NOAA's Weather Prediction Center has raised Colorado's mountains to a "moderate" heavy snow risk for Wednesday, a rare category for May, as a back to back Pacific storm targets I-70, I-80, US-50, US-395, and Wolf Creek Pass from Monday into Wednesday.
Two Act Storm
A two act late season storm will hit the western United States starting Monday night, with the second act bringing the harder punch.
California's Sierra Nevada gets struck first. NWS forecast offices in Sacramento, Reno, and Hanford are tracking a Pacific upper level low that pushes onshore Monday afternoon and grinds across the range through Tuesday. Snow levels are forecast to drop as low as 4,500 feet, putting accumulating snow into communities that rarely see it in May. The system then reloads over Colorado Tuesday night and intensifies through Wednesday, the day WPC has flagged as "moderate" risk for heavy snow.
The Corridors
I-80 over Donner Pass and US-50 over Echo Summit are the primary east west routes across the Northern Sierra. When both close at the same time, there is no realistic alternative across central California. Chain controls appear increasingly likely Monday night into Tuesday, with possible full closures during the heaviest hours.
US-395 along the Eastern Sierra, the only bypass around the range from south to north, faces high winds along the crest with side roads to Mammoth Mountain and June Mountain treacherous at best.
In Colorado, the heavy snow zone covers most major mountain corridors: I-70 over Loveland Pass and Eisenhower Tunnel, US-50 over Monarch Pass, US-160 over Wolf Creek, US-40 over Berthoud Pass, and I-25 approaching Raton Pass at the New Mexico border. Yosemite's Tioga Pass and Colorado's Independence Pass, both only just opening for the season, will likely close again.
Why This Matters
A "moderate" categorical risk this deep into spring is unusual. WPC issuing it five days out, instead of the typical 24 to 48 hours, signals that ensemble model agreement is unusually strong. NWS Pueblo's discussion already names Tuesday and Wednesday as the focus window.
For Colorado this is the third storm in two weeks, layering fresh heavy snow on a cold, saturated base. Avalanche risk for backcountry travelers will spike. Colorado's Traction Law stays in force through May 31. For California, where the April 1 Sierra snowpack came in at just 18% of normal, the second lowest on record, every spring system carries outsized weight for summer water supply.
If You're Traveling
Carry chains even if your vehicle is 4WD or AWD. Colorado's Traction Law applies regardless of drive type, and California's chain controls override AWD ratings above 4,000 to 5,000 feet. Keep a full tank of fuel before entering the mountains, since closures can strand drivers for hours and idling burns through reserves quickly.
Pack an emergency kit: blanket, flashlight, water, snacks, a folding shovel, and a portable phone charger. If you get stranded, stay with your vehicle. Run the engine in 15 minute intervals with a window cracked for ventilation, and keep the exhaust pipe clear of snow to avoid carbon monoxide buildup.
Check road conditions immediately before departure, not the night before. Mountain passes can flip from dry pavement to ice in under an hour during late season events. Allow extra following distance, slow down well before braking, and avoid using cruise control on snow or ice. If visibility drops below a quarter mile, exit at the nearest safe stop rather than pushing through. Check road status at cotrip.org for Colorado and quickmap.dot.ca.gov for California before you leave.
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