Today offered a nice contrast to D.C.’s typical sweltering summer heat, with delightful highs in the 70s. No one would mistake it with the frosty mornings of April and May, but for June weather, Washingtonians will take it.
Despite this splendid shot of relief, past summers in Washington, D.C. have brought stretches of cool temperatures far beyond compare. Sixteen or so warm season “cold snaps” stand out as cool antidotes to D.C.’s summer sweatfest.
A summer cold snap – subjectively defined – is a period longer than one day within which both the high and low temperatures are 10 or more degrees cooler than the long-term averages (except for one cold case in June, described in the “June ‘Cold Snaps’” graphs shown later in this post).
Over the course of Reagan (DCA) and Dulles (IAD) Airports’ recording history, these events have occurred roughly every four years and most frequently in June, the first summer month.
Many more of the qualifying cold snaps have occurred in June – rather than in July or August – because the jet stream pulls up to its most northern position for mid-late summer, reducing the chance for extreme fluctuations in temperature across our region.
Winter and early spring feature many more extremes overall, as the jet stream rides much farther south and can more easily lift up warm Gulf air and drag down cold air from Canada.
The patterns behind Washington’s cold snaps have similar themes, but because they support extraordinarily cool weather (relative to the season), the right timing and strength of associated pressure systems are rarely seen.
A strong Canadian high-pressure system – on the order of 1024 mb or higher – accompanied by a large dip in the jet stream that helps pull the Canadian chill southward are the primary features of a cold summer stretch. In some cases, a low pressure system to the south teams with the northern high to both wrap cold air and drive heavy rain into the region. It was just over 40 years ago – in June 1972 – that Hurricane Agnes brought this extremely cool, wet weather to Washington.
The other genre of cold snap – that of the dry variety – relies less on the dynamic interaction of high and low pressure and more on a major cold air press. The four-day stretch at the end of August 1986 – my birth month – illustrates this example quite well.
A large dome of high pressure, which measured about 1030 mb upon its approach to Washington, drove Reagan’s high and low temperatures downward to near 70 and 50, respectively, on Aug. 28, 29 and 30. Dulles cooled to incredibly low levels, into the lower-mid 40s from the 28th through the 31st (these minimum temperatures were around 20 degrees below average).
As June continues and the weather inevitably gets hotter during our interminable march toward the dog days of summer, many may pine for the elusive blockbuster cooling event. If this past spring was any indication, the season could surprise with an air mass worthy of “cold snap” status.