Today, NOAA researchers and meteorologists work hard to issue life-saving forecasts and tornado watches and warnings. Nearly 150 years ago, their predecessors were just beginning to figure out the “rules” for tornado forecasts and when tornados might form.
A map from John P. Finley’s booklet, “Tornadoes. What they are and how to observe them; with practical suggestions for the protection of life and property.” showing the path of the Enigma outbreak of tornadoes on February 19, 1884. (Image credit: Public Domain)
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In 1882, Sergeant John Park Finley of the U.S. Army Signal Corps’ Weather Bureau pioneered experimental tornado forecasts. During the Enigma tornado outbreak on February 19, 1884, he was able to establish rules for early tornado forecasting, which were updated and published in his winning essay for an American Meteorological Journal contest in 1888.
These are Finley's Rules:
1. Presence of a well-defined low pressure area.
2. Slow progression of the low increasing flow northward of heat and moisture into the southeast quadrant.
3. A north-south or northeast-southwest orientation of a trough-like low.
4. The descent of a well-marked anticyclone in the rear of the low.
5. High temperature gradients.
6. Increasing wind velocities of the southeast, southwest, and northwest quadrants of the low.
7. Northward curve of the isotherms in the southeast quadrant and eastern portion of the southwest quadrant of the low.
8. Southward curve of the isotherms in the northwest quadrant and the northern portion of the southwest quadrant
9. High temperature gradient between the noses of opposing curves of temperature.
10. Increasing high humidity in the southeast quadrant of the low.
11. Maximum areas of tornado frequency for each state.
12. Occurrence of tornadoes in certain parts of the country, in certain months of the year.
13. Tornadoes frequently occur in groups with parallel paths, within a few miles of each other.
14. Tornadoes always occur in the southeast quadrant of a low several miles southeast of its center.
15. Easterly curve in the southwest and northwest quadrants of a line separating the northerly and southerly surface winds of the low.
Finley’s work continued until 1885, when the chief signal officer of the U.S. Army Signal Corps banned the use of the word “tornado”in forecasts for fear of inciting public panic. Finley’s forecasting work for the Weather Bureau abruptly ended and the ban continued for several decades.
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