September 2021 Weather Recap: A Record-Setting 3rd Month in a Row with 10+ Inches of Rain
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:
During Hurricane Ida, a vicious tornado outbreak hit south of New York. My sister moved out of southern New Jersey to Kansas in late July, so she managed to avoid the worst tornado in the outbreak. An EF3 wedge tornado brought havoc on the county where she lived. While New York was drenched with seven inches of rain, it was spared from any tornadoes.
Posted by: Henry | 10/05/2021 at 11:13 AM
One thing I was curious about and had hoped to see mentioned in this recap: how did this Summer stack up against the rainiest Summers in NYC history? With the 3rd, 4th, and 6thd rainiest July, August and September I would assume it would be up near the top.
Posted by: Richard Arrigo | 10/05/2021 at 09:34 PM
With 24.03" of rain measured, the summer of 2021 became NYC's second wettest, behind 2011 (25.23"), and ahead of 1975 (22.40").
Posted by: Rob | 10/05/2021 at 11:53 PM
with a combined total of 31.44” of rain, this was the wettest Jul-Aug-Sep combination ever, beating 2011 by 0.07” and 1975 by 7.30”. all but three of the years since 2012 (the exceptions are 2018, 2020 and this year) have had less than a foot of rain in Jul-Aug-Sep combined.
Posted by: William | 10/06/2021 at 03:09 AM
Thanks for the reply. I was wondering though, unless I'm reading it incorrectly something about the math doesn't seem to add up. The recap states there was 10.32" 0f rain in August, 11.09" in July and 7.13" on 9/1, which would add up to over 28" all on it's own, without even counting the end of June or the rest of September up til the 21st. And yet in your reply you say this Summer only had 24.03" of rain measured, how do you account for the seeming discrepancy?
Posted by: Richard Arrigo | 10/06/2021 at 02:55 PM
Ok, I was able to answer my own question by Googling, I now see that meteorological Summer is counted from the beginning of June through the end of August, pardon my ignorance. I'm still curious how this past astronomical Summer ranked in NYC history as far as rain totals.
Posted by: Richard Arrigo | 10/06/2021 at 03:07 PM
Astronomical summer 2021 ranks as the rainiest on record, with nearly two inches more rain than 2011, which is ranked second.
Posted by: Rob | 10/07/2021 at 12:10 AM
the 16.19" of rain that fell from Aug. 19 to Sep. 1 is the wettest two week period ever.
Newark had more rain on Sep. 1 of this year alone than in all but three individual September calendar months. this was also Newark’s wettest September ever (after having its tenth wettest August and third wettest July), with 10.50” measured. its previous wettest September was 10.28” in 1944.
Posted by: William | 10/17/2021 at 03:03 AM
The information in the link below explains why 2021 was an exceptionally wet year in New York City.
Posted by: William | 11/10/2021 at 06:24 PM