Severe Weather Watcher Handbook

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72-77  W to N/NE  Sequence showing formation and evolution of a wall cloud. 72 shows an ordinary thunderstorm with flaking line towers leading up to the rain core at right. In 73, after 15 minutes, the updraft has become more compact. Although there is no typical lowering, the whole base is lower (like photo 69) and draws in some of the outflow (seen by tilt toward rain). The grey area lower left is another, unrelated shower. By 74, (7 minutes later), the base is even lower and a circular structure is emerging. The storm has become an LP supercell with a very short flank and a narrow updraft column at the back atop the lowered wall cloud. Only 7 minutes later (75), a fully-formed mesocyclone spins the air upward. Tufts of scud and a brief funnel were seen below the base. A line of scud from the rain flows into the wall cloud. This intense phase lasted about ten minutes, until a surge of outflow swept around the storm's south side and undercut the mesocyclone. The storm, meanwhile, moved past and, after another 13 minutes, in 76 it is much weaker. The lowering, squeezed forward (to the right) by outflow is elongated and no longer rotating. You can see the gust front axis as an arc from top left toward the lowering. But with a very unstable atmosphere, it recovered as soon as outflow weakened (77, 8 minutes later) and became an even larger, fully rounded, low wall cloud. The storm was becoming a more dangerous classic supercell. A dust cloud below it may have been a brief tornado touchdown.

78-83  NW/SE  Formation and dissipation of a classic supercell wall cloud. The rain-free base of this storm (78) has no lowering and is turned slightly forward from the rain core to its north. Two minutes later (79), a rain foot has kicked up a chunk of scud below the base. After another two minutes (80), a lowering is taking shape. It is already compact (likely circular) and has pointy tufts where the inflow air rises straight up into the base. A closer look in 81 (3 minutes later) still shows the rain foot connected to a gangly wall cloud that spins and gyrates against the bright sky beyond. But the wider view (82) tells another story. Outflow is also pushing south from the rain core (dark line, right centre) and the lowering is elongating as outflow there finally pushes beyond the strong updraft. By 83 (2 minutes later), the wall cloud is flattening and a gust front is beginning to separate from the rain area (note light behind forward edge, top right) as outflow takes over the scene. At peak intensity, this storm was a classic supercell. Later, it evolved into a raging HP supercell with a long swath of damaging winds and baseball hail.

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