On most storm days, small cumulus grow gradually larger, group together, decrease in number and then exhibit signs of a breakout as taller, larger towers rise here and there. This pacing of cloud growth gives you time to observe cloud evolution.  Occasionally, the atmosphere has a stable layer in place, preventing strong convection and sometimes even small cumulus until later in the afternoon.  When heating reaches the critical point or when cooling aloft erodes the stable layer, it breaks down rather suddenly, triggering explosive cloud development.  In such cases, the day's forecast, patience and a wary eye are all you can depend on to prepare.  Storms which break out in this abrupt fashion are more likely to be severe because the bottled-up heat and moisture are available for an immediate and concentrated release. 

A large bank of cumulus in 21 becomes a single, focused updraft in 22 (20 minutes later) in the first breakout phase.  Tops are hard and crunchy, but the new anvil (left) is still irregular.  By 23, only 5 minutes later, a very high tower signals the burst phase.  A minute later in 24, the burst has initiated a steady stream of close updraft pulses which form a persistent flaring crown 3 min. later in photo 25.  Now, the anvil has become more uniform and solid.  The new growth is shifting the storm to the right of the upper winds from the SW.  In photo 26 4 minutes later), an overshooting top over 39 Kft high persists above the anvil edge and this storm is now steady-state severe.  To its right is the large flange formed by the spreading of the burst phase.  Once begun (in 22) the change from small Cumulus to severe storm took less than 15 minutes! 

NOAA's
National WeatherService
Chicago Forecast Office
Amateur Radio Page