Extreme meteorological events heighten threats to life and property. Knowledge of expected frequency of such extreme events provides an important background for forecasters. Also, statements of potential for occurrence of extreme events are valuable to both public and private planners.
Annual extremes of temperatures (both maximum and minimum) and daily rainfall were compiled for several sites within the state of Missouri. These sites are served by a National Weather Service Forecast Office at either Kansas City, Springfield or St. Louis.
A theoretical Gumbel distribution was fitted to annual extreme data to estimate a probability that a given value will be equaled or exceeded within a specified period. Parameters of the fitted Gumbel distributions were estimated via the Method of Moments. Annual probabilities, return periods and median intervals were calculated for each meteorological variable. As an example, results for maximum temperature at St. Louis are given in Table 1.
Characteristics of the sampled data and variability in estimates are important in assisting users with the application of results. Indications of such variability were gained via calculation of annual probabilities, return periods, and median intervals for 10 random samples of 30-years annual maximum daily rainfall values at Springfield, Missouri. For example, the return period for having a maximum daily rainfall amount equal or greater than five inches ranged from 19.5 to 40.2 years. Similarly, the probability in one year ranged from .025 to .051, and the median interval ranged from 13.5 to 29.5 years.
Results provide useful information for the selected sites. Further, this study outlines a procedure that can be followed by newly-established offices to ascertain, or update, the statistical characteristics of extreme meteorological events within a local area of warning and forecast responsibility.
Table 1.
Frequency Statistics for Annual Maximum Temperature at St. Louis, Missouri. Period of record is 1946 -1990.
| Annual Maximum Temperature (o F) | Annual Probability | Return Period (years) | Median Interval (years) |
| 96 | .869 | 1.2 | 0.8 |
| 98 | .682 | 1.5 | 1.0 |
| 100 | .476 | 2.1 | 1.5 |
| 102 | .306 | 3.3 | 2.3 |
| 104 | .186 | 5.4 | 3.7 |
| 106 | .110 | 9.1 | 6.3 |
| 108 | .063 | 15.8 | 11.0 |
| 110 | .036 | 27.5 | 19.1 |
| 112 | .021 | 48.4 | 33.5 |
| 114 | .012 | 85.4 | 59.2 |
| 116 | .007 | 150.9 | 104.6 |
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