Summer-like temperatures prevailed across central Illinois for much of the first half of April. Temperatures averaged 10-15 degrees above normal through April 15th with many days seeing highs in the 80s and lows in the 50s. These readings were more typical of mid-June!
Normal highs for early April are in the upper 50s and lower 60s, while lows are typically in the upper 30s and lower 40s.
The reasons for the extended period of warm weather are a result of a "blocked" weather pattern which allowed high pressure at the surface and aloft. Several days of southerly to southwesterly winds allowed warm air to move into the region, and plenty of sunshine acting on relatively dry air produced further warming of the air mass.