Roger Smith

The historical highlights missed the biggest event on this date, namely the peak of the Pacific NW - BC heat dome in 2021. On June 30, 2021 it was 44 C (112 F) here, breaking our all-time record by probably five degrees. It was 49 C (120 F) at Lytton BC, a hotspot every summer but in this case, it burned to the ground at 5 p.m. due to a train spark induced brush fire fanned by the hot up-valley winds. The town (formerly home to 1,200 people) remains largely in ruins now as local governments wrangle about who will pay what to rebuild it, and native groups request careful handling of possible archaeological remnants. Most of the buildings were reduced to ground slabs and scattered debris, if you went in there now it would look like any other ghost town, but its population want to restore it. I don't have the U.S. numbers handy but I'm pretty sure the peak of the heat wave was on June 28th to July 1st at all recording sites. (later edit, 116F at Portland on June 28, and 108F at SEA-TAC ... 117 F at Pendleton and Omak on June 29 and 116F in Spokane) ... There were hundreds of heat prostration deaths in the region. Various locations in BC, WA and OR got well above 110 F, coming close to all-time records if not breaking them (1898 had some of those, 1941 had others). Looks like the severe heat broke down on the coast by the 29th but remained inland to July 1-2, I recall it sort of fizzled out gradually in the first week of July and then the severe smoke episode began later in July as fires began to spread through BC and WA. We were under 594 dm thickness here for several days, so this was just like the heat they get routinely in Las Vegas and Phoenix, and it felt the same, heat radiating into your face but no cooling sprays along the sidewalks like they have in Vegas (I managed to hit Las Vegas for their August record high of 118 F in 2011, just on my way to much less torrid places higher up in southern Utah but that heat wave was very similar to the heat dome, only where it was supposed to be).

+1.7 __ +1.5 __ +1.3 __ +1.2 __ +2.0 __ +2.0 ___ +0.3 __ +1.5 __ +0.7 June scoring is back in thread, everyone did well so there won't be big changes in the annual scoring race which I will update in another "back in thread" post set aside to take on that work (before all July forecasts except wxdude64's early submission) ...

Just my usual reminder to NYC forecast contest regulars to get a forecast in on Monday 30th. And have a great 4th of July everybody.

Today's min of 77F at NYC was 1F off the 2021 record which replaced 1870 (77F) so it was tied second warmest for the date.

Bonus coverage, here's a list of benchmark record high minimum values (for NYC) ... I reverse them at the point where the trend line reverses in early February ... (going up) 51F __ Feb 5, 1991 (mildest in period Jan 28 to Feb 15) 56F __ Feb 16, 2023 58F __ Feb 24, 2018 63F __ Mar 10, 2016 66F __ Mar 31, 1998 68F __ Apr 9, 1991 70F __ Apr 14, 2023 74F __ Apr 17, 2002 76F __ Apr 18, 2002 77F __ June 1, 1895 78F __ June 4, 1943 79F __ June 10, 1984 80F __ June 23, 2025 81F __ June 24, 2025 82F __ July 2, 1901 83F __ July 6, 1999 84F __ July 7, 1908 (84F also July 15, 1995 and July 22, 2001) 84F __ Aug 14, 1908 (heading down now) 81F __ Aug 29, 2018 79F __ Sep 7, 1881 78F __ Sep 11, 1983 77F __ Sep 23, 1970 75F __ Oct 5, 1898 72F __ Oct 8, 2017 71F __ Oct 10, 2018 69F __ Oct 25, 1908 67F __ Nov 2, 1971 66F __ Nov 6, 2015, 2022 64F __ Nov 11, 2002, 2020 63F __ Dec 24, 2015 59F __ Jan 4, 1950 56F __ Jan 14, 1932 54F __ Jan 15, 1995 53F __ Jan 27, 1916 (and back to 51F Feb 5, 1991). ... very few of these are concurrent with high max benchmarks (Mar 2016, Apr 2002, Sep 1881). ... the midsummer situation is different, a plateau of four equal values, not one peak as in July 1936.

My NYC list has a different structure through winter, Don has used the calendar year to generate his list, and if I did that it would add a few, making Dec 24, 2015 (72F) a legitimate entry, then 70F Dec 29, 1984, 65F Dec 30 1984, 63F Dec 31, 1965, 62F Jan 1, 1966, 68F Jan 2, 1876 and 72F Jan 6, 2007. (next as per my list, 73F in Feb 1949 etc). The equivalent to my mid-winter reversal method for JFK, I think, would be to skip all the entries of 70F or lower in Dec, then 71F Jan 6, 2007, followed by February's highest value, and on to the listed values from March onwards. Or reverse in late January if there is a higher value than February there.

Thanks Don, I was going to say that I didn't have a list but you probably would be able to generate one.

That 99 in 1983 tied 1931 (Sep 11) but yes it was the latest in season. The next benchmarks are 97F on Sep 23, 1895 and 94F Oct 5, 1941, 91F Oct 10, 1939, and 90F Oct 17, 1938. Benchmarks after that are Oct 22 1979 (88F), Oct 23, 1947 (85F), Nov 1 and 2 1950 (84F, 83F), Nov 15 1993 (80F), Nov 20, 1985 (77F) , Dec 7 1998 (75F) and possibly Dec 24 2015 (72F). (a benchmark as you probably know is a latest occurrence in season, or an earliest from midwinter to midsummer). Benchmarks before the 1881 101F are (by definition) the all-time record 106F July 9, 1936, then 104F Aug 7, 1918, 103F Aug 26, 1948 and 102F Sep 2, 1953, 101F Sep 7, 1881. 2015 (Dec 24) at 72F is a partially invalid benchmark as it was also 72F on Jan 6, 2007 and Jan 26, 1950. These are probably the real benchmarks eliminating Dec 24, 2015 but another view would be only Jan 25, 1950 is a benchmark (falling) because the coldest day of winter is on average in early February. This is when the benchmarks reverse, so the set of spring into summer benchmarks are 73F (Feb 15, 1949), 78F Feb 21, 2018, 79F Mar 10, 2016, 85F Mar 13, 1990, 86F Mar 29, 1945, 92F Apr 7, 2010; 96F Apr 17, 2002; 99F May 19, 1962; 100F June 26, 1952, 101F June 27, 1966, and 103F July 3, 1966 before the ultimate 106F July 9, 1936. These are the only daily records that "really matter" in the sense that all others are not as impressive in terms of being earliest or latest occurrences. It's interesting how few of these are recent. If you only count the 72F in Jan 1950, then there are 29 benchmarks (one tied 1931,1983, and two cases of two consecutive in same year 1950 and 1966) and the median of the 26 years involved (1950 counts three times, 1966 twice) is 1952. The years counted are 1881 1895 1918 1931 1936 1938 1939 1941 1945 1947 1948 1950** 1952 1953 1962 1966* 1979 1983 1985 1990 1993 1998 2002 2010 (2007, 2015 not counted) 2016 2018 ... The benchmarks cluster noticeably from 1936 to 1953 (12 of 29), and 1979 to 2002 (7 more). There is no significant bias towards recent decades, with 29 benchmarks over 156 years, one would expect 4 since 2000 which is the case. The number established by 1918 (3) is well below random expectation (9). The only year with two non-consecutive benchmarks from the same spell of weather is 1950 (one in January, two in early November). But if Dec 2015 counted, the two closest in time (non-associated) would be that one and March 10, 2016. I'm not sure what name you could give an almost-benchmark, which would be a case like April 18, 1976, tying a benchmark and clearly a near-equal outlier (86F Mar 31 1998 would be another, as would 94F on Sep 23, 1914). I guess almost-benchmark is probably as good as we'll get. A benchmark can be erased as even a daily record, for example, April 7, 1929 (89F) was a benchmark until 2010 replaced it as both a daily record and benchmark. 1991 held a 90F benchmark set April 8, that one is still a daily record but was removed as a benchmark by 2010. May is the only month with only one benchmark. That is a sign of the extreme significance of the 1976 and 2002 April 96 readings, no date in early to mid-May exceeded them (before 1962, the benchmarks were 92F (Apr 27, 1915), 93F (May 12, 1881), 95F (May 25, 1880), 96F (May 31 1895, 1939) and 99F (June 4, 1925).

It was very similar to 1953 in timing and duration but not quite as intense, highs were often 95 to 98 and failed to break records but it was very hot (I was actually in NYC for one day during it, on a road trip). The records it did set were ties for max and min on 30th (98/78, ties with 1953 for the max and 2018 for the min). So it has the highest daily mean for Aug 30th.

That is correct, in fact it has not even hit 99 F from July 24 to 28. (99F on July 29, 1949). There's a similar lull in August from 16th to 25th with highest value 97F and even more pronounced, records of only 92F and 94F on Aug 23rd-24th. After August 25th the records generally pick up considerably although most are in a few years like 1948, 53, 73 and later 1881. Perhaps these are expectable random variations that will fill in after 250-300 years of records.

(in response to Don's chart of the distribution of highs after low of 63) ... Probably the 63 "low" associated with the 95 in 1925 was at end of day as that heat wave ended, so it wasn't a case of 63 warming to 95, but 95 cooling down to 63? Not sure about that but it looks that way. Urelated factoid, I recently updated a data base for NYC and found that the highest average temperature for the entire period of record occurs on July 18 for both max and min. (86, 70).

I may not always need to know minute by minute temperature variations five miles east of JFK, but when I do, this is where I come.

New weekly records were set June 19-25 for mean (82.64) and average minimum (74.56), replacing 2024 in both cases. The value for average maximum, 90.71 (record 91.86 set in 1923) was not a record. The three-day super-hot interval at end of this weekly interval was the main reason for it to register, and cooler temperatures on 26th resulted in averages falling just below record values except for average minimum which tied 1909. Also 18th to 24th fell short as it was not able to use the last very hot day.

There was a considerable urban heat island in 1948, actually, you could rate it as 60 to 70 per cent of the present urban heat island. Perhaps most of its influence would be on overnight lows. But even by around 1890 to 1900 large cities were beginning to display an urban heat island. Vehicle traffic is only a small component of the cause of a heat island, the main components are altered surface albedos and escape of building heat. People were heating their houses before they had cars. Also they had transformed the urban environments to allow horse and carriage movement (thus changing heat retention). The strength of an urban heat island falls off rapidly after the first 100,000 of population in an urban area is reached and begins for towns as small as 2,000 population based on extensive research by many climatologists. You'd perhaps be surprised how quickly an urban heat island develops and how slowly it increases once developed, if you hadn't done active research or read the literature. For my Toronto data I estimated the urban heat island began in the 1881-1890 decade (rated at 0.1 C differential then) and I took an arbitrary 0.1 increase each decade to 1971-1980 (adding 1.0 then). For 1981-1990 and the past 35 years I estimated it had stabilized at 1.1 C (2.0 F) deg. For NYC data I have assumed the same pace of change except that I would expect an increase had already begun for 1869 to 1880 so if I had data as far back as Toronto's 1840 startup, I would start modifying 1861-1870 at 0.2 F or 0.1 C and adding that amount every decade so the stable period of 1981-2025 would be actually +1.3 C (+2.34 F) relative to what NYC might record in an unaltered "rural" state. Now some might instantly say, but NYC is in a megalopolis much larger than greater Toronto, would it not warm up even more? Perhaps, but as I said, past 100,000 the rate of increase is very slow and probably past 5 million it cannot increase because the environment is so substantially altered on a regional basis. The strength of the greater New York heat island is probably well over +1.5 F out into parts of west central NJ near the end of suburban sprawl. As to the urban park question and temperature reductions in hot weather, that probably washes out of data sets fairly quickly as there aren't all that many hot days and the nights in question remain more affected anyway. Urban heat islands tend to be something like 75% minimum boosts and 25% maximum boosts, an estimate of a +2.0 heat island really means +1.0 for average maximum and +3.0 for average minimum. Large stretches of cloudy wet weather show very small urban heat island effects, dry and clear months would have larger differentials.

2025 reduced 1888 in the roster of all-time records, from 2.0 to 0.5 daily max, and from 1.5 to 0.5 high daily min (it still has a tie with 2015 for a December high min, and it retains a tie with 2025 for June 23rd max. 2025 also reduced 1909 from 4.0 to 2.0 daily records for high minimum values, and reduced 1952 to a one-third share of monthly high min. The counts for June 2025 are 1.5 and 3.0 daily records (max, min) and two-thirds share of the monthly record high min.