Finley (1884) Tornado Forecasts

This is a classic example used in many textbooks, papers, and talks on forecast verification to illustrate the characteristics of the various categorical verification scores (e.g., Murphy, 1996; Stephenson, 2000).

In March 1884 Sergeant John Finley initiated twice daily tornado forecasts for eighteen regions in the United States, east of the Rocky Mountains. Finley claimed 95.6% to 98.6% overall accuracy for the first 3-month period, depending on the time and district, with some districts achieving 100% accuracy for all 3 months. A critic of the results pointed out that 98.2% accuracy could be had by merely forecasting "no tornado"! This clearly illustrates the need for more meaningful verification scores.

The contingency table for Finley's (1884) forecasts is:
    tornado no tornado  Total
Forecast tornado 28 72 100
  no tornado 23 2680 2703
Total   51 2752 2803

The standard categorical verification scores are computed and interpreted below:

Accuracy (fraction correct)accuracy for finley tornado forecasts    Overall, 96.6% of the forecasts were correct.

Bias score (frequency bias)bias score for finley tornado forecasts      Tornados were predicted roughly twice as often as they occurred.

Probability of detection (hit rate)Probability of detection for finley tornado forecasts     Slightly more than half of the tornados that occurred were correctly predicted to occur.

False alarm ratioFalse alarm ratio for finley tornado forecasts      72% of the forecasts for tornados turned out to be false alarms (no tornado occurred).

Threat score (critical success index)Threat score for finley tornado forecasts     Of the tornado events that were either forecast or observed, 23% of those were correctly forecast.

Equitable threat score (Gilbert skill score) -Equitable threat score for finley tornado forecasts
where Random hits for finley tornado forecasts         Since one could expect to get very few positive tornado forecasts correct due to random chance (only about 2 out of a total of 2803 forecasts), the ETS is almost the same as the TS.

Hanssen and Kuipers discriminant (true skill statistic, Pierce's skill score)Hanssen & Kuipers score for finley tornado forecasts

The forecasts were 52% able to separate the "yes" cases from the "no" cases. Because the correct negatives term dominates the others in the contingency table, the HK tends toward the POD when "yes" events are rare.

Heidke skill scoreHeidke skill score for finley tornado forecasts

There was a 36% improvement in forecast accuracy when compared to random chance.

Odds ratioOdds ratio for finley tornado forecasts     The odds were roughly 45 to 1 that a forecast for a tornado would be a hit as opposed to a false alarm.


Finley, J.P., 1884: Tornado predictions. Amer. Meteor. J., 1, 85-88.
Murphy, A.H., 1996: The Finley affair: A signal event in the history of forecast verification. Wea. Forecasting, 11, 3-20.
Stephenson, D.B., 2000: Use of the "odds ratio" for diagnosing forecast skill. Wea. Forecasting, 15, 221-232.