Severe Weather Watcher Handbook

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16-20  N/E  Sequence showing storm evolution by both regeneration and propagation. A thunderstorm has been growing southward by regeneration while moving east. In 16, there is a high-based, two-tiered wall cloud under the south end of a short flanking line (lower left). This strong phase weakened, then reintensified about 50 minutes later. As the storm comes closer (17), you can see the flaring anvil with a wide overshooting top above it. An aging anvil is partly left in the storm's wake (upper left) since the storm is growing forward rapidly in this second severe phase. A SE inflow, seen as a bank of cumulus (dark bases, lower right) enters a lowered area (light grey, bottom centre), while outflow fills the left side of the view (see diagrams). Note how the bases are higher at left where downdrafts evaporate the lowest parts. The big burst in 17 collapses in 18, ten minutes later, and a brief funnel cloud appears under the back edge of the storm. A period of weakening has begun as the outflow from this collapse undercuts inflow and forces new towers to grow slightly ahead (lower right) of the system's original core. The storm's anvil looks decidedly fuzzy, too. After 15 minutes, in 19, the previous big phase (lower left) has been left behind while a strong new phase begins. The system has propagated forward and is re-entering a steady state again. This same phase began from the small bank of growing cumulus in the previous photo. A half-hour later (20), the third sever phase is well underway. Updrafts again form a tall dome and the pulse spacing is quite small. It can be seen as regular bumps along the anvil. The older anvil material has drifted to the north, out of the way of this new bursst of activity. There are, again, hints of a small lowering (white tuft below the base). The storm complex survived for hours by regenerating until outflow forced it to propagate into the warmer air, where it reorganized and began regenerating again. The diagrams show inflow and outflow (top) and a top-down view of the main cloud and wind features (bottom). Dashed arrows represent air motion inside the cloud. The new updraft at right will become the next major phase and is initiated when the older upudraft weakens enough to let outflow undercut it and push SE.

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