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KEY MESSAGES

- Mountain snow overnight, becoming light through the week. *Winter Weather Advisory Issued*

- Breezy winds today and Wednesday.

- Cooling, below normal temperatures through week.

DISCUSSION

Current radar and visible satellite imagery shows light returns along the Cascade and Blue Mountains under mostly cloudy skies. This is in response to an upper level shortwave passing through the area. This shortwave has produced continued light mountain rain and brief showers across the Columbia Basin, Central Oregon, and John Day Basin. 12 hour rain totals are observed between 0.20-0.40" across the Cascade and Blue Mountains, 0.10-0.20" through the Blue Mountain foothills, and 0.01-0.05" across Central Oregon, Columbia Gorge, and the John Day Basin. Precipitation chances will persist across our mountain zones as an upper level low pressure drops south and offshore along the British Columbia coast. This enhances northwest flow and advects a cooler airmass into the region, effectively transitioning precipitation to snow across the Washington Cascades. Snow levels will plummet to around 3500 feet tonight into Wednesday morning as 3 to 7 inches of snowfall is expected at elevations at or above 4000 feet. Thus, a WINTER WEATHER ADVISORY has been issued for the upper slopes of the Washington Cascades through 5 PM Wednesday, which includes White Pass. Confidence in these snow amounts is high as the HREF and NBM advertises an 85-95% chance of 3 inches of snow, 75-85% chance of 5 inches or more, and 30-60% chance of 7 inches of snow or more at White Pass. Mountain snow showers taper off through Wednesday afternoon as the low pressure system continues to drop south along the Washington coast and weakens.

Along with cooler temperatures and mountain snowfall, the incoming system will also enhance winds as it tightens isobars on its approach. This will lead to sustained winds of 15-25 mph with gusts of 35-40 mph across portions of the Columbia Gorge and Simcoe Highlands. The NBM highlights a 60-80% chance of 35 mph wind gusts and a 40-60% chance of 40 mph wind gusts tonight through Wednesday morning across these areas. Winds are expected to peak between 11 PM tonight and 3 AM Wednesday morning.

Temperatures will continue to drop through the end of the week, bottoming out ahead of a low pressure passage on Saturday morning. Morning low temperatures are expected to drop to or below freezing across the Columbia Basin. The final Freeze Warnings of the season may be needed for areas still awaiting their first freeze of the season - which would include the Columbia Gorge and Oregon Blue Mountain foothills. The NBM suggests that the foothills have a 50-80% chance of freezing as early as Thursday morning, but the underlying concern will be related to cloud cover. This leads to the best chance of little to no cloud cover being on Saturday morning.

Guidance is in agreement that the low pressure will stall off the Oregon/Washington coasts Thursday, continuing to weaken on Friday as an upper level ridge behind this feature approaches and strengthens. However, this is where guidance begins to struggle in the incoming ridge's strength and subsequent weakening of the offshore low. It will also result in timing differences of when the weaken low pressure is pushed through the area, with current timing of late Saturday night into early afternoon on Sunday. Currently, 61% of ensembles favor an earlier more weakened system passage, bringing light rainfall to our mountain zones Saturday evening versus early morning on Sunday. After this passage, the upper level ridge moves through the area over the early part of the week.

Temperatures will continue to drop through the end of the week, bottoming out ahead of this low pressure passage on Saturday morning. Morning low temperatures are expected to drop to or below freezing across the Columbia Basin. The final Freeze Warnings of the season may be needed for areas still awaiting their first freeze of the season - which would include the Columbia Gorge and Oregon Blue Mountain foothills. The NBM suggests that the foothills have a 50-80% chance of freezing as early as Thursday morning, but the underlying concern will be related to cloud cover. This leads to the best chance of little to no cloud cover being on Saturday morning. 75

AVIATION /00Z TAFS THROUGH 00Z THURSDAY/

VFR conditions initialize for all sites, prevailing through the rest of the period. Reduced FEW and SCT clouds at 2500 to 3000 feet through most TAF sites. There's a 15-30% chance at each site that MVFR conditions develop and persist for a few hours. The greatest chances occur where the heaviest showers occur. Winds in general will be breezy with the strongest winds will be in PSC with gusts at 20 to 25 knots by the late afternoon to early evening hours.

Preliminary Point Temps/PoPs

PDT 33 54 32 51 / 20 40 10 0 ALW 39 53 35 53 / 40 60 30 0 PSC 39 58 30 54 / 20 20 0 0 YKM 34 56 29 52 / 20 20 0 0 HRI 38 56 30 51 / 10 20 10 0 ELN 30 48 22 47 / 60 50 10 0 RDM 31 48 20 48 / 0 10 0 0 LGD 31 51 30 54 / 60 30 30 0 GCD 29 49 28 54 / 20 0 0 0 DLS 40 55 34 54 / 70 90 50 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory until 5 PM PDT Wednesday for WAZ522. OR...None.