We often get questions about the three categories of risk defined by the Storm Prediction Center (SPC) in their daily convective outlooks. With some active weather expected today, this might be a good time to go over what those risks actually mean…
SLIGHT RISK: Implies well-organized severe thunderstorms are expected but in small numbers and/or low coverage. Within a slight risk area, 5-29 reports of 1 inch of larger hail, and/or 3-5 tornadoes, and/or 5-29 wind events are forecast.
MODERATE RISK: Implies a greater concentration of severe thunderstorms, and in most situations, greater magnitude of severe weather. Within a moderate risk area, at least 30 reports of hail 1 inch or larger, or 6-19 tornadoes, or numerous wind events (30 that might be associated with a squall line, bow echo or derecho) are forecast.
HIGH RISK: Almost always means a major severe weather outbreak is expected, with great coverage of severe weather and enhanced likelihood of extreme severe (i.e., violent tornadoes or extreme convective wind events over a large area). Within a high risk area, expect at least 20 tornadoes with at least 2 of them rated F3+, or an extreme derecho causing 50+ widespread wind events (50+) with numerous higher end wind (80+ mph) and structural damage reports.
The report criteria for each of those risks is valid for an area the size of Oklahoma without the panhandle, or about 50,000 square miles. As the size of the risk area increases (decreases) from 50,000 square miles, those expected severe weather numbers would increase (decrease) proportionally. Much of Alabama is in a moderate risk today, with the greatest risk posed by strong straight line winds and large hail. We will be watching the sky closely.
SLIGHT RISK: Implies well-organized severe thunderstorms are expected but in small numbers and/or low coverage. Within a slight risk area, 5-29 reports of 1 inch of larger hail, and/or 3-5 tornadoes, and/or 5-29 wind events are forecast.
MODERATE RISK: Implies a greater concentration of severe thunderstorms, and in most situations, greater magnitude of severe weather. Within a moderate risk area, at least 30 reports of hail 1 inch or larger, or 6-19 tornadoes, or numerous wind events (30 that might be associated with a squall line, bow echo or derecho) are forecast.
HIGH RISK: Almost always means a major severe weather outbreak is expected, with great coverage of severe weather and enhanced likelihood of extreme severe (i.e., violent tornadoes or extreme convective wind events over a large area). Within a high risk area, expect at least 20 tornadoes with at least 2 of them rated F3+, or an extreme derecho causing 50+ widespread wind events (50+) with numerous higher end wind (80+ mph) and structural damage reports.
The report criteria for each of those risks is valid for an area the size of Oklahoma without the panhandle, or about 50,000 square miles. As the size of the risk area increases (decreases) from 50,000 square miles, those expected severe weather numbers would increase (decrease) proportionally. Much of Alabama is in a moderate risk today, with the greatest risk posed by strong straight line winds and large hail. We will be watching the sky closely.
on April 21, 2005, 10:14 pm
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