Training storms are repeated convective cells that move over the same location one after another, like train cars crossing the same set of tracks, and are one of the most common causes of extreme rainfall totals from a single event. They are a leading driver of flash flooding.
How training happens
For storms to train, cells have to form or reach a fixed area and then move parallel to the line along which new cells are developing. In practice this happens when the mean wind aloft is aligned with a boundary, front, or terrain feature that is producing new cells at one end.
Each cell rides down the boundary, drops its rain over the same neighborhoods, and is replaced by another cell forming behind it. The result is that a location experiences repeated heavy rain cores instead of one cell that passes and moves on.
How forecasters watch for it
Warning meteorologists watch for radar echoes lined up parallel to storm motion, especially when new cells are forming on the upwind end of the line. Precipitable water values, warm cloud depth, and rainfall efficiency are pulled in to estimate how much water each pass may drop.
Once training is underway, the flash flood problem tends to grow faster than the meteorology alone would suggest, because soils saturate and small streams respond to each additional pass. Flash flood watches and warnings are typically escalated well before the last storm has crossed.
Important limits
Not every line of storms is training storms. Cells that move perpendicular to the line, or that quickly outrun their starting point, can look similar on a single radar image but do not deliver the same rainfall totals. Training is best identified in a loop, not a snapshot.
It also requires an environment where new cells keep forming. When the source of new convection shuts off, training ends even if the wind alignment persists. Flash flood risk in a training pattern can rise quickly, so the fact that a location has already been hit once is a reason to pay closer attention to the next cell, not a reason to relax.