Here’s How An Ice Storm Blanketed Oklahoma For Two Days Straight
This week began with an unusually potent early-season ice storm in parts of Oklahoma and Texas. Some communities could end up with more than three-quarters of an inch of ice accretion from freezing rain, which is more than enough to cause widespread tree damage and knock out power to hundreds of thousands of homes.
Why did the region see a prolonged ice storm instead of a thump of snow? Even though surface temperatures would remain below freezing for the duration of the storm, it all has to do with temperatures just a few thousand feet above the ground.
The four types of precipitation we can see during a winter storm are rain, snow, freezing rain, and sleet. While each type of precipitation forms through a similar process, it’s the journey from the cloud to the ground that makes all the difference.
Winter storms are dynamic systems. A classic storm sweeping over the United States can span more than a thousand miles across, triggering severe thunderstorms in humid air bathing the southern states while a blizzard rages over the Upper Midwest.
Temperatures at the surface are only part of the story. We have to look at the temperature profile through the bottom 10,000 feet of the atmosphere to get a full idea of what kind of wintry weather we’ll encounter during a storm.
Cold air is dense, so it tends to stubbornly hug close to the surface. If a winter storm blows warm air into an area with below-freezing temperatures, that warm air can simply rise above the denser cold air, leaving a pocket of above-freezing temperatures wedged a few thousand feet aloft. This scenario, known as an inversion, is the driving force behind the different types of precipitation.
Snow falls when temperatures between the surface and the clouds are at or below freezing through the entire atmospheric column. If there’s a layer of above-freezing air aloft, snowflakes falling through that warmer air will begin to melt. The amount of melting determines whether sleet or freezing rain reaches the surface.
A snowflake that only partially melts maintains a small amount of ice crystals within the droplet, providing a nucleus around which the water can refreeze. This results in an ice pellet, or sleet, hitting the ground. Sleet both looks and accumulates like snow when it reaches the surface.
If the snowflake completely melts before falling into the subfreezing air at the surface, leaving no ice crystals behind to facilitate refreezing, the raindrop can freeze on contact with any exposed surfaces. Freezing rain leads to a crust of ice developing on roads, vehicles, trees, and power lines.
Ice accretion from freezing rain that reaches one-quarter of an inch is typically enough to lead to tree and power line damage, with the damage increasing dramatically with thicker coats of ice.
This is what happened in Oklahoma and Texas this week. A thick, stubborn layer of cold air parked itself at the surface while winds a few thousand feet aloft blew above-freezing temperatures into the area. The cold air remained in place at the surface even as warmer air sat on top of it, allowing snowflakes to completely melt and enter the subfreezing air at the surface as freezing rain.
Precipitation types can change multiple times during the course of a storm. Communities near the changeover line can experience rain, snow, sleet, and freezing rain, complicating the cleanup and making travel even more difficult that it would have been otherwise. In the case of this week’s ice storm in the south, warmer air will eventually win out and precipitation will change over to normal rain and melt away the built-up ice.