August 10th wettest on record
After our all-time hottest month on record in July, it was certainly a relief to see a change in the jet stream pattern over the mid-Atlantic by mid-to-late August. The big hot heat dome that dominated June and July resided primarily over the South Central U.S. in August (consider Houston’s pain- only one day of the entire month saw a high temperature below 100 degrees!). However, a cooler August did not allow us to escape the status as the second hottest summer on record- behind only last year.
The average temperature of 79.9F is 2.3F hotter than the 1971-2000 baseline normal and 1.8F hotter than the new 1981-2010 baseline normal. August 2011 tied as the 13th hottest month with 2009 and 1973. Our hottest high temperature was the very first day of the month at an uncomfortable 100F.
The coolest high temperature of the month was Irene-induced last Saturday with only 79F. You can see how 2011 compared against 2010 and the new thirty year normals on the chart below. You can really see the seasonal decline in temperatures as we headed toward September in the graph above.
We caught up on precipitation in a very BIG way with the wettest August in 44 years. As a result of the big soggy surplus, our total rainfall since January 1 is running 0.90” above normal. We had three days of rainfall that measured over 1”, which was the highest frequency since August 1990. The 3.33” total last Saturday courtesy of hurricane Irene was the rainiest day since Aug 27, 1971 when tropical storm Doria passed through. In August, it’s all about the tropics.
The weather pattern
We were really saved by a significant retreat of the hot upper level ridging pattern that frequently dominated our weather in June-July. In fact, if it had not been for that very hot first week of August, the month would have been considerably cooler overall. You can see the prevailing jet stream situation in the Aug 1-28 upper level pattern shown below.
The June through August period was the second hottest on record, just two tenths of a degree behind last year’s record level. And July did most of the work by being the hottest month on record (of any month, of any year). The graph at the very top of the post illustrates the rankings of the top five hottest Washington, D.C. summers on record. The oldest one on the list is from 1980. 2011 bumped 1993 out of the top five.
Despite being the second hottest summer, 2011 tied for eighth place (with 1993) for the number of 90-degree or hotter days (47 days total). The “top dog” year for most 90-degree days is still 1988 with 55 days. This tells you that in order to achieve the spot for second hottest summer, we needed to see much warmer low temperatures. And so we did. 2010’s average summer low temperature was 72.4F. This year’s is 72.1F. And the hot 1988 summer saw an average temperature of only 68.9F. More humidity this summer probably kept more low temperatures warmer even if we didn’t get as many 90-degree days.
With a big boost from Irene, the summer precipitation came in at 13.63”, which is 3.19” above normal. Compare that to last year’s measly 9.63”, which was slightly drier than normal.
What were the best and worst days of the summer? I elect Tuesday, June 14th as the best day. A total “10” in my book with a high temperature of only 78F, a low of 64F, and an average temperature of 71F that made it the coolest day of the entire summer. You may debate me because we did get a trace of rain that day. For worst day, the “low-hanging” fruit is the brutal 104F day on Friday, July 29th. But I believe our “Sweat Ceiling” event the week before was more miserable. Even though we had two days at 102F, the humidity was higher and Saturday, July 23rd was the worst with the hottest average temperature of the summer at a painful 93F.
What does our new month ahead look like? With the prevailing summer pattern behind us, September 2011 should average close-to or slightly warmer than normal. Precipitation should average above normal partly due to contributions from a continued active Atlantic tropical season.
For further information
The National Weather Service publishes nice monthly assessments usually within a week of the close of each month (should be available shortly):
You can click on your closest airport location here:
Historical Washington, D.C. data provided by NOAA. See also Ian’s excellent rundown on August climatology.
Ian Livingston contributed to this post