We're coming to the close of Meteorological Summer 2013, which is comprised of June, July and August. Considering how hot it can get in the DC Metro area, we've actually had a pretty tepid summer! During those three months of Meteorological Summer, we only had 22 days with temperatures above 90. By amazing contrast, in 2012, we had 48 days of 90+ temperatures during those three months! We had as many days above 90 in July of last year as we have had in this entire summer period. Possibly even more shocking- we have had ZERO triple-digit days in Washington this year. Last year, we had eight. Eight sweltering days!
Of course, last year was an especially hot summer here in the DC Metro (the third-hottest on record), so the comparison looks especially stark. But if we give our data a bigger sample size- for instance, the current climate record used in our record-keeping- we can clearly see that we're still well below normal. In a typical summer, we have 36 days of 90+ temperatures in DC. So this year's 22 is well short of a normal, hot summer in Washington! So, if it hasn't been hot here in the DC area, where IS the summer heat? As it turns out, it's been mostly in the West.
I'll use Boise, Idaho as an example. Central Idaho has a much drier climate than we have here, so the summers generally have warm afternoons and cool, comfortable nights. This summer, however, has been scorching hot in Boise. This summer has featured 14 days with temperatures above 100. July was the hottest month, with 10 days of 100+ temperatures and, unbelievably, only one day with a high temperature below 90! In Salt Lake City, the heat has been even more intense, with 20 days since June 1st when the temperature topped 100. The city recorded its hottest July ever this year, with records dating back to 1874!
The reason for the heat in the West, and the relatively cool summer here in DC, are very much related. For nearly the entire summer, a huge ridge has been sitting over the western half of the country. The ridge allows heat to surge in from the south at the atmosphere's upper levels. In the lower levels, this warm airflow also has plenty of moisture in it, but the mountainous terrain in Arizona, Nevada and southern California wrings out the moisture before it can traverse the mountains and reach Utah and Idaho. As the ridge pushed northward toward the Canadian border, it forced a compensating trough into the eastern half of the USA, like the one in this picture:
The ridge is outlined in red, but it's clearly visible even without the extra detail. This bubble of hot air remained almost stationary for most of the summer, which not only kept our temperatures down, but in addition, we had to deal with more cloud cover and more frequent rain this summer than what we typically see (about 5.5" above normal since June 1st).
Thankfully, just in time for the end of summer, the pattern is changing. That massive ridge in the west is starting to flatten a bit and move east, toward the DC Metro area.
In this graphic above, you can clearly see how the ridge is no longer over Boise and Salt Lake City (BOI and SLC), but it's creeping toward Washington (DCA)! What does that mean for us? Well, if the ridge continues to progress toward DC, our weather will be warmer and drier. The upcoming Labor Day weekend will feel very much like summer, and the summery temperatures will probably stick around for most of September. And, since the ridge has thousands of miles of continental mass to traverse before reaching our area, the air should be pretty dry, too. A potential spoiler could be in the form of a tropical storm, and since we're entering the peak of the hurricane season, this is a possibility. But, if this ridge stays somewhat intact, we should expect some pretty pleasant weather this fall!