September 2021 will long be remembered for the torrential rain that fell the night of 9/1, submerging streets, highways, subway tunnels and residences. Nearly 50 deaths in the metro area were attributed to the storm (13 of them were in NYC). This unprecedented deluge was produced by the remnants of hurricane Ida pushing up against a mass of cool air. Between 5 PM and 1 AM, a little more than seven inches of rain poured down (with three inches falling between 9-10 PM).
Although heavy rain had been predicted, its intensity in such a brief period of time took residents by surprise. (And although hurricane Henri, two weeks earlier, produced an inch more of rain than Ida, it fell over the course of 48 hours).
Never before had a month started with so much rain, easily breaking the previous record amount on the first day of a month, 4.98" on Oct. 1, 1913. The 7.13" measured on 9/1 was the fifth greatest amount to fall on a calendar date. (Earlier this year, February began with a record amount of snow for the first day of any month, 14.8".)
The rest of the month had slightly below average rainfall, but just enough to bring September's total to 10.03". This was the most rain to fall in September since 2004, and it became the sixth rainiest September on record. It followed the fourth wettest August (10.32"), and third wettest July (11.09"). Never before had Central Park had three months in a row in which each month had more than ten inches of rain.
More than 90% of the month’s of rain was produced by two storms: 7.23” on 9/1-2 and 2.03” on 9/23-24. 0.77” fell on the other 26 days.
Looking at temperatures, September was 1.1 degrees milder than average, and was the 26th mildest September (tied with four other years). However, it ranked 13th for mildest average low, and 55th for warmest high. 12 days in a row (Sept. 12-23) had above average temperatures, a streak of days that was four degrees above average. The rest of the month was one degree cooler than average.
The warmest reading was 85° (on 9/15), the same warmest reading as last September (but it had two days with that high). Also, both Septembers had seven days with highs in the 80s (the average is eleven).
Half of the days in September had highs between 75°-79°, which was double the average number - and the most to be in this narrow temperature range on record (second most is 13 days in Sept. 1999, 1927 and 1908). Just five days had lows cooler than 60°, eight fewer than average. Only Sept. 1971, 1930, 1908, and 1881 had fewer days (all had four).
Finally, the month was also characterized by a narrow range in its temperature extremes. Typically, September's chilliest reading is around 50°, the hottest around 90°, but this September it ranged from 54° (on 9/29) to 85° (on 9/15). Only two other Septembers have had a range that was smaller. (The greatest range in temperatures in September was 55 degrees in 1929, ranging from 44° to 99°.)
Here are previous September recaps: