An atmospheric river is slamming into the west with up to a foot of rain in some places, but here in Montana, it's more like a trickle on a creek that needs hoot owl restrictions.
A foot of rain in one storm is a lot of water. Hard to imagine when parts of Montana barely see a foot of rain over the course of a year, but that's what California saw as an atmospheric river slammed into the west.
An atmospheric river is sorta like aiming a fire hose from the pacific ocean into the west. It's a concentrated plume of moisture that extends thousands of miles over the ocean in the lower levels of the atmosphere. Narrow bands of enhanced water vapor are transported into the west resulting in extremely heavy rain and mountain snow.
There are typically 3-5 of these rivers within a hemisphere at any given time, and a single one can carry a greater flux of water than the earth's largest river, the amazon river.
Atmospheric rivers, sometimes called a “pineapple express,” have a central role in the global water cycle, contributing about 22% of total global runoff. They are a major cause of extreme precipitation events that cause severe flooding in many mid-latitude, westerly coastal regions of the world including the west coast of North America, Western Europe, west coast of North Africa, the Iberian Peninsula and New Zealand.
Water water everywhere but why the trickle here in Montana? just like the fire hose, there's only so much water and pressure. Most of the moisture gets wrung out over coastal mountain ranges. Then the continental divide absorbs most of what's left after reaching Montana.