Thundercast - Non-Severe Thunderstorm Risk Categories
There are 4 risk categories for the non-severe thunderstorm forecast:
1. Isolated risk = less than 15% chance of hearing thunder
2. Small risk = 15-40% chance of hearing thunder at any given point
3. Moderate risk = 40-70% chance of hearing thunder at any given point
4. High risk = 70-100% chance of hearing thunder at any given point
Details:
An isolated risk is typically issued for situations where the risk for thunder is very low. Types of situations when this may be issued:
1. Isolated embedded thunder in an area of rain (isolated thunder in synoptic (large scale) rain events for example)… This is the most common scenario for an isolated risk to be issued.
2. Very isolated and slow-moving pop-up thundershowers.
Small, moderate and high risks are issued for events that are more widespread in nature. When a high risk is issued, this means it is almost a guarantee that thunderstorms will occur in the area.
Note that it is more difficult for higher risks to be issued in the day 3 and day 2 forecasts because there is more uncertainty than in the day 1 forecast. As a result, high risks in the day 3 forecast are rare and only occur a few times a year. High risks become more frequent in the day 2 forecast and are most frequent in the day 1 forecast when there is more certainty. As a result, moderate to high risks issued in the day 3 forecast indicate more confidence than usual that storm activity will occur in the highlighted area.
1. Isolated risk = less than 15% chance of hearing thunder
2. Small risk = 15-40% chance of hearing thunder at any given point
3. Moderate risk = 40-70% chance of hearing thunder at any given point
4. High risk = 70-100% chance of hearing thunder at any given point
Details:
An isolated risk is typically issued for situations where the risk for thunder is very low. Types of situations when this may be issued:
1. Isolated embedded thunder in an area of rain (isolated thunder in synoptic (large scale) rain events for example)… This is the most common scenario for an isolated risk to be issued.
2. Very isolated and slow-moving pop-up thundershowers.
Small, moderate and high risks are issued for events that are more widespread in nature. When a high risk is issued, this means it is almost a guarantee that thunderstorms will occur in the area.
Note that it is more difficult for higher risks to be issued in the day 3 and day 2 forecasts because there is more uncertainty than in the day 1 forecast. As a result, high risks in the day 3 forecast are rare and only occur a few times a year. High risks become more frequent in the day 2 forecast and are most frequent in the day 1 forecast when there is more certainty. As a result, moderate to high risks issued in the day 3 forecast indicate more confidence than usual that storm activity will occur in the highlighted area.