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In Canada, a severe thunderstorm is defined as having wind gusts of greater than 90 km/h (56 mph), hail with a diameter of greater than two centimetres (0.79 in), rainfall rates of greater than 50 millimetres (2.0 in) in one hour or greater than 75 millimetres (3.0 in) in three hours, or tornadoes. [14]
The scale of dBZ values can be seen along the bottom of the image. dBZ is a logarithmic dimensionless technical unit used in radar. It is mostly used in weather radar, to compare the equivalent reflectivity factor (Z) of a remote object (in mm 6 per m 3) to the return of a droplet of rain with a diameter of 1 mm (1 mm 6 per m 3). [1]
One can distinguish three stages in the evolution of a thunderstorm cell: [2] [3] formation: the upward current of the cell intensifies and allows the condensation of water vapor from the rising air parcel. This forms a cumulus congestus, then a cumulonimbus when ice crystals form at its apex which spreads horizontally in contact with the ...
A severe thunderstorm warning is issued if a thunderstorm becomes severe, or will soon turn severe. In Canada, a rainfall rate greater than 50 millimetres (2 in) in one hour, or 75 millimetres (3 in) in three hours, is also used to indicate severe thunderstorms. [38] Severe thunderstorms can occur from any type of storm cell.
A shelf cloud, such as this one, can be a sign that a squall is imminent.. A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more.
"At 8:03 p.m., severe thunderstorms were located along a line extending from near Newport to near Poolville, moving east at 25 mph," says the NWS. "Hail damage to vehicles is expected. Expect wind ...
Cumulonimbus storm cells can produce torrential rain of a convective nature (often in the form of a rain shaft) and flash flooding, as well as straight-line winds. Most storm cells die after about 20 minutes, when the precipitation causes more downdraft than updraft , causing the energy to dissipate.
This is because there could be severe hail from a strong updraft that lasts only a short period of time, with damaging winds. Rain is an important impact of such systems. The speed and direction at which the entire cluster of thunderstorms move downstream make the difference in the amount of rain received in any one location.