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, concentrated release. (Examples of storms that formed this way are seen in photos 31, 36 and 61.)
21-26 E/E Sequence of a developing severe storm.
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 the last
photo (26, 4 minutes later), an overshooting top over 12 km 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 Cb to severe storm took less than 15 minutes!
| Assessing the scene - does the storm have any of these features? | ||
| feature | what you want to see | this implies |
| longevity | long, flowing anvil | able to regenerate and persist |
| size | large, solid sloud mass large, dark rain curtain |
strong updrafts large core, very heavy precipitation |
| growth rate | boiling cloud tops backside of cloud almost vertical |
very rapid growth, sustained updrafts extreme growth, may be breaking out |
| organization | sharply defined flanking line nearby clear sky (west-south) overall compact appearance cloud bands/lines leading to storm |
sustained inflow spearate from outflow system has control over nearby flow controlled, focused inflow |
| structure | evidence of lowering overshooting tops above anvil |
focused, intense updraft intense updrafts, very tall storm |
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