Atmospheric rivers: read the timing, not just the total

The total can fool you

A storm can spread six inches of rain across three days—or dump most of it in a few nighttime hours. The second version can overwhelm streets and streams much faster. A rain band that stalls over one watershed, especially near a burn scar or steep slope, can turn a broad forecast into a very local emergency.

Put the storm on a clock

Find the expected start, the heaviest period and when conditions should ease. Then think about what will be happening at those hours: the commute, high tide, reservoir releases or simply whether people will be awake to see trouble developing.

• What alert is in effect, and exactly where
• The heaviest hourly rainfall—not just the storm total
• When winds rise and where the snow level sits
• Flood-prone roads, creeks and recent burn scars

What matters most

The most useful forecast tells you when the roughest window arrives, which places are most exposed and what officials want people there to do.