Severe Weather Watcher Handbook

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Damaging wind

If you are first at the scene of severe damage, survey the area for downed wires, gas leaks, fire threat, and injured people. First report these findings to local authorities and/or emergency personnel. Once this has been settled, call in your report to MSC.

Damage caused by a tornado will differ from that of regular high winds. It is very discrete, sudden, and confined to a narrow path, as tornadoes produce more abrupt and localized damage. Trees are often snapped off or twisted 2-5 metres above the ground.Structural damage will be total in one place yet almost nonexistent nearby. Heavy objects such as farm equipment, trailers, etc. may topple with straight-line winds but will be rolled, lifted and dropped, or twisted around by tornadoes. The tornado damage path is often narrow (50-200m) and clearly marked, while straight-line wind effects will be widespread but not as pervasive in total damage. The one exception is a derecho, which can cause widespread severe straight-line wind damage.

With the straight-line winds that occur when a wind squall precedes or accompanies the precipitation core,most of the damage is lined up with the wind direction. This is especially noticeable with fallen trees. Also, large trees will be uprooted or blown over, rather than snapped off.

Damage to shingles, tree limbs, etc. will occur at speeds of 70-90 km/hr, while uprooting of large trees, overturning of small planes and boats, loss of driving control on open highways, and larger-scale structural damage is likely with gusts over 90 km/hr.

A smaller-scale phenomenon is the microburst, a brief wind squall lasting from only seconds to a minute or so. Over dry earth, it will produce a puff of dirt that soon dissipates, but can be (and often is) mistaken for a tornado debris cloud. It occurs with severe storms, but also with smaller clouds in dry weather. Although very brief, microbursts can be strong enough to cause structural damage.

Many false reports of tornado sightings are the result of microbursts. Until recently, many wind damage events had been attributed to tornadoes but were more likely microbursts. A microburst damage swath is typically from 100m-1km wide and up to several kilometres long, with damage scattered in smaller swaths and clusters. The overall pattern is forward-leaning but there are signs of outward curvature too, as the microburst wind spreads out after contact (see diagram).

Flooding rains

The combination of torrential rains and local terrain conditions can give rise to flooding. The heaviest rains occur in slow-moving storms when the precipitation core passes directly overhead, or in lines of storms that regenerate over the same location. The potential for flooding depends on soil type, the presence of vegetation, land slope, saturation from previous rains, and the rainfall rate. Rates of 25-50 mm in one hour and continuous heavy rain for hours will cause street flooding, overflow of creeks, mudslides and washouts, etc. If you encounter any of these situations, report them. There are more deaths from flooding than there are from tornadoes. The greatest danger from flash flooding occurs when people attempt to drive through fast-flowing water crossing a road.

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