The evolution of a damaging tornado that struck Edgewater, Md., near Annapolis, Thursday evening

The storm complex, which focused on the District and areas to the north and east, produced several rotating storms that prompted tornado warnings in parts of Frederick, Carroll, Baltimore, Montgomery, Howard, Prince George’s and Anne Arundel counties.

The National Weather Service tweeted that Doppler radar confirmed the presence of a tornado about five miles southwest of Annapolis at 6:03 p.m. It then received a storm spotter report of damage coincident with the radar signature in Edgewater.

The twister downed multiples trees, peeled off siding and shingles from homes, and toppled a fence and two power poles.

Edgewater resident Robin Ward told WTOP News that the wind was “ferocious.”

The Weather Service surveyed the damage around Edgewater on Friday morning and rated the tornado an EF-1 on the 0 to 5 Enhanced Fujita Scale, estimating its peak winds at 90 mph. Its report on the storm said it touched down at 5:57 p.m. before lifting at 6:07 p.m., reaching a maximum width of 100 yards and carving a path of 6.1 miles.

The tornado evolution

Thursday’s volatile storm environment arose from a mix of an extremely humid air mass, significant wind shear (changing wind speeds and direction with altitude), a front draped over the region and associated zone of low pressure, and an approaching disturbance in the jet stream.

The Washington region was placed under an “enhanced risk” of severe storms, and a tornado watch was issued until the storms moved off between 8 and 9 p.m.

The wind shear was particularly critical to the tornado development.

Wind shear values in the range of 40 to 50 knots, as observed Thursday, are exceptional for summer in the Mid-Atlantic. The unstable atmosphere induces air to rise vigorously within cloud updrafts, but the shear contains an inherent degree of spin, which storms can tap into, developing storm-scale vortexes. The vortexes lead to more intense, long-lived cloud systems, including rotating thunderstorms or supercells.

The thunderstorm that spawned the Edgewater tornado was “messy” in terms of its structural appearance on radar. It contained a supercell with a core of rotation known as a mesocyclone, but a type embedded in heavy rain, termed an HP or “high precipitation” supercell. Two key radar snapshots in this storm’s evolution are shown below.

The radar features used to identify this supercell, shown above, were subtle. In the set of rain intensity images, shown along the top, there is no hook echo, or spiraling inward of heavy rainfall, which can be indicative of a tornado. Rather, an indentation or “cleft” was adjacent to a prominent, and persistent, bowing section of the squall line.

The radar panels showing the storm’s wind velocities, on the bottom of the above figure, reveal a fairly diffuse and spread-out mesocyclone initially. There is a small pocket of strong outbound flow (pink colors) over College Park, and a small region of inbound flow (green) just south of Calverton. The genesis of this circulation could be traced farther west, to near Potomac, Md., where a tornado warning was issued, but no twister confirmed.

The inbound portion of the mesocyclone corresponds to the inflow notch seen in the rain reflectivity (top panels), the outbound portion to the bulging part of the storm line, farther south.

The radar imagery at the later time (6 p.m.) takes on greater significance (right panels). Here, the mesocyclone was over the Annapolis-Londontowne region, and it is quite large and prominent. Velocities on both sides of the circulation (inbound and outbound) have intensified (lower right panel). This is also the time that tornado touchdown was noted. The mesocyclone remains wrapped in heavy rain (top right panel), making visual spotting of a tornado very difficult, if not impossible.

In addition to the supercell, which tracked from near Potomac to Annapolis, another took a more northern route from Frederick, Md., following the Interstate 70 corridor, into Baltimore and beyond.

For this additional supercell, radar implied a counterclockwise rotation indicative of a fairly broad mesocyclone. In the image below, the left radar panel displays a hook echo, while the right panel shows outbound winds by the red patch of color and inbound winds by the green.

To the best of our information, this supercell did not produce a tornado, but there is photographic evidence, at times, of storm rotation.

The longevity of both the southern and northern supercell storms is typical in high shear situations, as we experienced Thursday, and illustrates an interesting contrast between an isolated, “classic” supercell and the HP variety, which is often embedded in a heavily raining portion of the larger storm complex.

Anne Arundel County sky photos and video

Edgewater photos and video