Severe Weather Information

Transcription

1 Severe Weather Information

2 TORNADO FACTS Canada gets more tornadoes than any other country with the exception of the United States. Tornadoes are rotating columns of high winds. Sometimes they move quickly (up to 70 km/hour) and leave a long, wide path of destruction. At other times the tornado is small, touching down here and there. Large or small, they can uproot trees, flip cars and demolish houses. Tornadoes usually hit in the afternoon and early evening, but they have been known to strike at night too. Warning signs include: Severe thunderstorms, with frequent thunder and lightning An extremely dark sky, sometimes highlighted by green or yellow clouds A rumbling sound or a whistling sound. A funnel cloud at the rear base of a thundercloud, often behind a curtain of heavy rain or hail. SEVERE STORMS INDICATORS Many summer severe weather days begin with cumulus cloud that may develop as early as am. A general "rule of thumb" suggests that cumulus cloud forming during the early morning hours may produce thunderstorms during the afternoon. Cumulus clouds are characterized by small to medium vertical extent caused by convection with consistent flat bases sometimes darkened or shadowed by the clouds above them. These are quite often called "popcorn cumulus" for obvious reasons. In this picture you can see both cumulus clouds and higher cirrus clouds. As daytime heating occurs, convection increases causing cumulus to grow into taller towers. Not all cells grow at the same rate as you can clearly see in this image. The type of terrain beneath the cloud will often impact on it's growth. Rapid growth can be a sign that the atmosphere is ripe for thunderstorm develop. The cumulonimbus cloud (CB). This is the cloud that spotters normally watch for knowing it may or may not produce severe weather. Note the cauliflower appearance near the top. This type of crunchy detail is common with rapid growth CB s. Also note how the anvil has reached the level of the tropopause and has spread outward forming a large flange. Some thunderstorms do not possess enough vertical strength to punch through the tropopause and subsequently spread the energy sideways. Ones that do can often go on to produce severe weather events.

3 In this picture the main cell at the rear of the picture has developed quickly to it's full potential. It may continue this stage for quite some time, or it may collapse. Note the towering cumulus in the foreground. Sometimes a collapse of one storm can help propagate another. The towering cumulus in the foreground may grow on it's own and possibly join forces with the one in the background, or it may develop rapidly if the rear storm collapses sending large down drafts in the direction of the new storm. Note the remarkable similarity in this image to the Doswell visual schematic (Doswell, 1985) of a tornadic thunderstorm shown on the right. Study the features of these images and try and relate them to storms you may encounter. This shot from north-central Texas (looking WNW) again demonstrates large storm cell features. Rain-free base to the left...heavy rain and hail area to the right. The intersection of the rain and rain-free base is quite often where wall clouds will form directly below the tallest towers. To the left of the image can be seen flanking clusters of cumulus cloud and bright sunshine off to the west. Tornadoes are relatively common in Canada, but only in specific regions: southern Alberta; Manitoba and Saskatchewan; southern Ontario; southern Quebec; the interior of British Columbia; and western New Brunswick. Tornado season extends from April to September with peak months in June and July, but they can occur at any time of year.

4 ANATOMY OF A SUPER CELL Super cells derive their rotation through tilting of horizontal vorticity (an invisible horizontal vortex) caused by wind shear. Strong updrafts lift the air turning about a horizontal axis and cause this air to turn about a vertical axis. This forms the deep rotating updraft, the meso cyclone. A cap or capping inversion is usually required to form an updraft of sufficient strength. The cap puts an inverted (warm-above-cold) layer above a normal (cold-above-warm) boundary layer, and by preventing warm surface air from rising, allows one or both of the following: Air below the cap warms and/or becomes more moist Air above the cap cools This creates a warmer, moister layer below a cooler layer, which is increasingly unstable (because warm air is less dense and tends to rise). When the cap weakens or moves, explosive development follows. Wind shear (red) sets air spinning The updraft (blue) 'bends' the (green) spinning air upwards The updraft starts rotating with the spinning column of air The current conceptual model of a supercell was described in Severe Thunderstorm Evolution and Mesocyclone Structure as Related to Tornadogenesis by Leslie R. Lemon and Charles A. Doswell III.

5 CANADA S TORNADO WARNING SYSTEM Environment Canada is responsible for warning the public when conditions exist that may produce tornadoes. It does this through radio, television, newspapers, its internet site, as well as through its weather phone lines. If you live in one of Canada's high-risk areas, you should listen to your radio during severe thunderstorms. If you hear that a tornado warning has been issued for your area, find shelter and follow the instructions below. WHAT TO DO DURING A TORNADO? If you are in a house Go to the basement or take shelter in a small interior ground floor room such as a bathroom, closet or hallway. If you have no basement, protect yourself by taking shelter under a heavy table or desk. In all cases, stay away from windows, outside walls and doors. If you live on a farm Livestock hear and sense impending tornadoes. If your family or home is at risk, the livestock will be a non-issue. If your personal safety is not an issue, you may only have time to open routes of escape for your livestock. Open the gate, if you must, and then exit the area in a tangent direction away from the expected path of the twister. If you are in an office or apartment building Take shelter in an inner hallway or room, ideally in the basement or on the ground floor. Do not use the elevator. Stay away from windows. If you are in a gymnasium, church or auditorium Large buildings with wide-span roofs may collapse if a tornado hits. If possible, find shelter in another building. If you are in one of these buildings and cannot leave, take cover under a sturdy structure such as a table or desk. Avoid cars and mobile homes More than half of all deaths from tornadoes happen in mobile homes. Find shelter elsewhere, preferably in a building with a strong foundation. If no shelter is available, lie down in a ditch away from the car or mobile home. Beware of flooding from downpours and be prepared to move. If you are driving If you spot a tornado in the distance go to the nearest solid shelter. If the tornado is close, get out of your car and take cover in a low-lying area, such as a ditch.

6 In all cases Get as close to the ground as possible, protect your head and watch for flying debris. Do not chase tornadoes they are unpredictable and can change course abruptly. A tornado is deceptive. It may appear to be standing still but is, in fact, moving toward you. Acknowledgements WIKIPEDIA Supercells Government of Canada Environment Canada Saskatchewan Ministry of Health