WEATHER NEWS: Bowie tornado unleashed 90 mph winds. How it snuck up on forecasters.
Early Tuesday evening, a twister roared through Bowie in Prince George’s County, toppling power lines and trees — some smashing into homes.
While residents of Bowie had about 10 minutes of warning for the twister, it surprised forecasters by forming on a day when tornadoes were not considered a major threat. Several ingredients came together to support the twisting storm.
The National Weather Service in Sterling, Va., surveyed the storm damage around Bowie Tuesday night. James Lee, meteorologist-in-charge, said the twister earned an EF1 rating on the 0 to 5 scale for tornado intensity, unleashing peak winds estimated at 90 mph.
The storm survey for the Bowie tornado found that the twister was on the ground for one 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, Md.,” the survey said. “There was also one incidence where a tree had fallen on top of residence on Stafford Ln. The most concentrated areas of damage occurred between Stafford Ln and Saber Ln.”
While storm watches were not in effect in advance 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 disseminating the tornado warning issued … before the damage occurring, and took appropriate action to reduce their risk of injury from the tornado,” the storm report said. “There were no injuries or deaths reported 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 evaluating whether a tornado touched down there. The Weather Service may release additional information on its findings late Wednesday afternoon, Lee said.
About 15 miles southeast of Bowie, Capital Weather Gang contributor Matthew Cappucci videotaped evidence for weak tornado activity in a rural part of Anne Arundel County, which Lee said may support an EF0 rating.
Tuesday proved to be a perplexing day for forecasters. The Weather Service had placed the area under a level 2 out of 5 risk for severe storms but did not highlight tornadoes as a hazard of particular concern. When showers and a few storms swept through the area in the early afternoon, short-term computer models only simulated isolated additional storms in their wake, mainly north of Washington.
But twisting storms were able to flare up in a narrow corridor east of Washington where environmental ingredients gelled just after 5 p.m.
Inside the Bowie tornado and how it formed
The radar image below shows the isolated nature of the supercell storm. As shown in the left panel, it featured a core of very intense rain and embedded small hail. An appendage on the northwestern side — the “hook echo” — is a telltale sign of a rotating storm updraft or mesocyclone.
The right panel shows the radar-derived wind velocity. Red tones reveal strong flow outbound from the radar; green shades show opposing flow, inbound toward the radar. The 180-degree shift across a short distance, collocated with the hook echo, is termed a “velocity couplet” and reveals the location and strength of the counterclockwise-spinning mesocyclone.
The mesocyclone was not the tornado, per se, that tracked through Bowie. Rather, the tornado represents a tiny region of condensed spin that developed between the parent mesocyclone and the ground. The exact mechanisms behind this so-called “tornadogenesis” continue to elude meteorologists.
There was a very complex interaction between atmospheric elements, leading to the storm, and these elements coincided over a very small region and short window of time. It turns out that the high-resolution models relied-on by forecasters, all did a very poor job throughout the day — to the point of being misleading.
“Note, HiRes guidance hasn`t handled the storms well at all today,” the Weather Service wrote in a discussion Tuesday.
The poor model performance is one of the reasons a careful post-storm analysis of the 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 p.m. The geography should be familiar and the small purple diamond marks the spot of the Bowie supercell. The first key element, a warm front, was aligned with the Chesapeake Bay, and it helped to lift unstable air along and adjacent to the frontal orientation.
The warm front likely also provided a source of spin near the ground, which helped induce rotation in the thunderstorm. There are many historical instances of tornadoes being generated in the Washington region when parent thunderstorms lie in proximity to a warm front.
The small patch of red contours just west of the front shows a region of atmosphere that had destabilized rapidly in the wake of early afternoon shower and cloud activity. The extent and duration of cloud cover was one of the “wild cards” in the forecast for severe storms that meteorologists regarded as a very uncertain element.
Alas, a significantly unstable air mass evolved by 5 p.m. as shown in the center of the red “bull’s eye” and the supercell was able to tap into this buoyant air on southerly, low-level winds west of the warm front.
A third key element occurred through a deep layer of the atmosphere — a small pocket of strong wind shear (increase in winds with altitude) shown by the blue contours. The core of 40 to 50 knot (46 to 57 mph) wind shear values was of the proper strength to induce 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-bearing supercell.
But wait, there’s more! The image below shows what is perhaps the key element that helped tie all the foregoing aspects together. It’s called a mesoscale convective vortex (MCV) and it’s a compact, intense pocket of spin in the middle atmosphere that tracked into central Maryland out of Ohio. The MCV was generated inside a larger storm complex well northwest of the Washington region during the early morning hours — and in essence was the “ghost” of that former storm complex.
The MCV was not readily apparent from the widely-spaced network of weather balloon stations but was identifiable on satellite loops. Its arrival over the “trifecta zone” did several things. First, the pocket of spin induced strong rising motion on its southeast side. That helped stoke the storm’s updraft, contained within its mesocyclone.
Second, the MCV imported high amounts of spin energy into the genesis zone of the storm, which the storm’s mesocyclone likely tapped into.
These factors led to robust, persistent, strong mesocyclone with sufficient vigor to spawn a tornado (only 25 percent of mesocyclones ever generate a tornado).
Tuesday’s severe storm forecast was a challenging one for meteorologists across the region, given the demonstrably poor performance of our best models and uncertainty with the cloudiness forecast. This post-facto analysis of the environment in the vicinity of Prince George’s County clarifies the ingredients that came together for a rapidly evolving tornado setup, which unfortunately weren’t obvious in advance.