Indian Summer is defined as a warm spell that is preceded by at least one killing freeze. Since most of Southwest Lower Michigan has already had a killing freeze, our next warm spell would qualify as Indian Summer. Since October 1st, the warmest high temperature at Grand Rapids and Lansing has been 61 degrees, at Muskegon it was 60 degrees. Looking at Grand Rapids records going back to 1892, there has never been an October where the high temperature has failed to reach 70 degrees at least once.
The most recent October that Grand Rapids did not reach 70 degrees in the first 20 days of the month was in 1986. In that year it took until the 21st to reach 70 degrees. October of 1977 also did not reach 70 degrees at Grand Rapids until the 21st. In 1902, it took until October 24th before reaching 70 degrees. In 1917, it hit 70 degrees only once (October 18th). Then, the temperature never got out of the 40s for the rest of the month. The high temperature at Grand Rapids on Halloween that year was 32 degrees with 1.5 inches of snow!
Lansing has had only two Octobers on record when it failed to reach 70 degrees: 1981 (highest temperature 67 degrees on the 31st) and 1917 (highest temperature 69 degrees on the 18th). Muskegon, being closer to Lake Michigan, has experienced much colder Octobers. In 1917, for example, it never warmed above 58 degrees. More recently in 1987, it never got above 67 degrees. Other than that, Muskegon has reached 70 degrees in every October since 1988.
Our extended forecast does not include any 70 degree temperatures through the 21st of this month, but lower 60s are expected from the 19th through the 21st.