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August 2015

90-Degree Days in September

Hotweather.1940s

 

September is more likely than May to see 90-degree temperatures.  While May experiences a 90-degree day once every three years, September sees one about every two years.  The most such days in occurred in 1961, when there were eight, followed by seven in 1895, 1970 and 1983.  The most consecutive years to have a 90-degree reading in September is eleven, from 1936 to 1946.  (More recently, there was a seven-year streak between 2012-2018.)  The most consecutive years without one is six, from 1885 to 1890.  Finally, three days in September have never had a reading in the 90s: 9/28, 9/29 and 9/30 (9/24 was on this list until 2017). 

 

Only four of New York's ten warmest Septembers are among those with the most days in the 90s: 2015 (warmest September on record/six 90-degree days); 1961 (second warmest September/eight 90-degree days); 1931 (sixth warmest/five 90-degree days); and 1983 (tenth warmest/seven 90-degree days).  And one of the ten years with the most 90-degree days, 1988 (32 days), reported no 90-degree days in September.

 

Lastgaspofsummer

 

Finally, there have been five years with 90-degree readings in October - four were concentrated between 1938-1941, and the last time was in 2019.  Two of those years were back-to-back (1938 and 1939), and in both years there were no 90-degree days in September.  October 1941 had two 90-degree days, including the hottest October reading on record - 94° on 10/6.  In addition, there have been nine  years in which there were 90-degree days in the last ten days of September, most recently in 1980.

 

Most 90 Degree Days in September

 

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The 'Last Gasp' of Summer - Noteworthy Late Summer Warm Spells

Cool.dog

 

Late season heat waves can be great if your vacation happens to coincide with one of them, or if it occurs over the Labor Day weekend.  However, they can also be unwelcome, especially in years when a hot summer doesn't want to let go of its grip (as was the case in 1980, 1983, 1993 and 2010).  Since 1940 there have been more than two dozen of these last gasps of summer (of at least five days).  Some are distinguished by temperature, others by number of days.  For this analysis I discuss seventeen I found to be of greatest interest. (By "late season" I'm referring to any unseasonably warm spell that occurred between Aug. 20 thru about Sept. 20.) 

 

1947 (Aug. 22-26 and Sept. 7-15)

1947 is the only summer in this analysis with two late warm spells.  The first had an average high of 91°, which was nine degrees above average; the second's average high of 86° was ten degrees above average.  

1948 (Aug. 25-30)

This six-day sizzler merits attention because three of the days had highs in the triple digits and two others had highs of 95°.  With an average high/low of 97/76 this is the hottest of the late summer heat waves.

1953 (Aug. 24 - Sept. 5)

This one is the stand-out of all the late season hot spells - all twelve days were in the 90s, including six in a row with highs of 97° or hotter (two of which were in the triple digits).  This is also the longest heat wave in NYC history.

1959 (Aug. 25 - Sept. 9)

This sixteen-day stretch followed an eleven-day hot spell between Aug. 12-22 that ended when two days had unseasonably cool highs in the low 70s.  Then the heat returned beginning 8/25 and continued for sixteen days.  And there would be an early autumn warm streak of nineteen days from Sept. 22 thru Oct. 9.

1961 (Aug. 25 - Sept. 14) & 1964 (Aug. 22 - Sept. 11)

These two are grouped together because they were very similar.  Both lasted for 21 days, the lengthiest of any of the late summer warm spells.  And both had very similar dates.  1961's was a bit hotter than 1964's, with an average high/low of 89/71 versus 87/65; 1961 had nine 90-degree days while 1964 had seven.  However, unlike many of the periods of unseasonably warm weather, there were no highs hotter than 95° in either year.  1964's sultry period was notable because the first three weeks of August had been very cool, five degrees below average, with just one day reporting an above average mean temperature.  

1973 (Aug. 26 - Sept. 5)

Very similar to the late summer heat wave of 1953, this one featured eight days in a row with highs in the 90s, including highs of 98° on 8/28 and again on 8/30.  For the entire eleven-day stretch the average high was 93°.

1979 (Aug. 25 - Sept. 7)

Although three days were in the 90s, what was more striking was the twelve days in a row with lows in the 70s.  The average low during these days was 74°, nine degrees above average. This warm spell was also distinguished by having the smallest difference between the average high and low, twelve degrees.  The only late summer warm spell with a warmer average low would come six years later (but it lasted only five days).  This warm spell featured a cameo by the remnants of hurricane David, which dropped 1.29" of rain on the morning of 9/7.

