RodneyS

Most of us have complained at some point about rainy days seeming to occur more often on weekends than on weekdays. However, as the Capital Weather Gang recently noted, that has actually been the case this year in this area, assuming that the weekend is defined to include Friday; see https://www.washingtonpost.com/weather/2019/05/13/washingtons-wettest-days-history-inches-precipitation-have-fallen-past-year/?utm_term=.2bb2b4bf05c4. Specifically, through the first 19 weeks of 2019 at DCA, there have been 32 weekend days on which measurable precipitation has fallen vs. only 17 weekdays on which measurable precipitation has fallen. Moreover, because there are more weekdays than weekend days, the percentage of weekend days that have received measurable precipitation is 56% (32 of 57 days) vs. only 22% for weekdays (17 of 76 days). I just ran a two-tailed T-Test to determine the probability of this pattern occurring randomly, and it works out to be about one chance in 730. So, is this rainy weekend pattern here just a fluke, or is there more to it than that?

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.

I've been away for a few days, but I just discovered that IAD broke the annual precipitation record on Friday, with 1.08 inches. It has now added 0.52 inches today, and so the current total is 66.73 inches, which exceeds the previous record by 1.06 inches. Both IAD and DCA figure to get a little more rain before midnight, and so I'll post the final 2018 totals tomorrow morning.

That's correct. And nothing all that promising on the immediate horizon -- AccuWeather is now projecting only 0.2 inches at Dulles through year end.

Again, for what little it may be worth, AccuWeather is currently projecting 1.53 inches of precipitation through year's end at Dulles, which would exceed the annual record there by 0.21 inches.

Timely question, as the IAD record is 65.67 inches, set in 2003. The total through today for 2018 is 64.36 inches. So, IAD is closing in on the record, but still needs 1.32 inches to break it.

AccuWeather's shorter term forecast was too low, and DCA broke the all-time DC record between 5:52-6:52 this morning. We now (as of 7:52 AM) are up to 61.40 inches of precipitation for the year vs the previous record of 61.33 inches set in 1889.

That's right -- not a done deal yet, especially with the big southern snowstorm staying to the south of DC. For what it's worth, AccuWeather is currently projecting 1.23 inches at DCA through month's end. However, only 0.28 inches is projected there through December 19th, and long-range precipitation forecasts are not exactly sure things.

November 2018 precipitation at DCA appears to be finalized at 7.57 inches, leaving the annual total of 60.46 inches there just 0.87 inches shy of the 1877 DC annual record. As 138 of 147 previous Decembers in the official precipitation history of DC have exceeded 0.87 inches, the odds of breaking the record are about 94%. Also, the second half of 2018 (July-December) DCA precipitation total of 35.28 inches has already broken the second half record of 33.55 inches set in 1878. Additionally, November 2018 average temperature at DCA appears finalized at 46.5 degrees, which is 3.1 degrees below the 1981-2010 normal, and the lowest average November temperature there since 1997.

Not sure whether we're done for the day, but DCA already has recorded 0.44 inches of precipitation, bringing the November total to a DC record 7.55 inches. So, one record down, one to go -- DCA needs just 0.9 inches between now and the end of the year to break the annual record. Every December except nine in DC weather history has seen at least that much.

With today's 1.42 inches at DCA, the yearly total now stands at an even 60 inches -- still 4th all-time in DC, but now a strong likelihood to break the record. Even assuming no more precipitation in November, DCA would need only 1.34 inches in December to do so. In the previous 147 Decembers in official DC precipitation history, only 17 times has precipitation been less than that. Moreover, should DCA record only 0.08 inches more this month, that would break the all-time DC November record of 7.18 inches, set in 1877.

Not only within reach, but based on historical DC precipitation averages for November 18-December 31, we would receive another 4.28 inches through year's end, bringing the total to 62.86 inches. Further, AccuWeather currently is projecting an additional 4.39 inches in DC through year's end. Cautionary note: While, as Don says, 1889 was the all-time wettest in DC history, oddly enough December 1889 was the driest DC December ever, at only 0.19 inches.

September 2018 precipitation at DCA was 9.73 inches, good enough for 5th on the all-time DC list. January-September 2018 precipitation at DCA was 49.83 inches, third on the all-time list, behind only 1886 (50.65) and 1889 (50.63).

I don't have IAD figures at the moment, but DCA's 2018 astronomical summer (June 21-September 23) was interesting, as the average was 79.6 degrees, good enough for 5th highest all-time on the DC list (1980 was highest at 81.7). However, the 2018 DCA average astronomical summer maximum of 87.1 was only 26th on the all-time list, whereas the average minimum of 72.20 was 3rd, behind only 2016 (72.54) and 1980 (72.53).

We are now up to 7.54 inches of precipitation at DCA in September 2018. That means that the previous April-September DC record of 39.91 inches established in 1889 has fallen. The April-September 2018 total at DCA now stands at 39.99 inches, and the May-September 2018 total there is now 36.40 inches.