Beastly storms swept through portions of the Washington region Thursday afternoon and evening, surprising forecasters with their might and stunning eyewitnesses with their menacing clouds, bursts of wind and torrents of hail.

Forecasters had predicted scattered showers and storms but just an outside chance of severe weather. Yet for the second time this week, several storms blossomed to their full potential in the region, unleashing wind gusts up to 70 mph and hail up to an inch wide.

Montgomery County was the epicenter of Thursday’s severe storm activity. Two intense storm cells, which had produced strong winds and hail around Frederick, Md., as well as the eastern panhandle of West Virginia and northern Loudoun County, coalesced in the northern part of Montgomery County just after 6 p.m.

Downed trees were reported in Damascus, where the storm deposited enough hail to whiten the ground and stack 4 inches high in spots according to Robert Leffler, a National Weather Service cooperative observer.

Then the storm charged southeast along the Interstate 270 corridor. Wind gusts were clocked to 70 mph in Gaithersburg and 68 mph in Germantown, where the National Weather Service received multiple reports of downed trees and power lines. In Germantown, a tree fell on a home.

As the storm pressed farther southeast, the Weather Service received reports of multiple trees down in Rockville and north Bethesda.

“Wow, it’s really bad in Rockville right now. Sideways rain and flooding,” tweeted Karie Kirkpatrick.

As the storm moved inside the Beltway, multiple trees came down in Bethesda’s Battery Park damaging several homes.

“That was almost derecho level crazy!” commented Andrea Colantii Witt of Bethesda on Facebook.

The same storm reached as far south as Upper Northwest Washington, where there were additional reports of trees and wires down.

While the Montgomery County storm was most damaging, another storm tracked from Fauquier County to Charles County one to two hours later bringing down trees in Warrenton and Triangle while winds gusted to 63 mph in Quantico.

In all, the Weather Service logged several dozen reports of damaging winds and hail across the greater Washington region.

How did the storms become so intense?

While the storms that developed Thursday were widely scattered, the few that formed were impressive — relative giants that stood out by virtue of their strong radar signatures and staying power. Like the storms that caused damage in parts of the region Monday, they formed in an environment that was generally not conducive for widespread severe weather.

Radar view around 5 p.m. Thursday showing three lone but intense thunderstorms. (RadarScope)

Interestingly, the atmospheric conditions that gave rise to Monday’s storms could not have been much more different from those on Thursday. On Monday, very localized “pulse storms” were responsible for small pockets of damaging winds. They formed in a very unstable atmosphere, characterized by large buoyant energy, but meager wind shear, which is an increase in wind speed with altitude.

Monday’s pulse storms went up fast, formed top-heavy and intense rain cores, which then quickly collapsed as chilly blasts of strong wind. Many of those narrow, fingerlike storm cells rose to an impressive 50,000 feet.

But Thursday’s storm cells, by contrast, lacked strong instability, erupting thanks to exceptionally strong wind shear. They were short and stout, capped for the most part at 35,000 feet, but they contained vigorous cores that persisted for several hours. They were an entirely different breed of thunderstorm, what we call mini supercells.

Three-dimensional view of storm in northern Loudoun County at 5:23 p.m. Thursday. (Weather Lab)

Pulse storms are driven by a very unstable air mass; they are “one-hit wonders” that blow up, then collapse, and expend themselves in just tens of minutes. But on strong wind shear days, the cloud updraft and downdraft become tilted, so they do not interfere with each other. Supercell storms can hold together for hours. The instability need not be overwhelming to get a severe storm. And the shear can initiate a region of rotation in the updraft, called a mesocyclone.

One of these rotating storms with a mesocyclone traversed from near Bowie through Annapolis across the Chesapeake Bay on Thursday afternoon, developing a classic “hook echo,” the radar signature sometimes indicative of a tornado, along the western shore of the Chesapeake Bay, as shown below. While we received one unconfirmed report of a waterspout in Kent Narrows near the Chesapeake Bay Bridge, there has been no validation.

Strong thunderstorm containing some rotation near Annapolis late Thursday afternoon. (RadarScope)

Shown on the figure below is the radar view of the combined supercell-like storms that developed in west central Maryland and the West Virginia panhandle and merged over Montgomery County:

Severe storm over Montgomery County just after 6 p.m. Thursday. (RadarScope)

On Monday and Thursday, an approaching disturbance in the mid- and upper-level atmosphere helped trigger thunderstorm development. Thursday’s ripple, however, was associated with a very strong belt of winds aloft — strong by late June standards — which supercharged the atmosphere with significant wind shear.

It’s important to note that on Monday and Thursday, the National Weather Service Storm Prediction Center did not issue a severe thunderstorm watch despite eventual reports of severe weather in our region. The reason is that on both days, the setup was not favorable for widespread eruption of severe weather — but rather, hit or miss, isolated storms. It just happens that, on both days, isolated severe storms tracked through highly populated parts of the Washington metropolitan region, leaving behind some noteworthy damage.

Photos and videos

Storm structure

Hail and rain

Wind damage

Rainbows in the aftermath