Summer of Simmer: Semi-Seasonal Survey; First Half on Track for Record Hottest DC Summer

July 18 PM Update: Today's 95° has pushed the seasonal average temperature up another notch to 81.2°. The expected 90s through this week will likely push it up even further to 81.6°, increasing the chances of at least tying the record.

Original post:
Following a record hot June, July temperatures have been not quite as hot relative to average in the Washington, DC region. At the halfway point in meteorological summer (June-August), however, the official Washington temperature is well on its way to a place in the top 10 list of hottest summers. With a high today of 98°, the July average to date (TD) will rise unofficially to 82°, well above the long-term average for the month of 79.2°.

The seasonal average to date of 81.1°, as shown on the chart to the right, would be enough to break by over 1° the summer heat record set in 1980. This is statistically unlikely, since temperatures for August are 1.8° below July on average. What would happen if temperatures were only average for the rest of the summer? In that case, the season would still average 79.5°, enough to displace the tied 1991 and 2002 as the second hottest summer since records began in 1871. On the other hand, temperatures are expected to reach the 90s each day for the next week, further adding to the seasonal excess. Furthermore, the monthly outlook for August, issued yesterday by the Climate Prediction Center, shows at least a 40% chance for above-average temperatures.

Including today, there have now been 10 days in July with temperatures of 90° or higher. Together with the 18 days of 90°+ temperatures in June, this brings the summer total to 28, already 7 more than the final total for last year, but still 20 less than the 48 in 2002, which is currently tied as the second warmest summer. Along with the 2 days in April and 3 in May, there have been 33 days so far this year with 90°+ temperatures.

Every month this year, except February, has now been warmer than average.

The summer heat is being reflected in local residents' air conditioning bills. Today's 23 cooling degree days are over 50% above the average for the date. For the season to date (since June 1), they are slightly over 40% above average. Here are the daily, monthly and seasonal degree days and excess over normal:

July 16   23 +53%
Monthly  277 +20%
Seasonal 750 +40% 

Images (click to enlarge): Washington, DC summer average temperature to date and projected based on historical average vs. top 10 warmest summers; August 2010 temperature forecast from Climate Prediction Center; Number of 90° or warmer days, June-August, 2000-2010; 2010 monthly average temperature departures from normal; CapitalClimate charts from National Weather Service data, unless otherwise noted