Severe Weather Watcher Handbook

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84  NW/E,  85  NW/NE,   86  N/E,  87  NW/E  Four wall cloud examples, in order of increasing severity. In 84, a small but well-focused wall cloud feeds a hailstorm with an unusual green precipitation core (yes, it looked like that!). The same air entering this small place in the sky is forming the huge curtain to its right. The wall cloud was on the forward side of the storm, indicating an HP supercell. Roation in this wall cloud (85) has formed a concave underside with a lower edge (seen as prongs). The storm is a classic supercell with the core at right. The rain at lower left is from a different shower. A giant mesocyclone fills the fram in 86. Below it, you can see a very low wall cloud, seemingly dragging almost on the ground. This classic supercell began as an LP supercell earlier. It produced an F2 tornado a short while later (see photo 91). In 87, two large prongs mark the outer edges of a huge (3km dia.)classic supercell wall cloud, revealing the whole mesocyclone. There is steady rotation, with a tighter circulation in the right side where, moments later, a brief tornado poked down. The left side became a separate, larger tornado about ten minutes later.

Location relative to rest of storm

In the usual case, a wall cloud will be seen just S-SW of the rain core, under the rain-free base. This would be typical for storms moving to the E or NE. However, storms can move from other directions and the baseline will then be oriented differently. One common exception occurs with fairly strong SE winds ahead of a system. In this case, the wall cloud can be positioned to the S or SE, slightly ahead of the core. This is also true for some supercells which have rain around the back of the storm and the wall cloud just ahead of it. The closer the surface wind direction is to the upper flow and storm motion, the more likely you are to find the wall cloud at the SW-W back end, under the typical extended rain-free base.

88  SW/E  A small wall cloud sits at the back of this weakening LP supercell. The precipitation curtain at right acts as a barrier to the warm inflow, channelling it along a thick tail cloud (dark tube, lower right) into the lowering. Below this, rain and outflow curve sharply forward, becoming an elongated rain foot before being drawn into the updraft. Farther left, the claw-like cloud base leaning away from the rain suggests outflow (not visible there) has moved beyond the lowering and will likely soon choke it off complete-ly. This type of structure is similar to photos 69 and 74.

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