Lackluster Heat Waves
September 2018 - A Month With Unusually Mild Nights

August 2018 Weather Recap: Unusually Wet, With Unusually Warm Nights


Summer downpour


July's rainy and warm conditions carried over into August, which was even warmer and wetter, ending up as the ninth warmest August on record and the twelfth wettest.  It was 2.9 degrees above average and warmer than July (something that's happened in 22% of the years since records began in 1869).  Despite August being warmer than July (by 0.5 degrees), July's average high was slightly higher (84.9 vs 84.3); it was August's average low that made it warmer overall (72.0 vs. 70.4).  In terms of rainfall, August's 8.59" was about an inch more than what was measured in July (7.45"), with much of it falling during heavy downpours during thunderstorms.  Here are additional observations worth noting:


  • There were seven days in the 90s, which occurred during two heat waves - one was four days in length (8/5-8) and one lasted three days (8/28-30).  The four-day heat wave was one of the least impressive among those of this length. The hottest reading of the month was 94° on 8/28.  The coolest reading of the month, which was reported twice, was 65°, which tied August 2005 and 1906 for the mildest minimum temperature in the month of August.  The typical coolest low temperature in August is 59° (in the years since 1980).
  • This was the thirty-second warmest August in terms of average high, but it had the fourth warmest average low.  (The low was 4.2 degrees above average, the high was 1.7 degrees above.)  This produced the ninth warmest August overall (tied with Aug. 1955, which was even rainier than this August, with nearly eleven inches measured).  Three of the past four Augusts have now placed among the ten warmest.


Chart - warmest average lows in august

  • Twenty-three days had lows in the 70s, tying August 1980 and 2005 for the most in the month of August (the typical number of such days is 13).  The month's first 18 days all had lows in the 70s, only the second time it's occurred, joining August 1988.  It appeared the City was going to set an August record for most lows of 70 or warmer, but it was not to be as the mercury fell to 69° the evening of 8/31.  On 8/29 the low of 81° established a new record for the latest date for a low in the 80s, smashing the previous record by two weeks (which occurred in 1985 and 1988).  This summer was just the eighth in which both July and August had average lows above 70° (two of the other summers were in 2015 and 2016).
  • The month's diurnal variation (i.e., the difference between the high and low) was just 12.3 degrees, which was the smallest in August since 2000 (also 12.3 degrees).  The only variations smaller than this occurred more than 100 years ago, when eight Augusts in the years before 1911 had variances between 10.4 and 11.7 degrees (at the other end of the spectrum, the greatest diurnal variation in August is 20.4 degrees, which occurred in both 1964 and 1944.)
  • This year has had the twelfth wettest August, the fourth wettest July-August, and the twelfth wettest summer.  Almost all of July and August's rain fell in a soggy six-week period (July 12 - Aug. 22).  Ironically, while August is the year's wettest month (so far), it also had the longest streak with no measurable rainfall - eight days, from the 23rd to the 30th.


Chart - 10 wettest july-aug

  Storm approaches in august


Chart - 10 hottest augusts

Here are previous August recaps:








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not only was this the rainiest month since June 2013 (when just over ten inches fell), but this was also the tenth rainiest month since 2000. this month joined six others (most recently in July 2008 and August 2005) as the only ones in which the temperature went no lower than 65°. an extended streak of days with lows of 65°+ that began on July 13 continued through August. By 8/31, the streak had grown to fifty days, tying 2016 as the fourth longest such streak and the streak was expected to eclipse 2005's all-time streak of 58 days.


There were 107 days of measurable precipitation in the first eight months of this year (which was 14 fewer than ALL of last year), the most ever in the first two-thirds of a year, besting 1950 by one day. However, despite the number of days, the combined amount of precipitation that fell so far this year (41.64") didn't rank among the five wettest. Instead, 2018 was ranked as the seventh rainiest Jan-Aug on record. The night time warmth coupled with the wet weather is what really made this month stick out like a sore thumb. Among the thirteen Augusts with seven or more lows of 75°+, 2018 was the rainiest of them all (2.58" wetter than the second wettest).


While not the most total rain in any summer (ranked 6th). By my count, today was the 42nd time it rained during the Summer. The previous records was 40, in both 1915 and 2000.
Note: For ease of caluclation, I've defined summer as June 21st - September 20th. I acknowledge that this can change a bit from year to year (I believe this year, Fall starts on the 22nd). But this seemed to be the easiest way to define it. Looks like this will be the last rainy day of Summer.


Since it's such a pain in the ass to calculate season averages based on the astronomical calendar, the National Weather Service uses the "meteorological calendar", which uses entire months. According to this calendar, summer is considered to be June, July and August. Going by that metric I believe this summer is the 12th rainiest. But this doesn't mean your findings aren't valid - thanks for doing the research!


this month tied 2011 as the August with the most days of 0.50" or more of rain (eight days). however, this month had roughly ten and a half fewer inches of rain than what August 2011 experienced.


interesting fact - the low of 81 on 8/29 is the same as the average high

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