1980 (Aug. 23 - Sept. 7)

Half of the sixteen days were in the 90s (five of which were 95+) and three other days reached 89°.  The average high for this stretch f days was 90°, ten degrees above average.  (Fifteen days after this hot spell ended the high reached 94° degrees on 9/22.)

1983 (Sept. 3-12)

The highlight was a high of 99° on 9/11, the hottest reading of a hot summer.  And the mercury hit 97° the day before.  Overall, these ten days had an average high of 91°, which was fourteen degrees above average.

2005 (Sept. 12-23)

This was one of the latest of the late incursions of summertime heat; it was also part of 35 days in a row with highs in the 80s that began Aug. 20.  This five-week period was also very dry, with just 0.33" of rain measured in Central Park.

2010 (Aug. 29 - Sept. 2)

The hottest summer on record extended into the first few days of September during this five-day streak of temperatures in the 90s.  And Sept. 3 would probably have reached into the 90s as well if not for cloud cover brought by hurricane Earl (the AM low was a sticky 75°).  Five days after this hot stretch ended saw two days with highs of 89° and 90°.

2014 (Aug. 29 - Sept. 5)

In 2014 New York didn't experience an extended period of hot weather until late in the summer.  Seven of the eight days saw highs of 88° or hotter, with four of them in the low 90s.

2015 (Aug. 30 - Sept 9)

This eleven-day period began with a five-day heat wave and ended with a three-day heat wave, which included the hottest reading of the year, 97°, on Sept. 8. 

2016 (Aug 26-31)

For the third year in a row late summer featured unseasonably warm weather.   Earlier this summer there were two five-day heat waves, and while this six-day period didn't meet the definition of a "heat wave" (i.e., every day has a high of 90+), these days were eight degrees above average.  

 

Summer

 

2018 (Aug. 27 - Sept. 6)

This eleven-day stretch was comprised of two four-day periods of hot weather at the beginning and end, with three close to average days in between.  Each four-day period had three days in the 90s and three days with lows of 75°+.  The average high/low of the entire eleven days was 88/75°, eight degrees above average.  The average low is the warmest of any late summer hot spell of ten days or longer.

 

 Chart - Hottest Late Summer

Chart - Last Gasp of Summer

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July 2015 Weather Recap: A Very Warm Second Half

 Fireworks2

 

  • Although July 2015 was warmer than average (+2.2 degrees), three of the Julys since 2010 were hotter.  Overall it was the 19th hottest July since 1900.  Only six days were cooler than average.  After being close to average during the first two-and-a-half weeks of the month, temperatures from July 19-31 were nearly five degrees above average.
  • The number of 90-degree days was five, which is below the average number of eight.  However, eight days had highs of 88° or 89°, which tied July 1994 for the greatest number of upper-80s highs (going back to 1970).
  • All but two days had highs of 80 or warmer (an average July has six days with highs below 80).  Since 1970 only three Julys have had more 80-degree days (2011, 1994 and 1983). 
  • It appeared the month was going to be the driest July since 2002, but then 1.95" of rain poured down during the afternoon of 7/30, which was as much rain as fell in the previous four weeks.  This was the greatest daily rainfall since 1/18, when 2.10" fell.  It was also the first rain to fall on 7/30 in fifteen years.
  • The month's hottest temperature was 96° on 7/29; the warmest low was 82° on 7/20.  At the other end of the temperature spectrum, July's coolest reading was 64° on 7/16; the coolest high was 75° on 7/4. 
  • Finally, although July 2015 and 2012 had the same mean temperature, their diurnal variations (i.e., the difference between high and low) were very different, with 2015's being 14.2 degrees and 2012's at 17.0.  Although this year's average high was 1.4 degrees cooler than July 2012, the average low was 1.4 degrees warmer.

 

JULY TEMPERATURE AVERAGES SINCE 2010
             
  Average   Degrees Highs of
  High Low Mean +/- 90+ 88/89
2015 85.9 71.7 78.8 +2.3 5 8
2014 83.5 68.8 76.1 -0.4 3 3
2013 86.3 73.4 79.8 +3.3 10 3
2012 87.3 70.3 78.8 +2.3 10 6
2011 88.9 71.6 80.2 +3.7 14 3
2010 90.1 72.6 81.4 +4.9 20 5

 

 

Summertime

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