Precipitation in the Chicago area is projected to increase in winter and spring over 21st century, but projections for summer and fall precipitation are less clear

Mean annual precipitation is projected to only increase by 0.7 inches under the GFDL A1FI scenario for the final 30 years of the 21st century compared to the 1971 to 2000 baseline. By contrast, annual precipitation is projected to increase under the PCM B1 scenario by an average of 3.8 inches. Changes in precipitation are projected to vary greatly by season. Both models project an increase in precipitation in winter and spring. They differ in projections for summer and fall. PCM projects an increase of 1.8 inches, while GFDL projects a decrease of 5.8 inches in summer. In the fall, PCM projects a decrease of -0.9 inches, while GFDL projects an increase of 1.4 inches.