Severe weather events may not be common in some areas of Australia but a long history of 'surprises' demonstrates that we cannot take the weather for granted. Regularly check the radio and the Bureau's website (www.bom.gov.au) for local and regional weather forecasts and reports, and monitor the radar and satellite images. If thunderstorms are forecast for your area, stay alert for severe thunderstorm warnings which may be issued and updated later in the day and, of course, keep a lookout on the sky!
What Severe Thunderstorm Events Require
Severe thunderstorms produce large hail, damaging wind, very heavy rainfall and tornadoes. There is no absolute relationship between these events and visible thunderstorm features, but a few pointers are relevant.
Hailstones are fascinating ice sculptures which form in the strong updraughts within thunderstorms. Alternating clear and frosted rings inside the hailstone bring us back a crystalline time capsule from a long, turbulent ride on updraughts and downdraughts reaching speeds of 160 km/h. Small stones form and grow as they are swept up and held aloft by the strong air currents. Once the hail has grown too large to be supported by the updraughts, it falls to the ground. Large hail depends directly on updraught strength (and thunderstorm regeneration), which is indicated by a steep back side structure with boiling tops that overshoot the anvil. Hail is also more common in relatively cooler conditions aloft (less melting on descent) so the tropics rarely experience severe hail.
Damaging wind gusts will almost always originate from downdraughts originating in the core. These downdraughts will spread out as gusts as they reach the ground, usually inside or along the edge of the rain curtain. Ragged low clouds often accompany the on-rushing gust front, which usually pushes out ahead of the core region by several kilometres before weakening. These wind squalls are generated as follows. Inflow of drier air into the mid-levels of the cloud leads to evaporation of precipitation and cloud particles. This cools adjacent air which becomes denser and plummets toward the ground in the downdraught. The downdraught may be further enhanced by the frictional drag of the precipitation and by strong mid-level winds.
Flash flooding is the least predictable thunderstorm event. It depends on soil type, presence of vegetation, land slope, saturation from previous rains, and the rainfall rate. Three possibilities exist. An intense thunderstorm can cause flash flooding if the core passes directly overhead. Weaker or less organised thunderstorms may also induce flash flooding if they are moving very slowly. Thirdly, a series of thunderstorms may sometimes travel or regenerate along a single line, resulting in large amounts of rain in repeated downpours over the same area. These last two examples are unlikely to produce severe effects other than flash flooding.
Tornadoes and funnels almost always protrude from a cloud lowering under the cloud base, near the main updraught. Horizontal rotation of the lowering and/or the whole cloud base is often evident, usually in an anticlockwise direction when viewed from below. Tornadoes usually only happen with highly organised, large, sustained thunderstorm systems.
Thunderstorm Features to Check at a Distance
Watching separate parts of a thunderstorm system can provide valuable insights into its intensity and the probability of severe weather.
The anvil can tell us everything from age to strength and organisation. Unevenness on top indicates erratic growth, and a diffuse edge suggests weak updraughts, hence a weaker system. The anvil on long-lived regenerating thunderstorms is very long, with small notches or dents along the edge corresponding to the separate pulses or updraught stages.
If a thunderstorm is severe, the anvil will likely have a sharp edge (abrupt transition to clear sky), be smooth and high and mostly flat on top (Photo 6 is a good example). These are signs of a very powerful updraught which has spread out at the tropopause (the stable layer at the top of the weather-producing part of the atmosphere) and blown forward on strong winds aloft. This expanding cloud matter can sometimes push back against a strong upper flow to form a thick, backhanging anvil. The force of the updraught may also carry the cloud top above the anvil to form an overshooting top, which is an excellent sign of severity if the overshoot persists longer than several minutes. When a solid, new anvil section forms above an older one, we may be witnessing the onset of the severe phase, in which all aspects of the system are accelerated and magnified.
Photo 6. Anvil with crisp edge spreading out from storm cloud near Millthorpe,
New South Wales. Photograph courtesy of Rose Toomer.
The Core and Back Side of the Thunderstorm
There are several severe indicators in this part of the cloud system. All are the result of the updraught's large size, strength and persistence. A very steep, almost vertical mass of boiling towers at the back side of the thunderstorm is indicative of abrupt, explosive upward motion of air. This feature can persist in a steady-state supercell thunderstorm for many hours, or may be present for a short time during a new growth phase in a multicell thunderstorm, signalling the transition to a severe phase. The onset of a severe phase in a thunderstorm may also be characterised by a large, rising tower of convection that is larger and taller than any previous growth.
Another good indicator of a severe thunderstorm is a very short interval between successive updraught pulses, seen as individual rising domes within the main updraught, or as separate towers along the flanking line. Remember that perspective can be deceiving. If you look down the flanking line, towers may seem closer together than they really are.
One other feature deserves mention. Well-organised severe thunderstorms often have one or more bands of low clouds ahead of them leading up to the main updraught base. These inflow bands move with the low-level flow and indicate a focusing of warm air flowing into the thunderstorm.
