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

A close look at lowerings

If you could only look at one storm feature to determine its severe potential,it would be the lowering. That is rather fortunate since many days (in the East) are cloudy or choked with haze and the underside detail is all you have access to. The lowering is a small (½-2km diameter) cloud extension attached to the underside of the rain-free base, usually about a kilometre from the edge of the rain core.

What makes this cloud so special? For one thing, every strong storm seems to have a unique one. More practically, it is the site of the main, focused updraft into the system. Air enters the lowering from both the warm inflow and the cool outflow sides. Moist air moving out of the rain area is drawn into the updraft and condenses at a lower level below the original storm base.When this mixing of two different air masses is irregular the lowering is also irregular. When the updraft is stronger, more focused, or structurally more compact, it shows as a tighter, more complete lowering.When it is circular in shape (you have to imagine this since perspective squashes the circle visually) it indicates that this moist air has wrapped around the updraft, suggesting rotation. Every small change in the system's updraft strength, inflow/outflow balance, and flow control is manifested directly in the appearance of this small cloud feature.

The range of shapes and sizes is endless, but there are definite differences as we compare weaker storms to severe ones. Lowerings which are incomplete, tilted, elongated, or ragged are weaker. The incomplete shape or asymmetry is evidence of a transient stage (not in balance) which could be due to outflow pushing forward, lack of updraft focus, etc.- all signs that the storm is struggling to maintain a strong, steady inflow. Those that are very low, thick, large, or circular merit your serious attention. Some severe storms in dry weather may not have a lowering at all and some (seen in the West) have a high, small, "step-down" lowering instead.When a lowering does become organized, complete and circular, it is rotating and is known as a wall cloud, which may be the precursor to tornado development.

Before you panic at the sight of every low cloud under a storm, bear in mind that not every low, dark, ragged cloud counts. Check its location. If it is overhead, check the winds to see if you have cool W-NW outflow (no risk) or if the air is calm (under the main updraft area, possible risk). Check it against the storm's shape/structure, and watch the air motion in detail. If the air near the lowering - and associated cloud fragments - is moving horizontally or the cloud is elongated, it is being pushed forward by outflow and is currently harmless. On the other hand, if the ragged bits are all rising and a circular motion is present, it is a rotating wall cloud and should be reported!

66 NW/E, 67 W/SE, 68 NW/E Three lowerings in moist air. These lowerings are not yet rotaing but have enough structure to merit watching. In 66, a new updraft has drawn in rain-moistened air which condenses below the rain-free base. The structure is incomplete but the location (at the SW end of the storm) is right. 67 shows a larger, rounded lowering with tufts of rising scud adding in. It is on the east side of this storm and is continually being undercut and corced forward by outflow, preventing the updraft from rotating. It has the right shape and could begin rotating if outflow slows or the storm attains control over it. In 68, a small, potent storm (right) has a trailing cloud bank with a brushy lowered underside. This is both the flanking line and a slow-moving (right and forward) gust front, as seen in the banding above it. The main updraft is the pudgy cloud mass above the lowest-hanging bits of scud. The inflow-outflow balancing act usually favours the cool air but again, a brief tightening-up of a lowering can quickly change the scene.