flood stage, and will remain within action stage. Of note however, areas of the Yakima River up river from Cle Elum may see minor flooding impacts due to channel structure change from flooding in December.

As mentioned earlier, temperatures are expected to warm across the region as the upper level ridge moves overhead. By Tuesday, widespread afternoon temperatures in the lower to mid 70s are to develop across the lower elevations, warming further into the mid to upper 70s Thursday and Friday. There is a potential for the region to see afternoon temperatures warm even further into the 80s this week, with latest NBM guidance indicating a 60-80% chance of central OR and the John Day Basin reaching 80 degrees by Wednesday, and a 25-40% chance along the Blue Mountain Foothills and Columbia Basin on Thursday.

Friday into Saturday, the upper ridge will push into central CONUS as an upper level low deepens in the northeast Pacific. Ensemble cluster is in good agreement that weak shortwaves ejecting from the low will push across the PacNW, bringing a shot of cooler air and lowering snow levels across the Cascades and the northern Blues (confidence 55-70%). Each shortwave passage will bring light rain and snow to the Cascades and northern Blues Friday into Saturday, with dry conditions continuing in the lower elevations into next weekend. Lawhorn/82

AVIATION /06Z TAFS THROUGH 06Z TUESDAY/

VFR conditions are likely (80-95 percent confidence, varying by site) through the period.

Precipitation chances are low (20-30 percent) at PDT/ALW/PSC late morning into mid-afternoon Monday, and have included PROB30 groups for these sites. Elsewhere, confidence in any precipitation is too low (20 percent or less) to include mention in the 06Z TAFs.

CIGs have lowered to around 5-8 kft AGL at most sites this evening, and there is low confidence (20 percent or less) in MVFR or lower CIGs at all sites. By Monday afternoon, anticipating clearing skies with just FEW-SCT high cloud for the remainder of the period.

Sustained winds are forecast to remain mostly light (12 kts or less) through the period, but will mention medium confidence (40-60 percent) in gusts of 15-20 kts at RDM/BDN/PSC later Monday morning and afternoon. 86

Preliminary Point Temps/PoPs

PDT 40 66 45 70 / 30 20 0 10 ALW 42 63 49 66 / 40 40 10 20 PSC 40 66 48 71 / 20 20 0 10 YKM 33 63 42 68 / 40 20 0 10 HRI 40 66 46 71 / 20 20 0 10 ELN 31 55 38 60 / 60 30 10 30 RDM 37 71 38 75 / 10 10 0 0 LGD 37 61 42 68 / 70 40 10 10 GCD 37 67 41 71 / 40 10 0 0 DLS 40 68 45 66 / 30 10 0 20

PDT WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory until 5 AM PDT Monday for WAZ522. OR...None.