The average first true Fall Day with high less than 65 at Greater Lafayette (records back to 1864) is September 23.
When we look at the top 10 earliest & latest first occurrences of the first Fall day (after July 25)....it varies, but the meat of the days are in mid- to late September between September 21-27.
Two days could nearly account for the earliest first Fall day. However, I made it after July 25. These two coolest high temperatures before July 25, recorded 1864-present, were 63 on July 16, 1863. This was remarked about in local press about just how cool it was for a mid-July day. Now, my Greater Lafayette record dataset starts in 1864 due to chunks of missing data 1860 & 1862-63. Nonetheless, this was a remarkable high with a dark, low stratus overcast all day after 1.25” rain & 60s on July 15. Overnight lows were mid 50s to 60s, however & it was 72 on July 17.
Second coolest July day on record was 64 on July 22, 1869 with low, dark, gray overcast & lows mid- to upper 50s. This cool high followed an incredible 9.10” of rainfall June 27 to July 16. 1869 saw a remarkably cool, cloudy, very wet Summer with flooding. It had with similarities to 1875, but was cooler. The 1869-70 Winter was one of the snowiest on record with snow into April & the greatest October snowstorm on record occurred with 9” on October 23 & an extreme low of 7 there after.
The first truly Fall day in 1930 did not occur until October 18. It is particularly interesting that 5 of the last 15 first Fall days (in all of the 1864-present data set) have been in the top 10 latest. Taking the last 23 years & 7 of those first Fall days were in the top 10 latest, signaling a trend of later arrival of true Fall coolness.
A later than average first Fall day is not the best indicator of a very warm Fall or Winter.
However, a very early occurrence of <65F is a better indicator of a more harsh winter season. The 1915-16, 1917-18, 1963-64 & 1911-12 winters were all brutal with heavy snows, even blizzards......& bitter cold. 1866-67 Winter was very snowy.
It seems the farther you get away from late August to early September with the first <65 day, the less the correlation to a rough winter.
1918 saw the first <65 September 4 & the 1918-19 winter was very mild. The 1974-75 winter (first <65 September 2) was a windy, stormy, but overall mild winter.
1906 saw a late first cool day, but that first high was only 44. It was 39 on October 10th! This, after 77 on October 8!
Record accumulating snowfall occurred on the 10th with much of the foliage still green. An incredible low of 20 followed, causing a lot of the foliate to brown & fall highly prematurely.
1930-31 was a mild winter with a tie for the latest first cool Fall day on record.
However, the 1939-40 was rough with a very late first cool day for Fall.
1884-85 winter was one for the ages with record cold month after month & record snowfall, but a late occurrence of the first Fall-like day at October 9.
The Winter of 1875-76 was one of the warmest on record with a record 72 on New Year’s Day (when EF2 tornado hit Springfield, Illinois & t’storms occurred in the area!)
So when do the colder nights typically arrive? Well, below are the dates of the earliest low temperature occurrences of 45 all the way to to -20.
Earliest occurrences of 45, 40, 36, 32, 28, 24, 20, 16, 10, 0 to -20s at Southeast Lafayette (1864-78), Purdue (1879-1943) & the Purdue Airport (1944-2026) after July 30:
Earliest 45: August 4, 1963
40: August 14, 1964
36: September 8, 1956
32: September 14, 1881
30: September 23, 1995
28: September 29, 1871
24: September 30, 1899
20: October 11, 1906
16: October 24, 1869
10: October 24, 1869
0: November 18, 1880
-0: November 18, 1880
-10: November 19, 1880
-15: November 22, 1880
-20: December 18, 1884
Below are the dates of the earliest high temperature occurrences of 65 all the way to to -0s.
Earliest Highs of 65, 60, 55, 50, 45, 40, 35, 32, 30, 25, 20, etc. to -0s at Southeast Lafayette (1864-1878), Purdue (1879-1943) & the Purdue Airport (1944-2026) after July 30:
Earliest 65: August 1, 1875
Earliest 60: August 1, 1875
Earliest 55: September 12, 1902
Earliest 50: September 20, 1866
Earliest 45: October 10, 1906
Earliest 40: October 10, 1906
Earliest 35: October 23, 1869
Earliest 32: November 3, 1951
Earliest 30: November 3, 1951
Earliest 25: November 4, 1991
Earliest 20: November 12, 2019
Earliest 15: November 21, 1880
Earliest 10: December 4, 1871
Earliest -0: December 9, 1876
Here are the average first dates of various cool to cold temperatures in our forecast area
First 45:
Normal September 10-22:
40:
September 20-Oct. 1
36:
September 28-October 12:
32:
October 10-20:
28:
October 25-November 1:
24:
November 6-13:
20:
November 20-25:
16:
December 5-11:
10:
December 15-21:
0:
