RodneyS

Based on the metric I was using, 1917 wasn't quite as cold as this year in DC, with an average maximum temperature of 63.8 during April 14-May 13. However, May 1917 maximums averaged only 68.8, 4th coldest of all time. And the overall average May 1917 temperature was only 59.6, just 0.4 degrees above the all-time coldest DC May, which was 1907.

In response to a comment on the Capital Weather Gang webpage arguing that we had a normal Spring, I posted this reply at https://www.washingtonpost.com/weather/2020/05/13/warm-weather-arrive-abruptly-east-this-week-may-just-be-tease-some: I wouldn't exactly call this Spring normal, particularly for the last 30 days. The maximum temperature at DCA during April 14-May 13 averaged 63.6 degrees, the 6th coldest all-time in DC for that period and the coldest since 1882, which averaged 62.2. The coldest all-time was 1875, which averaged 60.8; and the warmest was 1985, which averaged 79.0.

April 2020 at DCA averaged 55.3 degrees -- the same as the 1871-2019 historical average. However, precipitation at 6.30 inches was double the DC April historical average of 3.15 inches. In fact, it was the 7th rainiest April in DC history, with the rainiest being 9.13 inches in 1889.

DCA _ NYC _ BOS __ ORD _ ATL _ IAH ___ DEN _ PHX _ SEA -1.7 -1.7 -1.2 -2.4 -0.3 1.6 3.0 3.4 1.4

I just posted this on the Capital Weather Gang webpage, in response to Ian Livingston's article at https://www.washingtonpost.com/weather/2020/04/28/pm-update-some-more-showers-tonight-much-warmer-wednesday Today marked the 15th consecutive day at DCA without a 70+ degree reading. The only other April 14-28 period where that happened in DC weather history was 1875. However, as Ian notes, the current run appears over, as tomorrow is forecast to be at least 70. In 1875, the run ended one day later, on April 30th, with a reading of 72. Still, there is a big difference between these two Aprils. April 1875 averaged only 48.0 degrees, the second coldest April in DC history, whereas this April has averaged 54.8 so far, only slightly below the 1871-2019 historical April average of 55.3. By the way, 1874 was the coldest April in DC history, at 47.1, and the only April day that reached 70+ that year was the 15th, at 72 degrees.

DCA _ NYC _ BOS __ ORD _ ATL _ IAH ___ DEN _ PHX _ SEA 1.8 1.5 1.2 0.7 2.8 4.5 -0.2 -0.2 -1.9

Eight years ago today, I posted on this thread -- http://www.americanw...h-on-record-dca: "The final numbers for the 2011/12 astronomical and composite winters are 45.9 and 45.6 degrees, respectively. Thus, the record for the warmest astronomical winter was broken by 2.2 degrees and the record for the warmest composite winter was broken by 1.6 degrees. When the astronomical winter warm record was last broken (in 1989-90), it was only by 0.2 degrees, and when the composite winter warm record was last broken (in 2001-02), it was only by 0.9 degrees." This season, the astronomical winter (December 21-March 20, on average) averaged 45.5 at DCA , and so it fell only 0.4 degrees short of 2011/12. And the composite winter (December 1-March 20, on average) averaged 44.6 at DCA, and so it fell 1.0 degree short of 2011/12. Both averages are good enough for second warmest in DC history.

February 2020 averaged 43.8 degrees at DCA, with the meteorological winter there at 42.8 degrees, each 7th on the all-time DC list, although February 1984 also averaged 43.8.

I have now done some additional analysis that shows that in several other eastern cities January temperatures during the most recent 30 years have peaked on or about January 13th and bottomed on or about January 21st: Raleigh-Durham: 46.9 vs 37.7 degrees Richmond: 43.6 vs 34.0 degrees Philadelphia: 38.4 vs 29.3 degrees New York: 37.3 vs 28.9 degrees Boston: 34.1 vs 26.2 degrees Portland: 26.4 vs 19.5 degrees

Time for another update. Once again this year at DCA, January 13th was much warmer than January 21st (48.5 degrees vs 31.5). So, for the most recent 30 years (1991-2020) at DCA, January 13th has averaged 41.6 degrees vs. 32.6 degrees on January 21st. A two-tailed T-Test shows that the odds of this being a random variation are less than 1 in 18,500. So, how will the NCDC handle this inconvenient fact when it calculates its new daily 30-year January temperature normals at DCA? The 1981-2010 January temperature normals there show temperatures bottoming out in mid-January, not January 21st. Further, even if the new DCA normals were to reflect a bottoming out on January 21st, the January 13th normal would likely be calculated using standard methodology as only 0.2-0.3 degrees higher than January 21st, and not the actual 9 degrees higher. Could someone at NCDC think way outside of the box and develop a radically new January temperature model, showing an annual thaw at DCA occurring around January 13th? And if s/he did, would s/he be given an award or fired? A thorny problem for NCDC, it seems to me.

October 2, 2019 is in the record books for the hottest October maximum in DC, Baltimore, and Dulles. The respective maximums today were 98, 98, and 96. The previous record for DC and Baltimore was set on October 5, 1941, with respective maximums of 96 and 97. The previous record for Dulles was set October 9, 2007, at 94.

Minor correction for DCA: According to the Capital Weather Gang, September 2019 was the 3rd warmest and 4th driest September in DC history; see https://www.washingtonpost.com/weather/2019/10/01/washington-just-posted-its-rd-warmest-th-driest-september-record

Another DC precipitation record has just fallen. The previous DC record precipitation from the second half of one year to the first half of the following year was 62.16 inches, set during July 1885 to June 1886. With 0.62 inches already recorded at DCA tonight, the July 2018-June 2019 total there is 62.53 inches and counting.

You're right that last April was relatively cool at DCA -- 54.9 degree average, which was 1.9 degrees below the 1981-2010 normal, and 7.5 degrees cooler than this year.

April 2019 averaged 62.4 degrees at DCA, second all-time in DC to the 63.8 recorded in April 2017. However, precipitation was only 2.24 inches -- the first below normal month at DCA since October 2018.

I had not noticed that, but had noticed that overall that snowstorm tied for 23rd on the all-time DC list (with February 22-23, 1987). Also, the 10.6 inches received at Dulles in that storm was 16th all-time there, displacing the 10.3 inches that fell at Dulles during the infamous (poorly-forecast) 10.3-inch snowstorm of January 25, 2000.

Today's high temperature of 74 at DCA was 50 degrees warmer than the high of 24 recorded there just four days ago. As best as I can tell, that 4-day warm-up of 50 degrees in the DC maximum was second only to the 52-degree 4-day warm-up in maximum temperatures that occurred from February 16, 1930 (24) to February 20, 1930 (76).

This morning, Dulles fell below zero for the first time since January 7, 2018. However, this morning's reading of -2 fell short of the January 31st record of -7 set in 1965.

DCA just recorded its first 1-inch or more precipitation day of 2019, after a record-breaking 24 such days last year. More remarkably, the amount of precipitation there since July 1, 2018 is now more than 44 inches. The previous record for any July-March period was only 43.81 inches, set during July 1934-March 1935. So, we have exceeded that total with more than two months still to go.

Time for an update. The 1984-2013 trend that I observed six years ago regarding January DCA temperatures peaking on or about the 13th and bottoming on or about the 21st has continued. If that trend during the period 1984-2013 were only a temporary aberration, since that time January 13th would be as likely to be colder than January 21st as it would be to be warmer. However, during 2014-19, January 13th DCA temperatures averaged 39.1 degrees, whereas January 21st DCA temperatures averaged only 34.4 degrees. That brings the average DCA temperature during 1984-2019 to 40.1 degrees on January 13th vs. only 32.6 degrees on January 21st. A statistical test (two-tailed T-test) indicates that it is highly improbable that this 7.5-degree difference for that 36-year period is a random variation, with the odds of that being only about 1 in 2200. Moreover, in the not-too-distant future, NOAA's National Climatic Data Center (NCDC, now part of the National Centers for Environmental Information in Asheville, NC) will begin to calculate its new daily 30-year temperature normals for the period 1991-2020. That period is now only one year shy of completion for January 13th and January 21st, and the average DCA temperature during 1991-2019 is 41.3 degrees on the former day and 32.6 degrees on the latter day. A two-tailed T-test of this 8.7-degree difference for that 29-year period shows that difference is even more unlikely to be a random variation, with the odds of that being only about 1 in 8500. So, how will the NCDC handle the fact that January temperatures at DCA now seem to peak around January 13th and bottom around January 21st? Does anyone here know someone at NCDC that works on temperature normals or know someone else (including yourself) that is knowledgeable in this area?

And the January 20, 2019 38-degree temperature spread at DCA (56 maximum, 18 minimum) was the greatest since the January 14, 2018 41-degree spread.

This morning both DCA and IAD recorded their daily high temperatures. However, DCA's high was 56 degrees vs only 40 at IAD. As best as I can tell, that 16-degree high temperature difference at the two airports was the second greatest ever, behind only December 9, 1975, when DCA recorded a high of 53 and IAD recorded a high of 36.

I can state with a high degree of confidence that the NWS does not smooth outliers in its 1981-2010 monthly and annual precipitation "normals" but rather uses the 1981-2010 averages, albeit with an unconventional rounding methodology. I state this because I have computed the 1981-2010 averages for each month at DCA, and they either match the DCA normals exactly for each month or they are rounded up by 0.01 inches. For example, if you average the January 1981-2010 precipitation totals at DCA, you get 2.803 inches. For some reason, NWS rounds this to 2.81, whereas using a conventional rounding methodology would result in 2.80. The annual 1981-2010 precipitation normal at DCA is shown as 39.74 inches, whereas I calculate 39.71 inches. However, there is no smoothing. So, for example, the August 1981-2010 normal at DCA is only 2.93 inches, which is rounded up from the 1981-2010 2.924 inch average. However, the historical August 1871-2018 DC precipitation average is 4.07 inches, and that is not taken into account. So, the 1981-2010 DCA August normal makes it appear that August is now a relatively dry month at DCA. But is it really, or was the 1981-2010 August average there an aberration? I would guess the latter, because beginning in 2011, August precipitation at DCA has averaged 3.77 inches -- closer to the long-term average than to the 1981-2010 average. In other words, you have to be careful with monthly precipitation normals because they may have little predictive value. Regarding annual precipitation normals, 30-year annual precipitation averages have ranged between 38.62 inches during 1961-1990 to 43.01 inches during 1871-2010, with an 1871-2018 average of 40.92 inches. So, again, using a 30-year annual precipitation "normal" can be misleading, as the most recent 30-years may be too short a period to conclude that anything other than a random variation is occurring.

The final 2018 figures are 66.74 inches at IAD and 66.28 inches at DCA. The IAD total broke the 2003 record by 1.07 inches and the DCA total broke the 1889 record by 4.95 inches.