Comment Early Tuesday afternoon, a tornado roared through Bowie in Prince George’s County, downing power lines and trees — some breaking into homes. While Bowie residents had about 10 minutes of warning about the twister, it surprised forecasters by forming on a day when tornadoes weren’t considered much of a threat. Many components came together to support the 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, meteorologist in charge, said the twister earned an EF1 rating on the 0 to 5 scale of tornado intensity, packing maximum winds estimated at 90 mph. The storm survey for the Bowie tornado found that 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 just north of Bowie, Manchester,” the survey said. “There was also an incident where a tree fell on the top of the residence on Stafford Ln. The most concentrated damage occurred between Stafford Ln and Saber Ln. While thunderstorm 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 lead time. “Residents in the area noted that they received the wireless emergency alert that disseminated the tornado warning that was issued … before damage occurred and took appropriate steps to reduce the risk of injury from the tornado,” the storm report said. “No injuries or deaths were reported from the tornado.” Before reaching Bowie, the storm left behind a trail of tree damage between Greenbelt and Laurel. Lee said the Weather Service is still assessing whether a tornado touched down down there. The National Weather Service may release additional information on its findings late Wednesday afternoon, Lee said. About 15 miles southeast of Bowie, Capital Weather Gang affiliate Matthew Cappucci videotaped evidence of weak tornado activity in a rural part of Anne Arundel County, which Lee said may support an EF0 rating. Tuesday proved to be a tricky day for forecasters. The Weather Service had put the area at a 2 out of 5 risk level for severe storms, but did not highlight tornadoes as a risk of particular concern. When showers and a few thunderstorms swept through the area in the early afternoon, short-term computer models simulated only isolated additional storms in their wake, mostly north of Washington. However, swirling storms were able to break out in a narrow corridor east of Washington, where the environmental components laughed 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 seen in the left panel, it contained a core of very heavy rain and embedded small hail. A component on the northwest side—the “hook echo”—is a telltale sign of a rotating storm updraft or mesocyclone. The right panel shows the wind speed derived from the radar. Red hues reveal strong radar outflow. Shades of green show counterflow, inbound toward the radar. The 180 degree shift over a short distance, combined with the hook echo, is called the “velocity couplet” and reveals the position and strength of the counterclockwise rotating mesocyclone. An in-depth guide to tracking tornadoes using radar The mesocyclone was not the tornado, per se, that was tracking Bowie. Rather, the tornado represents a tiny region of condensed rotation that has developed between the parent mesocyclone and the ground. The exact mechanisms behind this so-called “tornado” continue to elude meteorologists. There was a very complex interaction between atmospheric elements, which led to the storm, and these elements coincided in a very small area and a short period of time. It turns out that the high-resolution models that forecasters relied on all did a really bad job throughout the day — to the point of being misleading. “Note, HiRes guidance has not handled the storms very well today,” the Weather Service wrote in a discussion Tuesday. Poor model performance is one reason a 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 components, at 5 p.m. The first key element, a warm front, aligned with the Chesapeake Bay and helped lift unstable air along and adjacent to the frontal orientation. The warm front likely also provided a source of rotation near the ground, which helped induce rotation in the storm. There are many historical instances of tornadoes forming in the Washington area when the parent storms are near a warm front. The small patch of red contours just west of the front shows a region of the atmosphere that had quickly destabilized after early afternoon showers and cloud cover. The extent and duration of the cloud cover was one of the “wild cards” in the forecast for severe storms that forecasters viewed as highly uncertain. Unfortunately, a significantly unstable air mass developed by 5 PM, as seen in the center of the red “bull’s eye,” and the supercell was able to entrain this suspended air into the southerly, low-level winds west of the warm front. A third key feature emerged through a deep layer of the atmosphere – a small pocket of strong wind shear (increasing winds with altitude) shown by the blue contours. The core wind shear values of 40 to 50 knots (46 to 57 mph) were of sufficient strength to cause supercell development. Its alignment along the warm front and overlap with the region of unsettled air created a “triangle” of conditions ripe for a supercell carrying a tornado. But wait, there’s more! The image below shows what is perhaps the key element that helped tie all the previous aspects together. Called a mesoscale convective vortex (MCV), it is a compact, intense pocket of rotation in the mid-atmosphere heading into central Maryland off Ohio. The MCV formed within a larger storm complex far northwest of the Washington area during the early morning hours – and was essentially the “ghost” of that former storm complex. The MCV was not immediately visible from the widely spaced network of weather balloon stations, but was identifiable in satellite loops. His arrival above the “triphon belt” did many things. First, the gyre caused strong upward motion on its southeast side. This helped spark the storm’s updraft, contained within its mesocyclone. Second, the MCV introduced high amounts of spin energy into the storm’s genesis zone, which was likely used by the storm’s mesocyclone. These factors resulted in a strong, persistent, strong mesocyclone with sufficient vigor to spawn a tornado (only 25 percent of mesocyclones ever spawn a tornado). Tuesday’s strong storm forecast was a challenge for forecasters across the region, given the proven poor performance of our best models and the uncertainty with the cloud forecast. This retrospective analysis of the environment in the Prince George’s County area clarifies the ingredients that came together for a rapidly evolving tornado arrangement that were unfortunately not apparent beforehand.
