Following a rare winter storm that brought up to six inches of snow in parts of Williamson County on Sunday, another uncommon threat may follow long after the storm has departed.
Wet ground from heavy snowfall Sunday and melting on Monday will combine with clear skies and calm winds to form dense fog in parts of Central Texas. The fog should be most widespread in low-lying valleys and areas close to bodies of water.
Fog is a cloud that forms at ground-level when the air temperature drops to the dew point temperature. When these two variables equal one another, the relative humidity is 100% and the air is completely saturated.
In a cloud of fog, supercooled water droplets hang in the air. If fog forms when the air temperature is below freezing, as we expect in some areas tonight, those supercooled water droplets do not freeze unless they have something to stick to. Your railing, front porch, or roadways can serve as that object, instantly forming a thin rime of ice on that surface. Even more dangerous, this thin layer of ice can be very difficult for drivers to see.
Fog may begin forming as late as 11 p.m. tonight, especially in areas that received heavy snowfall on Sunday. Air temperatures will rise above freezing areawide by 10 a.m. Tuesday, eliminating the treat of icing on roadways.
From our local National Weather Service office:
A widespread hard freeze is expected tonight into tomorrow morning with overnight lows in the 20s across the entire [area]. With recent precipitation, as the cold air temperature draws closer to the dew point, freezing fog development is probable for our northern counties, and patchy for areas as far south as Bexar County. Freezing fog could potentially impact travel where the fog is more dense for an extended period of time as the suspended, supercooled water droplets freeze on contact with a frozen surface. At this time, significant impacts are not anticipated, but conditions will be closely monitored overnight for changes.