This morning, it is 32 degrees F in South Texas. From there, northward, the temperature falls with the latitude, reaching 35 below zero at the Canadian border.
The culprit is a deep trough (a dip in the jet stream) that extends from the Rockies to the Midwest. High pressure within that trough has brought Arctic air to the mid-section of the United States, extending down to the Southern Plains and, for the first time in decades, to southernmost Texas. Meanwhile, low pressure will move along the edge of the trough, sweeping warm, moist air from the Gulf of Mexico above the cold, Arctic air emplaced at the surface, setting the stage for an ice storm in the Deep South and a swath of significant snowfall farther north.
Freezing drizzle has already developed in the San Antonio-Austin region and crippling ice is forecast for southeast Texas, northern Louisiana and southern Arkansas, eventually extending northeastward across the Southeastern U.S. to the Mid-Atlantic region. Once this storm does its damage, another, more northern system will arrive from the Pacific Northwest.
