So, this is summer. Wait … this is summer? Many of us are beginning to doubt that the season has officially set in. It’s true that the last month has been extraordinary, all because of the overwhelming wetness that’s enveloped the area.
Almost 10 inches (9.97”) of rain fell at Reagan National Airport in June, the fourth-highest total for the month in Washington, D.C. since 1871. Only the double-digit monthly totals recorded in 2006 (14.02”), 1972 (11.53”), and 1900 (10.94”) surpass 2013’s mark.
This historically high-water number is drawing much attention, and deservedly so, given sharp memories of the recent hot DC summers and overall lack of rain prior to the June onslaught. All this rain – and the repetitive daily threat of heavy downpours – is not normal. Something is out of whack with the weather.
We can first point to the pattern about 20,000 feet above the surface. Brad Panovich, Chief Meteorologist at WCNC-TV in Charlotte, N.C., posted this map on his Facebook page:
It shows an atmospheric block, within which high pressure over the western U.S. and another high off the East Coast trap low pressure over the Midwest. Individual disturbances (or waves of energy) rotate through the main low and draw deep tropical moisture up into the Eastern states. These patterns are highly resistant to change in summer because the jet stream pulls well to the north, preventing a fast, sweeping steering flow that could clean out the moisture.
At the surface, weather fronts that would normally slice through the area get hung up, stalling out in the light flow. The southerly wind current formed between the low to the west and high to our east intersects the front. Because it is less dense, warm, humid air lifts up and over the frontal boundary, condensation occurs, and precipitation breaks out.
Injecting high humidity from the tropics is key, as an atmosphere that can hold more water (and produce more sweat on you and me) will maximize rainfall potential. This is what happened in late June 2006, when the tropics opened for business and soaked D.C. with over 13 inches of rain in the four-day period from the 24th through the 27th.
The predecessor to the CWG blog, CapitalWeather.com recalled the events of June 2006 in its “Greatest Hits” post from January 6, 2008:
Mudslides closed parts of the beltway, some metro stations were flooded, and a State of Emergency was declared in Washington, DC.
This is how CWG’s Jason Samenow described the behavior of moisture surges during the historic 2006 flood (sound similar to our forecasts for early this week?):
… due to the somewhat random nature in which these storms develop, some areas may receive less than this [2-4 inches] while some may receive more. And some areas may well experience significant periods of time with no rain and even some sunshine.”
Fortunately, a widespread flooding event has not developed over the past four weeks. 2013 is also different from 2006 in that the heavy rains last month were spread out (three separate events of 2.50” or slightly higher totals) – not compressed to a small period.
Before arguing that 2006 is an overly flawed comparison, consider the similar features: stalled-out front nearby, influx of tropical moisture via southerly flow, and a blocking pattern supporting extreme western heat.
Returning to the point I made earlier, it is quite unusual to get nearly three times the normal amount of rain in any month here in Washington, let alone in a summer month, when actual tropical storms or their remnants most frequently deliver the heavy rains. Just the right constellation of atmospheric features, however, leads to an excessively wet outcome for the District and much of the East Coast.
For those growing weary of this wet weather, an inland push of high pressure will bring more stable air to the region after today (though we won’t be able to completely escape storm chances and humidity will be at least moderate). The stable high pressure pattern will last through the weekend and into the first half of next week. But afterwards, the longer-range guidance is hinting at another wet pattern for the eastern third of the U.S.