Moisture transport is the movement of water vapor by the wind from one region to another. The product of the wind and the moisture at each level determines how much water vapor is imported into or exported from a region.
How the flux works
Moisture advection at a point is the wind vector dotted into the gradient in moisture. When the wind blows from a moister region toward a drier one, the drier location sees rising moisture. When it blows from drier toward moister air, the moist location dries out. Integrated through a layer, the same idea gives the moisture flux, which quantifies how much water vapor is crossing a boundary.
Because moisture usually decreases with height, the strongest transports are found in the lowest few kilometers where both wind and moisture are highest. This is why the low-level jet, a strong wind maximum typically between about 1,000 and 2,500 feet above the ground, dominates the moisture budget of the Great Plains warm sector.
How forecasters use it
Forecasters watch moisture transport to answer a specific question: is the air moving over the target area getting richer or drier over the next several hours? A strengthening southerly low-level jet advecting Gulf moisture northward is one of the standard signals that a warm sector may become supportive of severe weather overnight and into the next day.
Transport patterns also shape heavy rainfall. Persistent, deep moisture transport into a stationary front is a common setup for training storms, because the same air keeps feeding storms that redevelop over the same ground. When forecasters call a setup a moisture-rich pattern, they usually mean transport has been sustained long enough to build the column, not that a single hourly snapshot looks moist.
Important limits
Transport is a flux, not a state. A location can see strong inflow of moist air while local drying by mixing, subsidence, or downslope flow keeps the column dry. Transport tells forecasters what is arriving, not what will remain.
Transport also does not create instability by itself. Moisture is only one ingredient. The same moist inflow can produce a widespread severe outbreak under strong forcing and shear, or nothing more than a stratus deck under a cap and weak lift. Read transport alongside the rest of the environment before drawing conclusions.