RodneyS

It's interesting to note how widely rainfall varies between Baltimore and DC. Here are the DC figure for those same months, with the DC surplus or deficit relative to Baltimore shown in parentheses: Aug 1955 - 14.31 (-4.04) Aug 1933 - 9.91 (-3.92) Sept 2011 - 8.84 (-4.48) Sept 1934 - 17.45 (5.04) Aug 1911 - 7.27 (-5.01) Sept 1999 - 10.27 (-1.23) Jul 1889 - 8.13 (-2.90) Aug 1971 - 7.18 (-3.73) Jul 1905 - 9.95 (-0.70) Sept 1876 - 10.81 (0.29) Aug 2011 - 8.92 (-1.46) So far this month, DCA has 9.18 inches, but that still leaves it 4.84 inches short of the June record of 14.02 inches, set just nine years ago, when BWI had just 7.32 inches. So, in June 2006, DCA had a 6.70 inch surplus relative to BWI.

DCA is in the same range, with 7.86 inches of precipitation so far in June, placing it 7th all-time, even if no more were to fall this month. However, it's still a long way from the record June total of 14.02 inches, set in 2006.

The ultimate in this context would be to have a warmer May than June in the same calendar year. The closest DC has ever come was 1918, at 69.6 degrees in May and 70.8 degrees in June.

I think perception of heat depends on whether you're focusing on average maximum or average minimum temperatures. This year at DCA, the average May minimum was 63.8 vs. 62.8 in 1991, and even at IAD the relative numbers for those two Mays are 57.5 vs 57.1. On the other hand, IAD has yet to record its first 90+ degree day of 2015, whereas in 1991 maximums for the last seven days of May ranged between 90 and 95. By the way, the warmest May at IAD was actually 2004 at 69.8, followed by 69.3 in 1991 and 68.9 in 2015.

Warmest average DC May temperature (73.2 at DCA, vs 73.0 in 1991). However, average maximum temp. of 82.4 was exceeded by 83.1 in 1991. Also, current precipitation (assuming none this evening) of 1.92 inches ranks as only the 28th driest in DC history -- 1991 was only 1.57 inches. 1991 also finished with these three maximum temperatures: 97 on May 29th, 98 on May 30th, and 99 on May 31st.

I'm not sure that any meteorological organization uses weighted averages. Presumably, that is because in the old days -- prior to the widespread use of calculators -- it was a pain to compute anything but unweighted averages by hand, and the methodology was never updated. In general, this isn't a big deal, but when you have an abnormally cold or warm February, unweighted averages can result in slightly erroneous figures for both the first quarter and the meteorological winter.

I agree with your figures (I previously misstated this year's 1Q at IAD as 31.6 degrees). Bear in mind, though that these figures are unweighted averages of January, February, and March average monthly temperatures, which gives too much weight to February, as it has only 28 days (29 in leap years), compared to 31 days each in January and March. Weighting each of the first 90 days of each year equally results in 1Q 1970 being colder than 1Q 1978 at IAD (31.4 vs. 31.7). 2015 would still be third coldest, but at 32.0 degrees, rather than 31.8. Perhaps some day weighted averages will be used to calculate quarterly and seasonal averages.

Not so dramatic at DCA, but this year's first quarter average of 37.1 degrees was the lowest since 36.9 in 1994. IAD's first quarter average of 31.6 degrees was the lowest since 31.5 in 1978.

While DCA narrowly missed breaking the last half of February average low temperature record in DC, it has just narrowly broken the DC record for the lowest average temperature for the three weeks ending March 7th. February 15-March 7, 2015 averaged 28.1 degrees at DCA, breaking the record of 28.2 degrees set 101 years ago, February 15-March 7, 1914. Further, IAD averaged only 22.6 degrees during the last three weeks, shattering the previous record low average of 28.7 degrees for those three weeks set February 15-March 7, 1978.

