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What severe events will depend on
The four primary reportable summer severe events - hail, wind damage, flooding,
and tornadoes/funnels - are discussed in What to report, page 45. There
is no absolute relationship between these events and visible storm features,
but a few pointers are relevant.
Large hail is directly dependent on updraft strength (and storm regeneration)
and thus, on a steep backside structure with boiling tops that overshoot the
anvil. Hail is also more common in relatively cooler conditions aloft (less
melting on descent)so the Prairies - and Alberta in particular - experience
more hailfalls from regular storms than the East does. The largest hail falls
within a few kilometres of the core, near the inflow side of the precipitation.
Damaging wind will almost always originate in the core,inside or along
the edge of the heavy precipitation curtain. Low clouds and scud often accompany
the onrushing wind squall which usually pushes out ahead of the core region
by several kilometres before weakening. This can happen for two opposite reasons.
If the system is regenerating or maintaining its intensity, these bursts of
descending air accompany collapsing updraft pulses which have begun to precipitate.
In this case there are strong updrafts and a compact, organized structure. By
contrast, if a system undergoes a sudden, complete collapse, a large quantity
of suspended precipitation will plummet to the ground and a serious burst of
outflow wind (downburst) results. This often happens when a particularly large
updraft pulse matures into a broad dome, which then collapses down onto the
updraft region (as with a pulse storm,or the collapse of a supercell phase).
Flooding is the most difficult severe event to predict from visible
clues. Three possibilities exist. An intense storm can cause flooding if the
core passes directly overhead and the system has just peaked with respect to
the quantity of precipitation reaching the ground (often about 10-15 minutes
after a major updraft phase).Weaker or less organized storms may also induce
flooding if they are moving very slowly. Thirdly, sometimes a series of cells
travels or regenerates along a single axis (a train-echo storm), producing large
amounts of rain in repeated downpours over the same area. These last two examples
are unlikely to produce severe effects other than flooding.
Tornadoes and funnels almost always appear within a lowering under
the updraft/rain-free base. They usually only happen with highly organized,
large, sustained systems (usually supercells) and all the important clues and
features mentioned in this What to watch for section will be present. The appearance
of the sky during a tornadic supercell storm is so definitive and atypical that
your senses - after a little experience - will alert you to their presence.
89 NE/S A
slow-moving supercell with a tall mesocyclone is revealed by flashes of lightning
in the rain behind it. The light is blocked by most of the structure but comes
through thin clear leyers between the thicker cloud wafers that comprise the
rotating column. The base below it looks (and is) much like a slow-moving shelf
cloud, but not as low. Updrafts are most intense to the left of the bolt, where
there is a small dip in the base. The bolt has jumped out of the core at the
place where updrafts begin to flare forward and freeze. The striated forward
surface is caused by variations in moisture in those layers that are lifted
and bent by the converging flow ahead of the advancing column.
Severe weather at night
Severe weather at night is not too common, but there's no need to give up
and feel like a "sitting duck" when it happens! The most important clue you
have is the progression in precipitation and winds as the main updraft core
moves in. If the risk is serious - as when nearby watches and/or warnings are
active . then you can still recognize cloud details by lightning illumination.
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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/page31_e.cfm
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