textproduct: Pendleton

DISCUSSION

Key Messages:

1. Mountain snowfall persists across the Cascades. *Active Winter Weather Advisories*

2. Elevated river levels linger. *Hydrologic Statement Issued*

3. Widespread precipitation midweek, basin snow possible.

Current radar and infrared satellite imagery shows light returns extending across the Oregon Cascades and through Central Oregon under cloudy to mostly cloudy skies. This is in response to an approaching upper level shortwave and attendant cold front that will be slowly pushing onshore this morning before breaking up into the evening. This feature will keep mountain snow showers occurring across the Cascades through the day as an additional 4 to 8 inches of snow across the Oregon Cascades and 5 to 10 inches of snow across the Washington Cascades is expected. As a result, Winter Weather Advisories have been issued across both the Washington and Oregon Cascades through 4 AM Monday morning. Confidence in these snow amounts is high (70%) as the NBM suggests a 75-90% chance of 4 inches, 60-80% of 6 inches, and 35-60% chance of 8 inches of pass-level snow over the next 24 hours. Snow rates will be increasing through the afternoon hour before tapering off overnight into the early morning hours on Monday. A second shortwave is expected to spin off the upper level low pressure located off the British Columbia coast Monday, providing another 2-5 inches over the Washington Cascades during the afternoon and evening. Confidence in Monday's snow amounts is high (80%) as the NBM advertises an 80-90% chance of 3 inches and a 40-60% chance of 5 inches of pass-level snowfall.

The break in widespread precipitation and lower snow levels have allowed rivers time to recede, but some river reaches are still in action/bankfull stage. These include the Naches River at Cliffdell and Naches, and the Yakima River at Kiona. The latter reach is expected to drop below action stage this evening. However, the Naches River at both Cliffdell and Naches are forecast to stay in action stage until late in the week as levels slowly drop. Even with periods of widespread precipitation Tuesday through Thursday, ensembles highlight precipitable water (PW) amounts of 0.2-0.7" and 80-110% of normal. Thus, rain amounts this week will not contribute to area river rises and will allow rivers to continue to recede. Widespread precipitation is expected Tuesday, with lower elevations of the Columbia Basin, Yakima Valley, and the Blue Mountain foothills picking up 0.10-0.20", with 0.10" or less likely over Central Oregon. Confidence in these rain amounts is moderate to high (50-70%) as ensembles show an overall spread of 0.01-0.05" of 24 hour rainfall across the Columbia Basin.

Ensembles are still hinting at a low chance (10-30%) of measurable snowfall (0.01" or greater) across lower elevations of Central Oregon, Yakima Valley, Columbia Basin, and the Blue Mountain foothills during the morning hours Wednesday onward, with Wednesday morning being the best chance. Guidance still struggles with the overall strength and location of the upper level low pressure that will be dropping south from the British Columbia coast and along the east Pacific through the week. This relates to the amount of moisture available for the system to send into the Pacific Northwest and how efficient cold air advection will be via the initiation of northwest flow aloft. Currently, 26% of ensemble members suggest a weaker and closer upper low feature which would provide an 80-90% chance across Central Oregon and the Yakima Valley and a 50-60% chance over the northern Blue Mountains and the Tri-Cities area of measurable snowfall overnight Tuesday night into Wednesday morning. Chances currently look much lower Wednesday night into Thursday morning with only 30% of ensemble members providing a 25-40% chance of measurable snowfall over the northern Blue Mountain and Lower Columbia Basin, which slightly improves to 30% of ensembles providing a 40-60% chance of measurable snowfall Thursday night into Friday morning. Even if light snowfall occurs, minimal impacts are likely due to afternoon highs reaching into the mid-40s.

Confidence in midweek rain amounts are also low to moderate (30-60%) as discrepancies between system location and strength directly relate to the amount of available moisture. Ensembles showcase this as the spread between the 25th-75th percentile in 24 hour precipitation Wednesday is between 0.20-0.30" across the Columbia Basin. This improves slightly to around 0.10" Thursday and Friday, but current amounts are forecast to be minimal (0.04" or less) both days. Midweek high temperatures change very little across the region, but overnight lows are expected to slowly decrease through the week. 75

AVIATION...12Z TAFS

Latest radar showed precipitation across southwestern Oregon moving northeast. This precipitation will begin to overspread BDN/RDM shortly and then work northeastward throughout the day.

VFR conditions are expected to deteriorate to MVFR and IFR as the precipitation moves in. BDN and RDM could see SN or RASN at the start before changing to RA. All other sites will see RA as the precipitation will move in later and temperatures will be warmer.

There is a bit of uncertainty as to the timing of precipitation moving in at all sites, but latest forecasts should be within a couple of hours.

There will likely be a bit of a break in the early evening before more RA moves in overnight.

Winds will generally be 10 kts or less.

PRELIMINARY POINT TEMPS/POPS

PDT 43 33 50 33 / 80 70 10 0 ALW 44 36 50 37 / 80 80 20 10 PSC 43 33 49 32 / 40 60 0 0 YKM 39 26 42 27 / 20 40 0 30 HRI 42 33 49 32 / 70 70 0 0 ELN 35 23 37 24 / 20 60 10 50 RDM 44 27 48 27 / 90 70 10 10 LGD 40 31 43 30 / 100 80 30 10 GCD 43 30 45 30 / 100 80 20 10 DLS 43 35 45 34 / 40 80 30 70

PDT WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory until 4 AM PST Monday for ORZ509.

WA...Winter Weather Advisory until 4 AM PST Monday for WAZ522.