So far this century, the average date for the first reading of 70° or warmer in Central Park has been March 19 (and in the past six years it's been March 5). This is four weeks earlier than what the average date was during 1869-1899; and during the 20th century this first occurrence was at the end of March. The earliest date for a 70+ reading has been Jan. 6, which happened in 2007 (joining two other "first 70s" in January, in 1932 and 1950); the latest date is May 9, which happened in 1875 (and it's happened in May in five other years, the last time being in 1940).
The average high temperature on the day of the first 70+ has been 74°; the average low on that day, 48° (this diurnal variation of 24 degrees is much wider than the typical daily variation of 15 degrees). The average high of the day before the first 70+ is 60°; the average high on the day following the first 70+ day is 66°.
11 years' first 70+ reading also served as those years' first 80°+ reading, with the most recent occurrence being in 2003. And in 1927 the year's first 70+ served not only as the first 80+, but as the year's first reading in the 90s. Meanwhile, 15 other years had their first 80+ come the day after the first 70+ (the last time this happened was in 2013).
Back-to-back years with very early dates for the first 70+ are 1949 (Feb. 15) and 1950 (Jan. 26), and 2017 (Feb. 21) and 2018 (Feb. 24). And there have been two periods of three consecutive years with very late dates: 1883 (May 3), 1884 (May 2), and 1885 (April 21); and 1875 (May 9), 1876 (May 6) and 1877 (April 23).
The greatest difference in high temperature between the day before and the day of the first 70+ is 34 degrees, which was in March 1935 (from 43° to 77°), and 33 degrees in April 2001 (from 45° to 78°). And the biggest decline on the day after occurred in March 1934, 35 degrees (from 71° to 36°), and 34 degrees in Feb. 1874 (from 72° to 38°). Lastly, the biggest increase on the day after is 14 degrees in 1902 (from 70° to 84°).
The greatest diurnal variation on the day of the first 70+ is 42 degrees, with a high/low on March 13, 1990 of 85°/43°. The coldest reading to occur on the day of the first reading of 70+ is 30° on March 18, 1934. It came after the high of 71° was reached and a cold front moved thru mid-afternoon. (This is the also the only time a reading of 32° or colder occurred on the same day as the first 70°+.)
After a reading of 70°+, the average number of days before the next reading of 70°+ has been 11, with the greatest hiatus being 80 days in 1932 (when the first 70+ was on 1/14). Not surprisingly, the most days to elapse before the next 70+ high have occurred in Jan/Feb - with the average hiatus being 43 days. In about one-third of the years, the first 70°+ was followed the very next day by another high of 70+, with the longest streak being eight days, in April 1896 (including highs of 87°, 88° and 90°). The longest streak in the years since 1900 has been six, set in April 1952.
An early first 70+ isn't predictive of a hot summer as some very hot summers had their first 70+ at a late date. This includes the year with the second hottest summer, 1966, which didn't see its first reading in the 70s until April 25. And the fifth and sixth hottest summers, in 1983 and 1993, didn't have their first 70+ until April 25 and April 19, respectively.
About 20% of the dates of the first 70+ reported measurable rain; two-thirds of these years had amounts less than 0.10". (By contrast, 40% of the days with the first 60+ reading had rain.) Just one date had more than an inch, on April 6, 1937. On that day, 1.02" of rain fell before dawn and ushered in mild air (the high reached 72°).
And, finally, some more interesting occurrences:
> In 2018, after a high of 78° on Feb. 21 (the warmest reading ever in Jan/Feb), more than seven weeks passed before the mercury rose above the low 60s.
> In 1998 the first 70+ reading was 83°, and it was the first of five days in a row in the 80s - at the end of March (March 27-31), when the average high is in the mid-50s.
> The first high of 70+ in 1988, 76° on 3/24, came just two days after a low of 17°
> The day of 1980's first 70+ followed a big rainstorm of 3.42" the day before.
> In 1967, a week after the first 70+, on March 11, there was a week of harsh winter conditions, with an average high/low of only 31°/20°, including a reading of 8° on 3/19; and three snowfalls produced 15.4" of snow.
> 1947's first 70+ on 4/6 was the day after nearly two inches of rain fell.
> In 1929 the first 70+ came three days after a low of 12°. And in 2009 a low of 12° in March came four days before the year's first 70+.
> In 1874, two days after a high of 72° on Feb. 23, 7.5" of snow fell.
To read an analysis about the first readings of 60+ each year, double click here.