Severe Weather Watcher Handbook

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East-west differences

When sufficient heating, moisture, and instability are present, storms can form almost anywhere in the country. In Alberta, many storms form preferentially along the foothills, where plentiful moisture and convergence (where different airstreams meet) help initiate convection. Mountainous areas have storms forming early over the higher terrain. The approach of a cold front or low pressure trough will help to initiate convection but may carry layers of middle cloud that delay storm development or make spotting clouds difficult. Near large lakes or the ocean, storms often first form along the sea or lake breeze front, the leading edge of a cool wind blowing inland away from the water. This is important in Ontario, where converging lake breezes help to initiate storms by forcing air to rise where they meet.

Eastern Canada often has poor visibility in hazy, humid air masses and our attention is confined to watching the bases for cloud size and air motions. The Prairies, on the other hand, can boast visibilities exceeding 150km, in which you can clearly see the tops of distant storms that may never come near your location.

Moist air masses are more prevalent in Eastern regions. Storms there tend to be larger andoften evolve to the forward-propagating type, with damaging winds along the gust front. Supercells are more common in Southern Ontario than elsewhere, especially in late-spring and early summer. Severe weather there is most often associated with large-scale weather systems, but a few isolated summer events also occur. Farther west, isolated storms are more common, since the drier air limits convection to favoured times and locations. Prairie storms are more often "typical" in structure (with backside regeneration) and are more easily identified. In dry but unstable weather, storms may initiate from lines or lumps of mid-layer cloud which eventually grow large enough to become Cbs without any low cumulus present. Each part of the country has its own characteristics and special challenges for the spotter.

The majority of summer severe events occur from Alberta to Quebec. Farther north, storms are limited mainly by a lack of moisture. In B.C. there are only a few severe events a year because the mountains tend to inhibit storm organization inland and cool Pacific winds keep the coast stable. In Atlantic Canada, severe storms are much less frequent and less intense than areas farther west. They occur with trong frontal ystems, usually beginning farther inland (N.B.), then spreading toward the coast where cool ocean air reduces their strength.

Every storm is unique

All clouds are unique in the sense that their details don't ever repeat exactly, but the similarities predominate within each cloud type. Thunderstorm clouds extend vertically as well as horizontally and this produces an almost unlimited number of possible shapes, sizes, and colours to observe. Moisture affects instability, which affects cloud height, which is subjected to different winds at every level (wind shear), giving us a new result with each set of circumstances. And there are many other variables contributing to the outcome. Changing even one of them alters the recipe and the visible result. The atmosphere is always on the move, mixing and varying ingredients, forming each day's special environment to promote or deter a storm's development.

You can imagine a thunderstorm as a giant wringer that cycles huge volumes of air through itself, squeezing all the invisible moisture out, and depositing it along a narrow path. The powerful air motions and forces within the cloud initiate all the severe events we worry about. Thunderstorms may be responsible for danger and destruction, but the same mechanisms that give rise to tornadoes or hailstorms also distribute much needed moisture to dry regions, keep the daily heat balance regulated, and provide us all with an awe-inspiring,majestic example of one of Nature's great spectacles. We must pay a little for the privilege of a cool downpour on an otherwise hot summer afternoon or brilliant fireworks in the stillness of the night!

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