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Table 2

SPI classification (McKee et al. 1993)

SPI valueClassCumulative probabilityProbability of event (%)
SPI ≥ 2.00 Extreme wet 0.977–1.000 2.3% 
1.50 ≤ SPI < 2.00 Very wet 0.933–0.977 4.4% 
1.00 ≤ SPI < 1.50 Moderately wet 0.841–0.933 9.2% 
−1.00 ≤ SPI < 1.00 Near normal 0.159–0.841 68.2% 
−1.50 ≤ SPI < −1.00 Moderate dry 0.067–0.159 9.2% 
−2.00 ≤ SPI < −1.50 Severe dry 0.023–0.067 4.4% 
SPI < −2.00 Extreme dry 0.000–0.023 2.3% 
SPI valueClassCumulative probabilityProbability of event (%)
SPI ≥ 2.00 Extreme wet 0.977–1.000 2.3% 
1.50 ≤ SPI < 2.00 Very wet 0.933–0.977 4.4% 
1.00 ≤ SPI < 1.50 Moderately wet 0.841–0.933 9.2% 
−1.00 ≤ SPI < 1.00 Near normal 0.159–0.841 68.2% 
−1.50 ≤ SPI < −1.00 Moderate dry 0.067–0.159 9.2% 
−2.00 ≤ SPI < −1.50 Severe dry 0.023–0.067 4.4% 
SPI < −2.00 Extreme dry 0.000–0.023 2.3% 
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