Probability Categories
Five categories of temperature and precipitation departures have been defined by the National Climatic Data Center (NCDC) and are in widespread use. The table shows each category and the class limits for the Zscore (or standardized departure from average) categories.
Category

ZScore

Much Above Normal


Z

> 1.282

Above Normal

.524 <

Z

<= 1.282

Normal

0.524 <=

Z

<= 0.524

Below Normal

1.282 <=

Z

< 0.524

Much Below Normal


Z

< 1.282

Temperature Categories Used for Growing Season Calculations
Monthly and annual temperatures are usually well represented by the normal probability distribution; therefore, the Zscore was used to classify, by category, the growing season length. The growing season Zscore is calculated as z(i) = (T(i)  T(avg))/s, where T(i) is the growing season length associated with a given Zscore, z(i), T(avg) is the mean annual growing season length over the selected period (e.g., 19712000), and s is the standard deviation of the annual growing season lengths over the selected period (e.g., 19712000).
For example, Much Above Normal would represent any amount greater than a 1.282 standard departure above the mean. In a normal distribution, the Normal category will contain 40% of the values. The Above Normal and Below Normal categories will each contain 20% of the values, and the Much Above and the Much Below categories will each contain 10% of the values.
The 30% category shown in the WETS Table represents the class limit values associated with the NORMAL category Zvalues of 0.524 and 0.524.
Precipitation Category Definitions
The same Zscore categories apply to precipitation, however, monthly and annual precipitation exceedance probabilities are calculated from fitting the observed monthly data to a twoparameter gamma probability distribution.
The twoparameter gamma distribution is asymmetrical and is used with continuous random variables such as precipitation. Its probability density function has a lower limit of 0 and an upper limit of infinity. The distribution was fit using the method outlined by the Soil Conservation Service (1985).