Probabilites are computed for the amount of precipitation expected at different probability levels (0.1, 0.2, 0.4, 0.5, 0.6, 0.8, 0.9). If the January precipitation probability at 0.9 is 1.49 inches, this means that 9 out of 10 years will have 1.49 inches of precipitation, or less. Conversly, it also means that 1 out of 10 years will have more than 1.49 inches of precipitation.

Quintiles show expected precipitation values at 5 quintile levels:
Q1: 0 - 20%
Q2: 20 - 40%
Q3: 40 - 60%
Q4: 60 - 80%
Q5: 80 - 100%

The Q0 value corresponds to the minimum precipitation value, and the Q5 value corresponds to the maximum precipitation value.