RodneyS

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

I don't have monthly minimum/maximum temperatures at Dulles at my fingertips, but I'll try and dig some out. I do know that some recent months have not featured such extreme minimum and maximum temperature differentials between the two airports. For example, in September 2011 the high temperature differential was 0.8 degrees and the low temperature differential was 3.6 degrees. Still 2.8 degrees greater on the low side, but that compares with 3.8 degrees greater on the low side this September.

The beat goes on -- September 2014 averaged 73.9 degrees at DCA (2.9 degrees above the 1981-2010 normal), but only 67.5 at IAD (0.4 degrees below the 1981-2010 normal). The 6.4 degree monthly differential is the greatest since that same differential in January 1984.

With respect to overall temperatures, the 2014 astronomical summer at DCA was a virtual carbon copy of 2013, averaging a minimum of 69.5 degrees and a maximum of 85.8 degrees. And, believe it or not, the overall average was identical no matter how many decimal points you carry it out -- 77.66 degrees to two decimal points, which is the 26th warmest in DC history.

I believe that was just three years ago -- July 19-25, 2011 averaged 89.2 degrees at DCA, making it also the hottest week ever in DC history. July 2-8, 2012 gave that week a good run for its money, averaging 89.1 degrees. July 7, 2012 featured DC's hottest average daily temperature ever at 93.5 degrees, with a high of 105 and a low of 82.

Also, the week of August 31-September 6 was the hottest seven consecutive day period of the year at DCA, averaging 84.7 degrees. During the 2014 meteorological summer, the hottest week was June 27-July 3, which averaged just 81.8 degrees. Eyeballing historical DC temperature records to determine which year had the latest hottest week, 1985 may be the champion. September 4-10, 1985 averaged 84.3 degrees, which was hotter than the hottest meteorological summer week of August 10-16, 1985, at 82.6 degrees. Also, in 1881, which is not only the hottest September in DC history, but also the only year in which September was the hottest month of the year in DC, September 3-9, 1881 averaged 82.8 degrees, which was slightly hotter than the hottest meteorological summer week of July 10-16, 1881, at 82.6 degrees.

Remarkable August 2014 temperature differential between DCA and IAD. DCA averaged 77.7 degrees -- warmer than last August and only the 22nd coldest August there since 1963, when records began at Dulles. IAD, on the other hand, averaged just 71.8 degrees -- the coldest since the record of 70.4 in August 1992, and the third coldest August ever there. The 5.9 degree August 2014 differential between the airports was the greatest since that same differential in October 1997. The greatest monthly differential between the airports was December 1975, at 7.3 degrees.

I have data for DC only -- that's where the aspiring Ph.D meteorology student or meteorology professor comes in. But you're right -- if meteor dust is involved, one would think it would be a worldwide phenomenon. And if there is a bona fide connection, maybe there actually could be some merit in those weather planning services that advise folks six months or a year in advance as to the best day to hold an outdoor wedding: "Stay away from August 12th in the mid-Atlantic unless you have an ample supply of rafts for the reception."