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August 2018

Lackluster Heat Waves

Lackluster

 

This post was inspired by this week's rather tepid four-day heat wave (Aug. 5-8, 2018), which had an average high of 90.8 degrees (with highs of 91°, 92°, 90° and 90°).  Of the close to 70 four-day heat waves that have occurred since 1870 (thru June 2021) only one, in July 1896, had a lower average high, 90.3 degrees.  (2018's low-grade heat wave matched one in August 2009 and would be matched again in June 2021.)  On average, four-day heat waves have had an average high close to 94°.  (The hottest four-day heat wave on record took place in the summer of 2010, when the high temperature from July 4-7 was 99.5°.)

 

Chart- most tepid 4-day heat waves

However, the story changes when low temperatures are included in the analysis.  For example, this August's heat wave had an average low of 75.0 degrees, which was 1.3 degrees warmer than the average four-day streak and warmer than two-thirds of the four-day heat waves examined.  When combined with the average high, the mean temperature ranked as 27th coolest - quite a difference in ranking compared to its average high alone.  (This follows the weather storyline of this century, whereby nighttime temperatures in New York are warming more than daytime temperatures.)

 

Chart - coolest 4-day heat waves based on mean temp

 

Finally, while the typical four-day heat wave had a 20-degree difference between its high and low, this August's was 15.8 degrees apart, which was the third smallest diurnal variation of the heat waves studied (a heat wave in July 1995 had the smallest, 14.7 degrees, while the second smallest was in July 1870).  


Stuck in the 70s

Gray skies new york skyline

 

Despite what the headline suggests, this post isn't about people obsessed with 1970s pop culture.  Rather, it's a discussion about days that have had both their high and low temperatures in the 70s.  These type of days have occurred infrequently - about once or twice each year.  33 years (since 1869) have never registered such a day, 53 have had one instance; the most days in one year was ten, which happened in 1905.  In 2017 and 2018 there were five occurrences, joining just ten other years that have had five or more (see chart below).  Temperatures stuck in the 70s usually occur when tropical air is in place, the day is hazy/overcast and rain is in the air. 

FURTHER DETAILS

  • The warmest low in the 70s to be accompanied by a high temperature below 80° is 74°, which has happened twice - first in August 1901 and then seventy-two years later in August 1973.  Both dates had a high of 79°.  (For reference, the average high when lows are between 70°-74° is around 86°).
  • The most consecutive days with temperatures in the 70s all day was five, in 1905 (Aug. 30-Sept. 3).
  • The most consecutive years without any "all-70" days is four, from 1962 to 1965 (and six out of seven years from 1962 to 1968).  By contrast, there were 21 of these days in the three-year period 1905, 1906 and 1907; they had ten, five and six of these days, respectively.
  • The first decade of the 20th century had more "stuck in the 70s" days than the next three decades combined (38 vs. 37).  The 1890s had the second greatest number - 34. The decade that just ended, i.e., the 2010s had 26 occurrences.
  • The latest occurrences for a high/low in the 70s were in 2017, on Oct. 8 and 9 (at the time the 9th was the latest date for a low in the 70s - until the following year).  The earliest occurrences were on June 4 and 5 in 1960.
  • The dates most likely to have experienced a day entirely in the 70s are 7/23, 7/29, 8/9 and 8/12, all of which have had seven such occurrences.
  • One out of four of these days either had no measurable rain (half were before 1935) while another one out of four received more than a half-inch of rain.  The greatest rainfall to occur was in 1990 when 4.64" fell on Aug. 10 (the high/low was 76°/70°).  Three other days with substantial rainfall were associated with Hurricanes Connie and Irene, in Aug. 1955 (2.70") and Aug. 2011 (2.88"), respectively, and tropical storm Fay in July 2020 (2.54").

 

Chart - stuck in the 70s

 

Stuck_in_the_70s_button-r0f7cc4f51c6543a9893da93dbb3d5e21_k94rk_307


July 2018 Weather Recap: Hot Temperatures, Followed by Torrents of Rain

Rain slicker

 

If July 2018 were a cocktail its ingredients would be one part heat, three parts rain.  After a hot start, with temperatures nine degrees above average during the month's first five days (average high/low was 92°/78°), the rest of July was seasonably warm, which moderated the month's average temperature to 1.1 degree above average.  As for the rain, after a dry start, with just 0.24" measured through July 11, the skies opened up and the rest of the month was very rainy, with 7.21" measured.  With 7.45" in total this was the rainiest month in more than four years, since April 2014, and the rainiest July in fourteen (and the fourteenth wettest July on record).

 

In late July there were seven days in a row (July 21-27) in which measurable rain fell.  This was the fourth streak of six days or more this year, joining 1989 as the only two years since 1900 with four streaks of six days or longer.  (Besides the persistent rain, dew points throughout this week-long period were in the uncomfortable 72°-74° range.)

 

Chart - 6+ Days of Rain in 2018

 

The seven-day streak with rain was the first since 2012, but this year's had considerably more rain (2.89" vs. 0.86").  The last streak of seven days or more with more rain was in May 2009, when 3.81" fell.

 

Downpours on 7/12 and 7/30 were confined to upper Manhattan; Central Park had 0.71" and 0.37", respectively, but my neighborhood, Greenwich Village, four miles south of the park, was dry.  And during the afternoon of 7/17 Manhattan was the bulls-eye for torrential rain from a severe thunderstorm, with 2.24" pouring down in little more than an hour.  Conversely, on 7/27 thunderstorms during late afternoon thru evening produced more than an inch of rain in most parts of the metro area (and 3-5" in parts of east-central NJ) but Central Park had only 0.25". 

 

July's rainfall in Central Park was considerably more than the three other reporting sites in the metro area:

 Chart - July 2018 Rainfall

Other July observations: 

  • Four of the first five days in July had highs in the 90s; the only one that didn't was the 4th of July, but its high of 86° was the warmest reading on this holiday in five years.  However, due to its very warm low of 77°, the mean temperature was the warmest since 2010.

 

Chart - 4th of July

 

  • Thirteen days in a row (July 8-20) had highs of 80° or warmer, the longest such streak since one of 19 days in August 2016.  (The record is 62 days in a row, set three years ago.)
  • What was most impressive about the hot beginning to July were the low temperatures, all  which were 76° or warmer.  This happens almost as infrequently as five-day streaks with highs of 95°+.
  • This was the ninth July in a row with no lows in the 50s, the longest such streak in the years since 1900.  Before 1980 more than half of Julys had at least one low in the 50s, but since 1980 that portion has fallen to 25%.
  • By July 25 the month had picked up more rain than the combined amount in May and June (6.80" vs 6.64").

 

Here are links to previous July recaps:

2017

2016

2015

2014