The Flanking Line
A thunderstorm's main axis of inflow, along which air is guided towards the system, is usually marked by a line of separate towers on a continuous baseline - the flanking line (Photo 3). When viewed from a distance, the flanking line is an extension of the rain-free, updraught base under the main cloud. The line is often narrow but can also be a very wide, dark cloud bank which fills the sky when overhead. A severe thunderstorm that controls the inflow pattern results in a flanking line with orderly, successively taller towers leading towards the core. If the system weakens, towers will begin growing independently and may mature into separate thunderstorms. The Australian
Gust Fronts and Wind Squalls
A great variety of cloud features may be seen along the leading edge of gust fronts and wind squalls. Advancing outflow air acts like a plough, mixing the cool, moist air at its boundary with warmer inflow air and forcing it to rise. This can result in a spectacular low cloud bank (called a shelf cloud) (Photo 7) on the leading edge of the thunderstorm. The shelf often has a smooth, laminar or banded surface and a black, turbulent base. If it pushes too far ahead, it will slow down, decrease in size, and sometimes become detached as a horizontal cloud tube, or roll cloud. Very humid conditions will promote a thick, low cloud bank while a sharp, strong gust front will cause the lowest part of the leading edge to be ragged and lined with rising scud. In a severe case there will be vortices along the edge, seen as twisting masses of scud that may reach to the ground or be accompanied by rising dust. An extreme example of this phenomenon looks almost like a tornado and is known as a gustnado. A very low shelf cloud accompanied by pockets of dense, rising scud is a good indicator of a potentially violent wind squall to come.
Photo 7. Shelf cloud outflowing from beneath a storm, Bribie Island, Queensland.
Photograph courtesy of Barbara Burkhardt.
Wind squalls may also be generated by downbursts: concentrated severe downdraughts which may accompany a descending deluge of precipitation. These induce an outward (horizontal) burst of damaging wind at the surface. Outwardly curved rain shafts are a good sign of these strong outflow winds and the steeper the angle, the stronger the flow. On a smaller scale, this wind feature is known as a microburst: a brief, intense wind surge (Photo 8). Microbursts need not be accompanied by a thunderstorm situation. Dry regions like inland Australia often experience these brief, damaging wind gusts below cumulus, small showers, or streaks of virga (rain which evaporates before reaching the ground). Evaporation of falling raindrops cools the air which then plunges earthward, arriving almost 'out of the blue' at times. At a distance they are often made visible by a puff of dust rising from the ground.
Photo 8. A wet microburst. Notice the lower-level cloud spreading out away
from the main rain region. Photograph copyright Jimmy Deguara.
When the Thunderstorm Has Arrived
Once it is raining and clouds have engulfed the area, we must rely on other features to assess the thunderstorm's behaviour. Two things to pay attention to are precipitation and lightning and/or thunder.
The expected precipitation pattern for a well-developed or severe thunderstorm is a steady transition from lighter rain to heavy rain as the core approaches, and finally rain mixed with or changing to hail. If this occurs, hailstones will become larger and less frequent, then cease abruptly. When rain (or small hail) begins as brief, separate showers or a sudden cloudburst, the thunderstorm is unlikely to be severe. The one exception would be if the cloudburst brings strong wind squalls. For a severe thunderstorm, the heaviest rain and any large hail will usually, but not always, occur last.
Thunder and lightning can be good indicators of severity when combined with other observations. An active core will have very intense lightning, with multiple flashes and deep booms. If frequent lightning occurs without any precipitation (or only a few hailstones) below a dark cloud base with no wind, you may be beneath the main updraught of the thunderstorm.
One small cloud feature, which is particularly valuable in assessing a thunderstorm's potential severity, can sometimes be found beneath the rain-free cloud base toward the rear of the thunderstorm. This localised cloud-base lowering is the site of the main, focused updraught into the system. It forms when cool, moist air from the rain area is drawn into the updraught and condenses below the main cloud base in a process similar to that which forms a shelf cloud at the leading edge of the thunderstorm.
The range of shapes and sizes of the lowering is endless but there are definite differences between weaker thunderstorms and severe ones. Lowerings which are incomplete, tilted, elongated or ragged indicate weaker thunderstorms. Lowerings that become organised, complete and circular are known as wall clouds and may be the precursor to tornado development (Photo 9). However, some severe thunderstorms in dry weather may not have one at all or have a high, small, 'step-down' lowering.
Photo 9. A wall cloud at the base of a severe thunderstorm, Hornsby,
New South Wales. Photograph courtesy of Andrew Treloar.
The Wall Cloud and Rotation
A cloud-base lowering is more likely to be a true wall cloud if it:
- has a circular shape indicating rotation, and/or is visibly rotating;
- shows laminar bands above, where it joins the main cloud base;
- has prongs - lower portions on the edges - or a 'tail' pointing toward the rain curtain;
- has a shadowy curl curving up into it from the edge of the rain curtain;
- shows a tiered structure, stepping down in stages below the main cloud base.
Ordinary updraughts contain no rotation. However, as they become stronger and develop an organised inflow, slight rotation may exist. This is sometimes reflected in broad rotation of the cloud base beneath the main updraught or the circular nature of the wall cloud, if present. In most cases in the southern hemisphere, rotation will be in an anticlockwise direction when viewed from below.
Many thunderstorms will exhibit this rotating motion without forming tornadoes or funnel clouds. However, if the rotation increases in speed or seems to have a single rotating 'hot-spot' within the wall cloud, a funnel may soon form and a tornado can follow.
Photo 10. Rotating cloud base indicative of a supercell thunderstorm that
produced seven-centimetre hailstones in the western suburbs of Adelaide.
Photograph courtesy of Peter Geytenbeek.