Severe Weather Watcher Handbook/Manuel de l'observateur de temps violent - Severe Weather Watcher Handbook - [Meteorological Service of Canada

Reading the clouds

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