RodneyS

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.

Breaking news -- literally. With 2.76 inches of precipitation already recorded at DCA today and 3.57 so far for September 2018, the previous May-September DC record total of 32.26 inches set in 1934 has now been exceeded, at 32.43 inches. Let's see how high we go by September 30th.

August 2018 continued the rainy trend at DCA, at 5.19 inches. That brings the May-August 2018 precipitation total there to 28.86 inches, tied with May-August 1878 for second on the all-time DC list, behind only May-August 1886, at 30.41 inches.

The beat goes on. This year, DCA recorded 0.36 inches of precipitation on August 12th and nothing on August 15th. So, the updated totals, 1871-2018, are: August 12: 43.43 inches total; 21 days with a trace, 54 days with measurable, 12 days with at least an inch, 5 days with 2+ inches. August 15: 12.45 inches total; 16 days with a trace, 47 days with measurable, 5 days with at least an inch, no days with 2+ inches.

While DCA could not match BWI's record-breaking July 2018 precipitation, the 9.73 inches recorded there was 4th on the all-time DC July list, and the most since the record 11.06 inches in July 1945. Also, the May-July 2018 total of 23.67 inches at DCA was third on the all-time list for those three calendar months, and the most in 129 years. Only May-July 1889, at 23.83 inches, and May-July 1886, at 27.98 inches, had more precipitation in those three months.

And May 2018 has the distinction of being an exceptionally warm, and yet still rainy, month in DC. Average temperature of 72.4 degrees at DCA places it third on the all-time DC list behind only 2015 (73.2) and 1991 (73.0). Further, the warm-up from April 2018 was 17.5 degrees, second only to April-May 1911 (18.8 degrees). And yet precipitation was 8.73 inches, which is 6th on the all-time DC May list (May 1953 is # 1, at 10.69 inches).

You're on to something. May 12-19, 2018 was indeed the first time in DC weather history that eight straight days each recorded at least 0.15" of precipitation. The previous record for that amount or more was seven straight days, set in July 1938 and equaled in July 1941; see http://xmacis.rcc-acis.org/#. Click on that link, then select "Single Station" and click on "Consecutive Days." For "Criteria" select "Precipitation" then type in (more or equal to) "0.15." Then select the "Washington Area" station and click "Go." As I just posted to an article on the Washington Post's Capital Weather Gang: "Rainy to be sure, but to put things in perspective, DC's record single-day rainfall is 6.39 inches, on August 23, 1933. More recently, DCA recorded 9.08 inches during the five-day period September 22-26, 1975."

I have just discovered that the above statement is not true for 0.4", as only 2011, 1975, and 1937 also had 5-day streaks of at least 0.4 inches. However, I would have thought that the statement would have more logically read: "It has been an unusually wet pattern over the last week. For DCA (Reagan National), there has been at least 0.4" of rain each of the last 7 days. This is the longest streak on record. The previous longest was 5 days in 2011, 1975, and 1937. Precipitation records date back to 1871."

You can view the May 2018 daily precipitation totals at DCA here under "Preliminary Monthly Climate Data" (Column 7 "WTR")-- https://w2.weather.gov/climate/index.php?wfo=lwx

The week of Sunday, May 13, 2018 through Saturday, May 19, 2018 saw at least 0.4 inches of precipitation at DCA each of those seven days. Previously, no more than five consecutive days in DC weather history recorded at least 0.4 inches each day.