A downed tree branch from overnight storms at Congressional Cemetery on April 15. (Jim Havard/Twitter)

A vigorous spring storm system, which caused extensive damage and eight fatalities in the South, swept through the Washington region Sunday evening into the predawn hours Monday. The system was not nearly as violent in the Washington region as it was to the south, but it still generated some intense storms, which led to some damaging gusts.

The storms came in two rounds, the second of which was most widespread and intense. It raced through the region between 1 and 4 a.m. Monday, unleashing gusts to around 60 mph. The National Weather Service logged about a dozen reports of downed trees.

Some of the peak gusts reported to the Weather Service include:

  • Fredericksburg: 63 mph
  • Seat Pleasant in Prince George’s County: 62 mph
  • Reagan National Airport: 55 mph

Map shows reports of strong winds (wind icon) and downed trees (tree icon) from storms Sunday evening into Monday morning. (Iowa Environment Mesonet)

The first round of storms, which hit Sunday evening, was somewhat tamer and mainly affected areas west and northwest of the Beltway. Although it triggered several severe thunderstorm and two tornado warnings (in Frederick and Carroll counties), damage reports were scarce.

How it happened

The part of the large storm system that moved through Washington during the evening and overnight was called the warm sector — a wedge-shaped region in the southeast section of the storm, ahead of the cold front. Within this sector, strong low-level winds from the south imported a very humid and moderately unstable air mass from the Gulf of Mexico.

In the middle and upper levels of the atmosphere, a pronounced dip in the jet stream, called a trough, overspread vigorous winds that changed direction with altitude. This created an impressive wind shear within the unstable air mass.

The unstable air enabled air rising along the cold front to soar to great heights, triggering the strong line of storms in the predawn hours Monday. Meanwhile, the shear created zones of spin within storm clouds and helped generate corridors of wind damage.

A radar image of that convective line is shown below at 2:45 a.m., consisting of multiple bands and short segments. The wavelike appearance was generated by small vortices and regions where strong winds caused parts of the line to surge forward, creating small arcs and bow shapes.

Radar view of storms at 2:45 a.m. Monday. (RadarScope)

In the two hours it took for these bands to sweep through the region, a total of 12 severe thunderstorms and two tornado warnings were triggered.

This was an impressive line that was well forecast, down to nearly the hour, from earlier in the day Sunday. Such overnight occurrences of severe storms are uncommon in our region. But what the sun could not provide in terms of destabilizing the atmosphere, the surge of warmth and humidity from the south in part made up that difference. The strong uplift within the jet stream and from along the intense cold front also contributed to the air’s upward vigor.

Damage pictures