WHAT TO WATCH FOR

Being prepared

Severe events may not be that common in some areas but we cannot take summer weather for granted. There has been a long history of surprises. Your best defence is to use a good set of eyes and an enthusiastic dedication to keep you tuned up on every potential storm day.

Check local and regional weather forecasts regularly.If Weatheradio service is available in your area,keep it tuned for watches and radar updates.Listen to amateur radio frequencies in use in your area (check your local club) or to predetermined CANWARN frequencies.After storm activity has begun, try locating radio stations to the west (or wherever the storms will come from) to monitor their local weather conditions.
And, of course, keep an eye on the sky!

A day to watch

To make a good storm you need the right ingredients, plus atmospheric instability. Ingredients include moisture (indicated by low clouds in the morning and/or many cumulus later), good heating (at least some sunshine), and a steady SW-SE breeze. Instability, which will assist vertical development in clouds,is usually provided by cooling at the mid-levels of the atmosphere and an upward push of air from near the surface brought on by an approaching cold front, low pressure trough,or smaller-scale disturbance. If the atmosphere is lacking in one of the ingredients, severe weather is unlikely.Dry days cause high-based, weaker storms;too much cloudiness may delay or even inhibit development;light winds all day may indicate a weak wind profile, no "push" to get things going, or poor vertical wind shear.

Once conditions are in place,the exact location of storm initiation is often determined by topography or by the presence of a boundar (where two different airmasses meet).There, updrafts get a little extra lift from the converging flows to help them break through the inversion. Fronts are obvious boundaries, but old outflow boundaries (even from the day before), valley winds, or lake breezes have all been shown to greatly increase the chance of a severe storm in that particular location. Even weak boundaries can be recognized by a small change in temperature and wind direction across them, and by the location of lines of larger clouds.

The most likely severe weather days will have two essential factors in place. First, winds up high are sufficiently strong (good vertical wind shear) to carry much of the leftover cloud matter and outflow well downwind and out of the way of warm air entering the system; and second, the atmosphere is unstable enough to permit very strong updrafts to rise rapidly (very buoyant) into colder air aloft. Conditions conducive to severe events vary greatly across the country and exceptions are common. In Ontario and Quebec, most spring severe storms accompany large lows sweeping up from the SW, while summer evere weather is mostly cold-frontal and occasionally along an active warm front. On the Prairies, storms precede cold fronts but also occur in pockets of instability or as isolated events.

Radio static

Many of you are no doubt aware of the crackling sounds on a.m. radio during thunderstorms.This noisy interference can become beautiful music if you train yourself to interpret the crackling and relate it to the lightning you see. Every crunch corresponds to a flash that just occurred somewhere in the area. The louder, the closer the flash was;the more frequent, the more intense the system. You can fine-tune your interpretation by matching crackles to flashes in the distance,and you will discover that long, uniform ones are more common in large, old storms with little new activity. By comparison, short, fast crackles represent quick bolts or sparks inside the cloud, both of which are associated with new, growing cells. Supercells can create deafening quantities of static,within which steady, long noises (from the large anvil) are complemented by many short zips and crackles from the constantly regenerating core of the storm. When visibility is poor or clouds are unclear, static can be the best indicator of faraway storm activity.

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Meteorological Service of Canada - Environment Canada - Government of Canada

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