September 2018 - A Month With Unusually Mild Nights

 

 

September 2018 was 2.7 degrees warmer than average.  Like August, its average low was considerably more above average (+4.4 degrees) than its average high (+1.0).  This resulted in a vast difference in their historical rankings, with the average high ranking 60th warmest while the average low was ensconced among the ten warmest, at #4 (overall, the month was the 21st warmest).  The month's other significant story was its rainfall, which amounted to 6.19" - the most in September in seven years.  Furthermore, this was the sixth month this year with more than five inches of precipitation, joining ten other years with this many or more.  And if that wasn't impressive enough, 2018 became just the second year to have these six months concentrated in the first nine months of the year (1998 was the other year). 

Additional points of interest:  

• The remnants of two hurricanes brought 1.38" of rain on 9/9-10 (Gordon) and 1.19" on 9/18 (Florence).  However, the month's biggest rainstorm was on 9/25 when 1.95" fell. Nearly half of that day's rain poured down in about an hour early in the afternoon.

• The first full week of the month (week of 9/3) had four days in a row with highs that ranged from 89° to 93°.  The average high/low of 91/76 during these days was twelve degrees above average.  The first day of this hot spell fell on Labor Day, which had a high in the 90s for just the fourth time since 1980. 

• This was the fourth September in a row with an average temperature warmer than 70.0 degrees, the longest such streak on record.  What's more, the month set a September record for the most days with lows of 67° or warmer (seventeen; the average number is six).  Despite this surfeit of warm nights, the number of days with highs of 80° or warmer was a bit below average (nine).

• The month's average diurnal variation (the gap between the average high and low) was the smallest for September in more than 100 years (since 1908).  In 2018 it was 11 degrees, well below the average of 16 degrees (based on the past 100 years).

• This was the record-tying seventh year in a row in which September experienced highs in the 90s, equaling in length the streak of 1940-1946.

• Like September, August's average low also ranked fourth (its average high ranked 32nd).

• Typically September is 2.5 degrees cooler than June, but this year it was one degree warmer.  This is the fifth time it's happened since 1980 (most recently in 2015).

 

 

Here are recaps of the four previous Septembers:

2017

2016

2015

2014

 

Comments

[Ken K. in NJ]

Hi Rob. Always look forward to these monthly summaries, thanks! Surely the mild nights were due to the consistently high humidity, limiting radiational cooling. I know there are not comprehensive or accurate humidity records, but it would seem that September 2018 would be near the top of a list of highest September average nighttime humidity.

One question: Were we anywhere near a record for total precipitation in July, August, and September? I know the total was well over 20 inches.

Thanks again. --Ken

[Rob]

Hi Ken,

Unless there's rain during the daytime, a day's highest humidity levels are usually in the overnight hours so I don't know much a factor the humidity was to the month's warm lows. Instead, I think persistent cloud cover as well as the jet stream played a role.

Regarding rainfall during July-Aug-Sept, this year's amount (22.23") ranks fifth for those three months (and 28th for any three-month period). The wettest July-Sept was in 2011 when 31.37" fell; #2 was in 1975 (24.14"); #3 occurred in 1971 (23.93"); and fourth place is occupied by 1933 (22.66"). This year's July-Sept bested 2004's amount by 0.05".

[William]

this month saw the end of a fifty-seven day streak with a low temperature of 65° or warmer (7/13 to 9/7), second only to 2005's such streak of fifty-eight days. then, just two days later (on 9/9), the high was just 62°. the last time any of September's first ten days had a high this chilly was in 1963 when the high was 56° on 9/6. this was the fifth September in a row with no lows in the forties, tying 2001 to 2005 as the longest such September streak on record. this month's twelve days of measurable rain was the sixth time, in just this century alone, that September had at least ten such days, four of which have occurred from 2000 to 2006. this was also the tenth rainiest fall month (Sep-Nov) since 1996.

[Biebs]

By my count we’re now up to 119 days with precipitation, 2 behind the record of 121 (set in 1950).