The images coming out of Forth Worth, Texas this week are horrific. According to numerous media reports, 130 cars were involved in a pileup along a stretch of Interstate 35. At the time of writing, six fatalities had been reported as well as numerous injuries. Pileups were also reported in Austin, Texas from the same weather system. Icy road conditions were a likely culprit. Earlier in the week, I watched, in horror, a video of a car plunging seventy feet off an icy overpass in Milwaukee, Wisconsin. Thankfully the driver survived. Though the official investigations in Fort Worth and Milwaukee are ongoing, my initial assessment points to something in common between both incidents – elevated roadways.
Charlotte-based TV Meteorologist Brad Panovich, one of the most skilled and astute colleagues in our field, also noticed it. He tweeted a series of photos of where the pileup took place in Forth Worth, Texas. He pointed out that freezing rain and drizzle (we call it black ice in the South) were present. However, it is important that he also pointed out in the Tweet below about the blind elevated hill before sunrise. These are conditions that would further intensify dangerous freezing conditions on the roadway.
Most roads are made from from materials like asphalt, which are poor conductors of heat whereas elevated surfaces like bridges are often made from steel or concrete, which are better conductors of heat. Unlike sleet or snow, freezing rain or drizzle falls to the surface as liquid. If the surface temperatures are below freezing, the water freezes to form a dangerous glaze of ice on roadways and other surfaces. Because bridge materials are good conductors of heat, the heat energy within them moves quickly to the surface. Elevated surfaces will freeze more easily because they exchange more energy from exposed surfaces (see first graphic above). A non-elevated surface like an asphalt roadway has compensating heat from the ground to serve as an insulator. As Brad Panovich noted in his Tweet, the Fort Worth situation was likely made worse by reduced heating associated with the delayed insolation as the sun was rising and the sheltered nature of the location.
Freezing rain will likely be in the forecast over the next few days as bitterly cold air continues to plunge into the Great Plains and Deep South. The NOAA Weather Prediction Center map below depicts anticipated weather conditions in the United States. There is a significant swath of freezing rain predicted from Texas into the mid-Atlantic region. Extreme care is recommended in these regions because a road that appears to be wet may actually contain a coating of ice.
Climatologically speaking, the regions with enhanced risk for freezing rain this week are not unusual. According to the NOAA map below, these particular regions do receive several days a year of freezing precipitation. The problem, however, is that it is still not frequent enough for many drivers to develop the proper driving awareness for freezing rain. Additionally, many southern locations lack the large volume of ice or snow mitigation infrastructure found in many northern locations.