North Texans brace for "measurable" snow starting Wednesday night as an Arctic air mass brings below-freezing temperatures and a wintery mix of precipitation.
The blend of rain-snow is expected to last into Friday, according to the National Weather Service in Fort Worth. A forecast Monday morning showed some areas of North Texas getting between 1 to 4 inches of snow with some areas further west possibly getting more than 5 inches of accumulation during the winter storm.
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Up until early Monday, it was uncertain how much snowfall Dallas-Fort Worth could expect. For meteorologists, forecasting snow days before it happens is challenging because of the complex interplay between fluctuating weather conditions, said Sarah Barnes, a meteorologist at the National Weather Service in Fort Worth.
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As the Arctic polar vortex spins over North America, it brings with it low pressure and ultra-cold air resulting in temperature and atmospheric conditions ripe for snow. The polar vortex typically stays around the North Pole, but it sometimes hurdles southward. (Studies show that warming global temperatures may be influencing the frequency and stretch of the polar vortex.)
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Exactly how much snow any particular area in North Texas will see depends on something called snow banding, Barnes explained. Snow banding refers to the varying concentrations of snowfall in an area.
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"Essentially, what we're expecting is widespread snow, so it will exist across a large area," Barnes said. "Embedded in that widespread snow, we could have heavier pockets of snowfall."
Snow bands need three main ingredients to form: moisture, air lift and atmospheric instability. These conditions, which can change at any point, will determine how thick the pockets of snow-producing clouds will be, according to the National Weather Service, and explain why some areas of North Texas will see more snow than others.
To monitor these and other environmental conditions, Barnes said, the National Weather Service launches weather balloons twice a day into the atmosphere. These balloons are outfitted with instruments that measure atmospheric temperature, humidity, air pressure and wind speed. Data collected from the balloons are run through weather models that predict what the most likely snowfall outcome might be.
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Meteorologists also factor in historic weather observations when making their assessment, said Dan DePodwin, senior director of forecasting operations at AccuWeather.
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"We have to ground ourselves with what's happening now, so we look at satellite and radar observations and we combine those things with what has happened in the past," DePodwin said. For instance, meteorologists can look at a city's past snowfall history and compare that to its current weather conditions to see if a repeat event, or something similar, is more or less likely.
Both Barnes and DePodwin said the closer the measurements and modeling are to when the snowfall is expected, the easier it is to predict what will happen.
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"As we get closer to the event, we rely heavily on surface weather stations to tell us exactly where the cold air is at the surface," Barnes said. "Then we can look at the weather balloon data to help us assess if we have any layers of warm air which could melt the ice crystals and give us a different type of precipitation ... If you spend any time here in North Texas, we usually end up somewhere on that rain-snow line."
While it might be frustrating not knowing whether to anticipate an inch or half a foot of snow, Barnes recommended staying up to date by checking your local weather forecast.
"In North Texas, we often see weather events and details surrounding winter weather events change," she said. "Because we're still expecting some uncertainties in the temperature forecast, that's going to ultimately affect what ends up happening later in the week. So monitoring the forecast multiple times a day is the best thing you can do to be prepared."
Staff reporter Lana Ferguson contributed to this report.
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Miriam Fauzia is a science reporting fellow at The Dallas Morning News. Her fellowship is supported by the University of Texas at Dallas. The News makes all editorial decisions.
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