That depends on what probability assumptions you make. For example, during February 2011-October 2012 inclusive, every month (21 straight) at BWI was above normal, leading some to believe that the probability of a below normal month there had fallen well below 50% -- possibly as low as only 20-30%. However, since then only 13 of 28 months have been above normal at BWI. If you do a straight binomial calculation, the odds of 10 of the last 16 months being below normal there are as follows, given these probabilities of a below normal month: 50% chance of below normal month: 22.7% chance of at least 10 months being below normal. 40% chance of below normal month: 5.8% chance of at least 10 months being below normal. 30% chance of below normal month: 0.7% chance of at least 10 months being below normal. 20% chance of below normal month: 0.02% chance of at least 10 months being below normal. So, I would conclude that the probability of a below normal month at present is much closer to 50% than it is to 20%. However, we could simply be in an unusual run of below normal months. Nonetheless, the probability of 21 straight months being above normal followed by only 13 of 28 months being above normal simply due to random factors is unlikely in the extreme.

February 2015 appears to be finalized at 30.3 degrees for DCA and at 25.4 degrees for IAD. That makes each airport the coldest in February since 1979 (DCA: 28.4, IAD: 23.3), and it places DCA 14th coldest of 145 years in DC history and IAD second coldest of 53 years in Dulles history. February 2015 was also the coldest month experienced at DCA since January 1994 (28.8) and at IAD since December 1989 (23.1). For the meteorological winter as a whole, DCA averaged 36.5 -- pretty much in the middle at 68th coldest of 144 seasons and coldest only since 2009-10 (35.8). However, IAD averaged just 31.0 -- the 7th coldest of 53 seasons there and coldest since 2002-03 (30.7). Regarding the last half of February 2015, it was a case of "missed it by that much" at DCA. It averaged an even 25.0 degrees from the 15th to the 28th, which was just a tenth of a degree warmer than February 15-28, 1934, and 1934 also holds the record for the coldest February as a whole, at 24.6 degrees. However, the last half of February 2015 was 7.3 degrees colder than the last half of February 1979 at DCA, and IAD's February 2015 last half average of 19.5 degrees was 7.5 degrees colder than the last half of February 1979 there.

Dulles is a lock for the second coldest February there, but 1979 is now out of reach due to the warm-up yesterday afternoon and today. As Ian notes, DCA will finish in the Top 15 coldest in DC history, but will also finish warmer than in 1979. However, DCA still has a chance for the coldest last half of February in DC history. Further, the second half of February at both IAD and DCA will finish way colder than in 1979.

Last Thursday didn't work out, but this morning did: -4 at Dulles. That's the coldest ever there this late in the winter.

The coldest IAD and DCA Februaries since 1979 are now pretty well assured. Further, both airports are headed for the coldest second half of February (15th-28th or 29th in leap years) in their respective histories. Finally, and most remarkably, DCA is very likely to break the record for the coldest second half of February in DC history. The current record holder is February 15-28, 1934, which averaged 24.9 degrees at the Weather Bureau location at 24th and M Streets, NW. If National Weather Service, Weather Channel, and AccuWeather forecasts through the end of the month come even close to verifying, that record will be broken by at least a degree or two.

Weather Underground currently forecasts -6 at Dulles Airport on Thursday morning. If that holds -- or even if the minimum reaches -3 -- that would be the coldest there since -9 on February 6, 1996. The all-time low at Dulles was -18 on January 22, 1984. The all-time February low there was -14 on February 18, 1979 -- the Sunday of the first Presidents Day storm.

It looks as if IAD is headed for its coldest February since 1979. DCA could achieve that as well, but February 2007 featured a negative departure relative to the 1981-2010 normals of 8.1 degrees (15th coldest February in DC history), so that could be a close call.

January 2015 continued the trend of a big DCA/IAD average temperature differential. DCA averaged 35.6 degrees, while IAD averaged just 29.7. The 5.9 degree differential is the largest in January since 1984, when the differential was 6.4 degrees. The largest January differential was 6.9 degrees in 1975, while the smallest was 1.3 degrees in both 2005 and 2006.

I just ran a two-tailed T-Test for the periods 1872-1983 and 1984-2015 to determine the probability that the temperature differences between January 13th and January 22nd in DC for each period are attributable to random factors, or statistical noise. For the first period, the probability is 0.375%, or about one chance in 267. For the second period, the probability is 0.088%, or about one chance in 1132. The results for the second period are particularly interesting, as the 1981-2010 "normals" for DC indicate that temperatures bottom out in mid-January. Thus, if those normals are taken at face value, January 22nd should be slightly warmer than January 13th, not eight degrees colder.

