A mountain pass can be open in the morning and gated by early afternoon. The decision is made against live conditions rather than a calendar, which is why advance notice is short.
Closure is a safety judgement, not a schedule
Highway agencies close a pass when they judge that they can no longer keep it passable or clear it if something goes wrong. That judgement depends on what the weather is doing now.
Forecasts give hours of warning at best in complex terrain, because a storm that stalls or shifts a few miles changes the outcome on one side of a ridge and not the other.
Publishing a closure early would mean guessing, and a pass closed unnecessarily strands freight and residents. Agencies therefore wait until the call is clear, which compresses the warning.
Snow is only part of the problem
Accumulation on the road surface is the visible issue, but wind is often the binding one. Drifting can refill a cleared lane faster than a plow can complete a circuit.
Visibility collapses in blowing snow even when little is falling. A pass with a clear surface can be undriveable because a driver cannot see the edge of the road.
Ice forms where meltwater refreezes at the shaded side of a curve, so the hazard is uneven along the route and difficult to communicate as a single condition.
Avalanche control forces planned shutdowns
Slopes above a pass load with snow over a storm cycle and must be released deliberately before traffic is allowed underneath. That work requires the road below to be empty.
Control work is scheduled around weather windows and daylight, both of which move. A shoot planned for one morning slips to the next when cloud sits on the start zones.
These closures are announced when the window firms up, often the evening before, and they can extend if the slope does not respond as expected.
Plow capacity sets the limit
Each agency has a finite number of machines and operators for a given corridor, and crews work in shifts with mandated rest. Sustained storms exhaust that capacity.
When the rate of accumulation exceeds what the fleet can remove, the road loses ground with every pass. Closing lets crews get ahead rather than fall further behind.
Reopening then depends on clearing the full length, checking for slides and restoring barriers, which is why a pass often stays shut for hours after the snow stops.
What this means for routing a trip
A pass is best treated as a conditional link in a route rather than a fixed one, with the detour identified before departure rather than at the gate.
Detours around a high crossing are frequently long, because the terrain that makes the pass useful also makes alternatives scarce. Adding hours is normal.
Travelling early in the day gives the most flexibility, since closures cluster as storms build through the afternoon and crews lose daylight.