Thundercast - Severe Thunderstorm Risk Categories
Marginal, slight, moderate and high risk categories are used to identify the likelihood of experiencing severe thunderstorms at a given location. These categories consider how widespread and intense severe thunderstorm activity is expected to be. These depend partially on the type of thunderstorms that are expected to develop (isolated pop-up storms, multicell complexes, supercells or squall lines for example).
Marginal severe risk: Typically issued for unorganized pulse thunderstorms that pop up and become severe only briefly and over a small area. Alternatively, it is a lower end risk that may be issued for an area where advancing thunderstorms are expected to be weakening and therefore, the severe risk would be more isolated in nature.
Slight risk: Typically used for organized severe thunderstorm events. Severe weather coverage is not exceptional but severe weather lasts longer and is more widespread than associated with a pulse storm. Squall lines, isolated supercells and multicell cluster events fit into this category. Types of conditions possible in these scenarios: large hail (nickel to golf ball sized, but locally/very briefly larger hail possible), strong wind gusts over 90 km/h, a tornado or two and flash flooding.
Moderate risk: Used for widespread and intense organized severe thunderstom events. Severe weather coverage is larger than in a slight risk scenario but not as large as in a high risk scenario. Types of thunderstorm events that would fit into this category: strong squall lines, bow echoes and supercells. Types of conditions possible in these scenarios: very large hail (tennis/baseball sized or larger), a significant likelihood of tornadoes and widespread damaging winds over 90 km/h with some gusts over 100 km/h. In some cases, a moderate risk may also be issued if flash flooding is likely in an area that is already experiencing significant flooding problems and cannot handle another deluge.
High risk: Only issued for day 1 forecasts due to the inherent higher uncertainty in the day 2 and day 3 forecasts. High risks are extremely rare and are typically issued for exceptional events featuring significant tornadic or widespread damaging wind events. They aren’t usually issued for large hail events because these are much more common than widespread tornadic and damaging wind events. The types of thunderstorms that may develop in high risk scenarios include: derechos and widespread super cellular events. Types of conditions possible in these scenarios: numerous tornadoes and/or widespread damaging winds over 100 km/h with some gusts over 120 km/h. It is quite difficult to predict these types of wind and tornado events, another reason why high risks are so rarely issued. Obviously, these types of events are just naturally extremely rare in Manitoba.
Marginal severe risk: Typically issued for unorganized pulse thunderstorms that pop up and become severe only briefly and over a small area. Alternatively, it is a lower end risk that may be issued for an area where advancing thunderstorms are expected to be weakening and therefore, the severe risk would be more isolated in nature.
Slight risk: Typically used for organized severe thunderstorm events. Severe weather coverage is not exceptional but severe weather lasts longer and is more widespread than associated with a pulse storm. Squall lines, isolated supercells and multicell cluster events fit into this category. Types of conditions possible in these scenarios: large hail (nickel to golf ball sized, but locally/very briefly larger hail possible), strong wind gusts over 90 km/h, a tornado or two and flash flooding.
Moderate risk: Used for widespread and intense organized severe thunderstom events. Severe weather coverage is larger than in a slight risk scenario but not as large as in a high risk scenario. Types of thunderstorm events that would fit into this category: strong squall lines, bow echoes and supercells. Types of conditions possible in these scenarios: very large hail (tennis/baseball sized or larger), a significant likelihood of tornadoes and widespread damaging winds over 90 km/h with some gusts over 100 km/h. In some cases, a moderate risk may also be issued if flash flooding is likely in an area that is already experiencing significant flooding problems and cannot handle another deluge.
High risk: Only issued for day 1 forecasts due to the inherent higher uncertainty in the day 2 and day 3 forecasts. High risks are extremely rare and are typically issued for exceptional events featuring significant tornadic or widespread damaging wind events. They aren’t usually issued for large hail events because these are much more common than widespread tornadic and damaging wind events. The types of thunderstorms that may develop in high risk scenarios include: derechos and widespread super cellular events. Types of conditions possible in these scenarios: numerous tornadoes and/or widespread damaging winds over 100 km/h with some gusts over 120 km/h. It is quite difficult to predict these types of wind and tornado events, another reason why high risks are so rarely issued. Obviously, these types of events are just naturally extremely rare in Manitoba.