Comment on this story
Comment
A twister ripped through Bowie in Prince George’s County early Tuesday night, downing power lines and trees — some smashing into homes.
Although Bowie residents had about 10 minutes of warning about the twister, it surprised forecasters by forming on a day when the tornado was not considered a major threat. Several ingredients came together to support the swirling storm.
Tornado in the Bowie area, with reports of tree and structure damage
The National Weather Service in Sterling, Va., surveyed storm damage around Bowie Tuesday night. James Lee, the meteorologist in charge, said the twister was rated EF1 on the 0 to 5 scale of tornado intensity, unleashing peak winds estimated at 90 miles per hour.
The storm survey for the Bowie tornado found the twister was on the ground for a mile between 5:31 and 5:34 p.m., reaching a maximum width of 125 yards.
“The tornado caused extensive tree damage in the Somerset subdivision, north of Bowie, Maryland,” the study said. “There was also one incident where a tree had fallen on a house on Stafford Ln. The most concentrated areas of damage occurred between Stafford Ln and Saber Ln.”
While storm watches were not in effect ahead of the storm, the weather service issued a tornado warning for the area at 5:21 p.m., offering about 10 minutes of advance time.
“Residents in the area noted that they had received the wireless emergency alert disseminating the tornado warning issued … before the damage occurred and had taken appropriate actions to reduce the risk of injury from the tornado,” the report said. the storm. “There were no reports of injuries or deaths from the tornado.”
Before reaching Bowie, the storm left behind a pocket of tree damage between Greenbelt and Laurel. Lee said the weather service is still assessing whether a tornado touched down down there. The weather service may release more information about its findings late Wednesday afternoon, Lee said.
About 15 miles southeast of Bowie, Capital Weather Gang contributor Matthew Cappucci captured on video evidence of weak tornado activity in a rural part of Anne Arundel County, which Lee said could support an EF0 rating.
Tuesday proved to be a confusing day for forecasters. The weather service had placed the area under a 2 out of 5 risk for severe storms, but did not highlight the tornado as a hazard of particular concern. As showers and a few storms moved through the area in the early afternoon, short-term computer models only simulated isolated additional storms in their wake, mainly north of Washington.
But the swirling storms managed to burst into a narrow corridor east of Washington, where the environmental ingredients gelled shortly after 5 p.m.
Inside Tornado Bowie and how it formed
The radar image below shows the isolated nature of the supercell storm. As shown in the left panel, it includes a core of very intense rain and embedded small hail. An appendage on the northwest side – the “hook echo” – is a telltale sign of an up-spinning storm or mesocyclone.
The right panel shows the radar-derived wind speed. The red tones reveal a strong stream coming out of the radar; green shades indicate the opposite flow entering the radar. The short-distance 180-degree shift, combined with the hook echo, is called a “velocity couplet” and reveals the location and strength of the counterclockwise rotating mesocyclone.
An in-depth guide to tracking tornadoes using radar
The mesocyclone itself was not the tornado that passed through Bowie. Rather, a tornado is a small region of condensed rotation that has developed between the main mesocyclone and the ground. The exact mechanisms behind this so-called “tornadogenesis” continue to elude meteorologists.
There was a very complex interaction between the atmospheric elements that produced the storm, and these elements coincided in a very small region and a short period of time. It turns out that all the high-resolution models forecasters relied on did a very poor job throughout the day – to the point of being misleading.
“Note that the HiRes guidance did not handle the storms well at all today,” the weather service wrote in a discussion Tuesday.
Poor model performance is one reason why careful analysis of post-storm environmental conditions is so important as part of the learning experience.
The image below shows a composite weather map of the interacting atmospheric elements at 5:00 p.m. The geography should be familiar, and the small purple diamond marks the location of Bowie’s supercell. The first key element, a warm front, was aligned with the Chesapeake Bay and helped to raise unstable air along and adjacent to the frontal orientation.
The warm front probably also provided a source of near-ground rotation that helped induce rotation in the thunderstorm. There are many historical instances of tornado generation in the Washington area when the main thunderstorms are located near a warm front.
The small patch of red contours just west of the front indicates an area of the atmosphere that has destabilized rapidly as a result of early afternoon rain and cloud activity. The extent and duration of cloud cover was one of the wild cards in severe storm forecasting, which meteorologists considered a highly uncertain element.
Alas, a significantly unstable air mass had developed by 17:00, as shown in the center of the red bull’s eye, and the supercell was able to touch this buoyant air in southerly, light winds west of the warm front.
A third key feature appeared deep in the atmosphere—a small pocket of strong wind shear (increasing winds with altitude), shown by the blue contours. Baseline wind shear values of 40 to 50 knots (46 to 57 mph) were strong enough to trigger supercell development. Its alignment along the warm front and overlap with the region of unstable air created a “trifecta” of conditions ripe for a tornado-carrying supercell.
But wait, there’s more! The image below shows perhaps the key element that helped connect all the above aspects. It’s called a mesoscale convective vortex (MCV), and it’s a compact, intense pocket of rotation in the middle atmosphere that tracks across central Maryland from Ohio. The MCV was generated in a larger storm complex northwest of the Washington area in the early morning hours – and was essentially the “ghost” of that former storm complex.
The MCV was not easily visible from the widely spaced network of weather balloon stations, but it was recognizable on satellite tracks. His arrival above the “trifecta zone” did several things. First, the gyre pocket produced strong upward motion on its southeast side. This helped strengthen the storm’s updraft contained within its mesocyclone.
Second, the MCV introduced large amounts of vortical energy into the storm’s genesis zone, toward which the storm’s mesocyclone likely entered.
These factors resulted in a stable, persistent, strong mesocyclone with sufficient strength to spawn a tornado (only 25 percent of mesocyclones ever generate a tornado).
Tuesday’s severe storm forecast was a challenge for meteorologists across the region, given the apparent poor performance of our best models and uncertainty with the cloud forecast. This post facto analysis of the environment around Prince George’s County elucidates the ingredients that came together for a rapidly developing tornado setup that were unfortunately not apparent beforehand.
Add Comment