Some SPC Watch Statistics

The numbers below are for 20 years of watches (1980-1999). Watches with no reports are included in the averages and probabilities. First, all reports:

                         WATCH TYPE
                         TOR     SVR
Average # of tornadoes:  2.2     0.5
Average # of wind:       7.1     6.0
Average # of hail reps:  6.3     3.8

                                      TOR  SVR
Probability of 1 or more tornadoes:   56.1 22.7
Probability of 1 or more wind events: 75.5 72.5
Probability of 1 or mail hail reps:   67.0 62.1

Probability of 5 or more tornadoes:   15.1 1.8
Probability of 10 or more wind:       22.9 19.1
Probability of 10 or more hail reps:  22.4 11.7

Conclusion: Tornadoes are much more likely in tornado watches than severe thunderstorm watches.

Next -- same thing but for significant reports (2" or larger hail, 65 kt or greater wind speed reported, and F2 or stronger tornadoes):

                        TOR  SVR
Average # of tornadoes: 0.5  0.1
Average # of wind:      0.4  0.3
Average # of hail:      0.7  0.3

                                    TOR  SVR
Probability of 1 or more tornadoes: 21.2  4.8
Probability of 1 or more wind:      19.5 15.5
Probability of 1 or more hail:      25.0 13.9

Probability of 5 or more tornadoes:  2.5  0.2
Probability of 10 or more wind:      0.5  0.3
Probability of 10 or more hail:      0.7  0.1

In summary: tornado probabilities are much higher in tornado watches than severe thunderstorm watches, as are the probabilities of any type of significant severe report (most notably tornadoes and large hail).

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