Severe Weather Watcher Handbook

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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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