Thanks for the response, Roger. However, DC temperature data show much more than a handful of anomalous warm days recently on January 13th. I've already compared that day to January 21st for the periods 1872-1983 and 1984-2015. Here now is the same comparison for January 13th and January 22nd: During the period 1872-1983, January 13th averaged 33.3 degrees (coldest January day, and third coldest day of the year), compared with 37.2 degrees on January 22nd (warmest January day). January 13th was the colder day 69 of those 112 years, or 61.6% of the time. During the period 1984-2015, January 13th averaged 40.3 degrees (warmest January day), compared with 32.1 degrees on January 22nd (coldest day of the year). January 13th was the colder day just 7 of those 32 years, or 21.9% of the time.

January 21st did manage to nose out January 13th in DC for a warmer average temperature this year, 36.5 to 35.5. So, the updated DC numbers for the two days are as follows: During the first 112 years of official DC temperature records, January 13th had a lower average daily temperature than January 21st 70 times (62.5%), with three ties (2.7%). During the most recent 32 years, January 13th had a lower average temperature than January 21st seven times (21.9%), with no ties. January 22nd has also grown much colder in DC since 1984, although this year it was a balmy 42.5. So, perhaps that's some support for your position. Still, it's striking how few cold waves have occurred in DC in mid-January for more than 30 years, while they have been common in the latter third of the month during that same period. Regarding your point about smoothing of temperature data, that's actually my point as well. In other words, the "normal" temperatures reflect underlying assumptions about temperature trends, which may not be valid in all instances.

December 1880 was only the fourth coldest December in DC history, with an average temperature of 29.0 degrees (1876: 27.1; 1917: 27.9; and, as some of us remember, 1989: 27.9), but it did end on a rather chilly note, with a memorable New Year's Day morning low: Dec 30, 1880: Max 9, Min -7 Dec 31, 1880: Max 13, Min -13 Jan 1, 1881: Max 19, Min -14 In contrast, December 1989 managed a low of only 5 above zero (on the 22nd), and was relatively balmy on New Year's Eve (Max 46, Min 33) and New Year's Day 1990 (Max 46, Min 35).

December 2014 averaged 43.7 degrees at DCA to bring the yearly average to 58.5 degrees, but only 38.3 degrees at IAD to bring the yearly average to 53.0 degrees. Thus, the 2014 differential was 5.5 degrees, tying 1975 for the second greatest differential ever, behind the 5.9 degree differential of 1976. The 58.5 degree average at DCA was the coldest only since 57.4 degrees in 2009, whereas the 53.0 degree average at IAD was the coldest since 51.9 degrees in 1976.

Same relative deal for November: November 2014 averaged 48.0 degrees at DCA (1.6 degrees below the 1981-2010 normal), but only 41.8 at IAD (4.6 degrees below the 1981-2010 normal). The 6.2 degree monthly differential brings the 11-month average differential to 5.45 degrees -- again the second largest ever. 1976 averaged a 6.05 degree differential through November.

October 2014 averaged 62.9 degrees at DCA (3.4 degrees above the 1981-2010 normal), but only 57.0 at IAD (1.0 degree above the 1981-2010 normal). The 5.9 degree monthly differential brings the 10-month average differential to 5.4 degrees -- the second largest ever. The largest differential was in 1976, which averaged 6.2 degrees through October and 5.9 degrees for the year as a whole.

I have put together a decade-by-decade comparison of the average daily difference in maximum and minimum temperatures at Reagan and Dulles Airports. Overall, since IAD opened in November 1962, DCA has averaged 1.4 degrees higher in maximum temperature and 5.8 degrees higher in minimum temperature. Decade by decade it looks like this, with the first number the degrees by which the DCA average maximum has exceeded IAD's, and the second number the degrees by which the DCA average minimum has exceeded IAD's: 11-17-62/12-31-70: 2.1/5.3 1-1-71/12-31-80: 2.1/6.9 1-1-81/12-31-90: 0.8/6.4 1-1-91/12-31-2000: 1.0/5.7 1-1-2001/12-31-2010: 0.7/4.6 1-1-2011/09-30-2014: 1.9/5.6