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20 years after La Plata disaster, D.C. area’s tornado risk has grown

The violent tornado was Maryland’s deadliest in the modern record. It could have been worse and probably would be today.

The track of the La Plata tornado can be seen from satellite on May 1, 2002. (NASA)

Two decades ago, the La Plata tornado gouged a long and highly destructive track across southern Maryland. It was rated Level 4 on a 0 to 5 scale and became the second-strongest twister to strike a state along the East Coast. The rotating thunderstorm that produced the twister on April 28, 2002, originated on the Kentucky-West Virginia border and tracked nearly to the Atlantic Ocean. Three people died, making it Maryland’s deadliest tornado in the modern record, and 122 were injured.

Remembering the La Plata tornado

While it has been 20 years since the La Plata disaster, its occurrence is a reminder that the D.C. region is vulnerable to devastating whirlwinds on par with those of famed tornado alleys in the Great Plains and Deep South. The D.C. region also sees much more frequent tornadoes of lesser strength.

The region has been fortunate that an intense tornado, rated 3 out of 5 or higher, hasn’t struck since the La Plata disaster. But because of population growth and development since then, the region faces a greater threat of a large, intense twister than before.

Although the ingredients that produced the La Plata tornado could easily come together again, we also have improved tools for forecasting and detecting them.

When are we most vulnerable?

In the Washington region, April is arguably the most dangerous month for tornadoes, although they pose a persistent risk through the summer and fall when thunderstorms are most numerous.

April has seen the most killer tornadoes and the most “long-track” tornadoes, or the type that stays on the ground for an extended distance, like the La Plata twister.

Among all months, April has produced the second-most tornadoes, rated 2 out of 5 or higher, behind September.

July has produced the most tornadoes overall, although summer tornadoes tend to be weaker and more short-lived compared with those in the spring and fall.

There have been numerous local tornado events of note over the past decade, including:

  • 2022 tornadoes in Tysons and Centreville: Two weak tornadoes, rated 0 out of 5, caused minor damage in Tysons Corner and Centreville on March 31.
  • 2021 Annapolis tornado: On Sept. 1, 2021, a strong tornado, rated 2 out of 5, with maximum winds of 125 mph, sliced an 11.25-mile path through parts of Annapolis, damaging dozens of buildings. It was generated in part by the remnants of Hurricane Ida, which also spawned a Level 3 tornado in New Jersey.
  • 2021 and 2017 tornadoes inside the District: The city itself is not immune from tornadoes, and several have touched down in recent years. Two weak tornadoes were confirmed in the District in 2021, and a pair of 0-rated twisters struck in 2017.
  • Regional outbreaks in June 2012 and April 2011: Sometimes tornadoes come in swarms. June 2012 featured a localized tornado outbreak in which a dozen tornadoes touched down in the region. Numerous tornadoes also struck the region on April 27-28, 2011, amid the “Super Outbreak,” in which hundreds of twisters touched down in the central and eastern United States. Most of the tornadoes locally were weak, although a Level 2 twister tore through the Shenandoah Valley.

Patterns to watch

While most of the tornadoes that have affected the region over the past decade have been weak and short-lived, the local environment has the potential to fuel more severe twisters.

Nearly all high-end tornadoes are generated by supercells — long-lived, rotating thunderstorms that can last for hours. Supercells require an optimum balance of wind shear — or a change in wind speed and direction with altitude — and an unstable air mass fueled by hot, humid and swiftly ascending air.

The ideal setup in the D.C. area for these conditions is a springtime “warm sector,” in which a large-scale region of low pressure tracks to the north and west of our region. Warm, humid winds from the south glide beneath chilled air at higher levels of the atmosphere. This vertical stratification of air mass properties destabilizes the atmosphere.

Wind shear is a prerequisite for tornadic thunderstorms. In the Mid-Atlantic, it is typically created by a combination of winds flowing from the south at low levels, which turn clockwise and strengthen with altitude, up to jet stream height, where winds can roar in excess of 100 mph from the west. These strong winds aloft help “vent” the storm, to keep it alive a long while.

The weather analysis for April 28, 2002, the date of the La Plata twister, presented a textbook springtime tornado setup for the region, given a warm front to the north and a cold front to the west. Moisture pooled to the east of the mountains sat in wait for a storm to arrive. Once it did, the sky was the limit. The La Plata tornado was one of several from the supercell.

Similarly potent setups with different origins come relatively often from the remnants of tropical storms and hurricanes. In these cases, extreme low-level shear often helps power relatively short-lived tornadoes. At times, tropical systems can produce stronger tornadoes, as well; such was the case in Annapolis during the remnants of Ida last year.

Strong tornado risk

About 85 percent of tornadoes are rated 0 or 1 locally, which is considered weak despite the risk of damage they present.

Eleven percent of D.C.-area tornadoes have been rated 2 (strong), with 3.5 percent rated 3 (intense). Only 0.3 percent of tornadoes in the region have been rated 4 or violent.

Shortly before La Plata, a Level 3 tornado ripped through Prince George’s County, Md., causing severe damage in College Park. That twister developed on the afternoon of Sept. 24, 2001, leading to two fatalities and 55 injuries.

But since the College Park and La Plata disasters, the region has avoided intense or violent twisters.

In the meantime, the area has undergone significant growth. In La Plata, a population just north of 7,300 in 2002 has blossomed to about 9,500 today.

Walker Ashley, an atmospheric scientist and disaster geographer at Northern Illinois University, has documented an acute increase in tornado vulnerability in the Southeast because of what he calls the “expanding bull’s eye effect.” In other words, development creates larger areas for tornadoes to damage.

Furthermore, La Plata — even now — is relatively undeveloped compared with locations just to its north. If the 2002 La Plata tornado were to occur 20 miles farther north, it would follow a path along the southern branch of the Beltway through multiple large population centers, causing devastation perhaps on an entirely different level.

There’s no reason to believe a violent tornado can’t impact the D.C. area. In the Northeast, they have been recorded as far north as central New York and as far east as east-central Massachusetts.

Improved forecasting and detection

Despite the real and escalating tornado damage risk, forecasting tools offer improved storm lead time.

Weather models can routinely spot dangerous thunderstorm potential up to a week ahead. On the day of potentially hazardous storms, the areas most at risk can typically be identified. Weather satellites rapidly scanning the skies mean we can see supercell development in real time.

Forecasters used to have to rely on a telltale signature in the radar’s mapping of rain patterns, along with a coincident wind velocity signature. False-alarm rates approached 75 percent.

An in-depth guide to tracking tornadoes using radar

While the over-warning problem has not been resolved, several years ago, radars nationwide were upgraded to image storms with finer resolution, and a technology known as dual polarization was introduced.

Dual polarization enables forecasters to better distinguish among various types of meteorological and non-meteorological “targets” within the radar beam. With this new capability, tornado debris can often be detected as a twister begins. It has become increasingly rare not to have warning before a significant tornado.

There are also more trained storm spotters and storm chasers every year, who aid in detection.