Maryland has already seen five tornadoes this year

Tornadoes confirmed in Howard and Frederick counties on Thursday were the fourth and fifth in Maryland this year, two more signs that this is one of the most active severe weather seasons in years, both locally and nationally. They followed a moderate tornado in the Clarksville area May 23, and weak tornadoes in Monkton and Federalsburg in April.

Already, this year is Maryland’s most active for tornadoes since 2013 — the last in a string of three years in which the tornado tally hit double digits, with as many as 18 in 2011. Across the country, there have been more than 1,000 tornadoes so far in 2019 — third-most since 2005, behind only 2008 and 2011, according to the Storm Prediction Center.

May was a particularly active month, and potentially record-setting. There were at least eight confirmed tornadoes across the country every day for 13 consecutive days, according to AccuWeather.com, one of the longest such streaks of severe weather on record. And there were more than 500 preliminary tornado reports during the month. After each is investigated, the final count for the month could surpass a record of 414 set in May 2015, AccuWeather meteorologists said.

Meteorologists say the flurry of cyclones is the result of an active weather pattern in which the jet stream buckles over the United States, creating what they call ridges of high pressure and troughs of low pressure. The jet stream steers weather from west to east across the country. But when it weakens and bends — potentially a consequence of warming in the Arctic, scientists say — those pressure differences clash, often spawning severe storms.

That pattern was unusually persistent throughout the second half of May, contributing not just to the spate of tornadoes but severe flooding across the Midwest, AccuWeather meteorologist Alyson Hoegg said.