A Wild Weather Ride in May 2016

The weather during the first two weeks of May 2016 couldn’t be more different than what Washingtonians experienced during the same period last year.  May 2015 was the warmest May on record and the 5^th drier than average May since 2010.  Meanwhile, this May has featured a continuation of the cooler and wetter than average weather in the Mid-Atlantic Region that began late last month.

Washington, D.C. has broken its all-time record for consecutive days with measurable precipitation.  There has been at least 0.01” of rain every day in the Nation’s Capital for the last 15 days since April 27.  The previous record was 10 consecutive days of measurable rainfall that occurred twice – most recently in July 1938.  Rainfall chances are less today so the streak could end at 15 days.  However, rainfall chances increase again tomorrow and Saturday.

Unlike some past streaks of consecutive days with rainfall, the current streak hasn’t featured an unusual amount of rain.  Although a total of 4.03” of rain has accumulated at National Airport since April 27, there is a still a rainfall deficit of 0.24” in 2016 as of May 11.  That helps illustrate how dry 2016 had been prior to this record stretch of measureable rainfall.  The cloudy and rainy weather has also helped keep temperatures cooler than average on 12 of the last 15 days.

While the Mid-Atlantic Region has been cooler and wetter than average, there has been warmer than average weather across much of the central United States and cooler than average weather across a lot of the western United States.  Initially, this stagnant weather pattern was due to an “omega block” that causes the jet stream to flow in a pattern similar to the Greek letter “omega.”  It’s characterized by an area of high pressure that’s blocked by two areas of low pressure.  Areas of high pressure feature sinking air that suppresses cloud formation and promotes dry weather.  That’s acerbated the wildfires that have been getting headlines recently in the Canadian province of Alberta.

Conditions have also been conducive for a fair amount of severe weather in the central United States during the past few days.  From May 7 through May 10 there has been a preliminary total of more than 650 cases of severe weather – including roughly 50 tornadoes nationally.  That’s not unusual, though, since the United States averages more severe weather in May than any other month.  The DC Metro Region also experienced severe weather earlier this month.  On May 2, Rockville, Maryland saw hail 2.5” to 2.75” in diameter – roughly the size of a tennis ball.  That set a new record for largest hail in Montgomery County according to NOAA.

Despite being roughly four degrees cooler than average during the first half of May, temperatures can still rebound during the second half of the month.  It’s interesting to note that 90-degree heat has been felt at least once in the Nation’s Capital every May since 2010 so summer-like heat can arrive quickly this time of year.  By the time the hazy, hot and humid weather characteristic of July and August arrives many area residents will remember this cooler and wetter than average weather quite fondly.