Most Texans have given up on the idea of ever seeing a white Christmas. But sometimes, just sometimes, a brief snow blankets the Lone Star State.
What may be surprising is how much water is generated once that snow melts.
Water vapor evaporated from oceans, lakes, and plants, among other things, condenses and returns as precipitation, once again replenishing reservoirs and other sources of water.
Sometimes as that water falls, it is in the form of a snowflake.
The general rule is every 10 inches of fallen snow equals one inch of liquid water equivalent, Brazos River Authority Water Services Manager Aaron Abel said. But things are rarely that simple.
To meet that equivalent, the snow has to be pretty wet, Abel said.
“As you decrease the temperature, say you’re in Colorado or in mountains where the air is really dry, then you have much drier, powdery snow, and that snow has less water content,” he said.
Powdery, or drier, snow may have a water content as low as 5% or have a 20-to-1 snow to water ratio, he said.
Sleet, on the other hand, has an almost 1-to-1 ratio, Abel said.
In 2010, roughly eight inches of snow fell around the BRA reservoir Possum Kingdom Lake. It took a day or two to melt, and there was a slight increase in inflows to the reservoir, Abel said.
“Since it equates to such a minor amount of water, we don’t have to worry about it all that much,” he said. “It’s really rare. Typically, in Central and North Texas when we get snow, it’s a pretty small amount.”
An inch of snow falling evenly on 1 acre of ground is equivalent to about 2,715 gallons of water, according to the U.S. Geological Survey. If the snow has a high-water content, that could equate to more than 5,400 gallons of water across the 1 acre.
The 10-to-1 ratio, of course, can vary. And it takes the right combination of ingredients to bring about a winter storm.
According to the National Oceanic & Atmospheric Administration National Severe Storms Laboratory, the magic number is three: cold air, lift and moisture. First, there needs to be below freezing temperatures in the clouds and near the ground are necessary to make snow and or ice. Then, there needs to be something to raise the moist air to form the clouds and cause precipitation. Next, air blowing across a body of water, such as a large lake or the ocean, provides moisture.
While much snow may not be in the forecast for the Brazos River basin, there are other winter weather conditions. Some of those important winter weather notices to be familiar with as temperatures hover around freezing according to the National Severe Storms Laboratory include:
- Winter Storm Warning: Issued when a combination of hazardous winter weather in the form of heavy snow, heavy freezing rain, or heavy sleet is imminent or occurring. Winter Storm Warnings are usually issued 12 to 24 hours before the event is expected to begin.
- Winter Storm Watch: Issued 12-48 hours in advance of the onset of severe winter conditions. The watch may or may not be upgraded to a winter storm warning, depending on how the weather system moves or how it is developing.
- Winter Storm Outlook: Issued prior to a Winter Storm Watch. The Outlook is given when forecasters believe winter storm conditions are possible and are usually issued 3 to 5 days in advance of a winter storm.
- Winter Weather Advisories: Issued for accumulations of snow, freezing rain, freezing drizzle, and sleet which will cause significant inconveniences and, if caution is not exercised, could lead to life-threatening situations.
- Wind Chill Warning: Issued when wind chill temperatures are expected to be hazardous to life within several minutes of exposure.
- Wind Chill Advisory: Issued for a wind chill situation that could cause significant inconveniences, but do not meet warning criteria. Criteria for issuing Windchill Warnings and Advisories are set locally.
- Dense Fog Advisory: Issued when fog will reduce visibility to ¼ mile or less over a widespread area.