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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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Created :
2002-08-26
Modified :
2002-12-19
Reviewed :
2002-12-19
Url of this page : http://www.msc.ec.gc.ca /education/severe_weather/page46_e.cfm
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