The National Weather Service has Winter Storm Warnings active for the South Washington Cascades and the Northern and Central Cascades of Oregon, plus the East Slopes of the Oregon Cascades. What is heading for the mountains starting this afternoon is not a routine April shower. A deep Pacific trough is pushing cold air and heavy moisture into the range, and the snow level is dropping fast. Totals above 2,500 feet could reach 21 inches from Marion County to Lane County in Oregon, with 6 to 12 inches on the east slopes above 4,000 feet. Wind gusts are forecast at 45 mph across exposed terrain. The NWS says to “consider delaying all travel.”
I-90 over Snoqualmie Pass, Highway 2 over Stevens Pass, Highway 58 over Willamette Pass and US-26 over Mount Hood are all in the direct path. Chain controls are virtually guaranteed. Full closures are on the table. Snoqualmie Pass shut down as recently as mid-March during a similar late-season event and stranded drivers for hours. If you do not need to be on the other side of the mountains before Thursday morning, do not go.
The Storm
A deep upper-level trough is digging into the Pacific Northwest from the Gulf of Alaska. It will push onshore Tuesday afternoon and slowly grind through the region over the next 36 hours before conditions improve Thursday morning. The combination of strong dynamics and a deep moisture plume will deliver sustained heavy snowfall across the full length of the Cascade Range from southern Washington to south-central Oregon.
The critical detail for drivers is the snow level. It starts around 3,000 to 3,500 feet Tuesday afternoon, which means the highest passes get snow right away while lower elevations initially see rain. By Tuesday night the snow level drops to 2,500 feet, and it could dip as low as 1,000 feet in some areas. That means nearly every pass in the Cascades will be dealing with heavy accumulation by Wednesday morning. The heaviest period is forecast from late Tuesday night through Wednesday morning. Snow will taper to showers Wednesday afternoon and evening before the system clears Thursday…