This week across the Tennessee Valley is Winter Weather Awareness Week for the National Weather Service. Today’s topic will be focused on the criteria for winter weather alerts for our area and the type of wintry precipitation. The frequency of winter weather here in the Tennessee Valley is small but when it occurs it can cause significant property damage, injury, and even death.
The winter season is from December to February, but winter weather can also occur even during the months of November and March. The earliest measurable snowfall in Huntsville, of one inch or greater, was on November 2, 1966 and the latest was March 22, 1966.
Winter Weather Alert Criteria
The three main weather alerts are a Winter Weather Advisory, Winter Storm Watch, and a Winter Storm Warning. These alerts are all issued by the local National Weather Service in Huntsville ahead of a winter weather event that could cause hazardous conditions.
A Winter Weather Advisory is issued for a winter weather event that could cause significant inconveniences but does not reach the warning criteria. If one or more types of hazardous winter weather are expected, is occuring, or is imminent a Winter Weather Advisory will be issued. The criteria is 1 to 2 inches of snow or sleet and/or up to a quarter inch of ice during a 12 to 24 hours period.
A Winter Storm Watch is generally issued 24 to 48 hours in advance of the threat of a significant weather event. The National Weather Service in Huntsville issues a watch when there is a 50 percent chance the criteria for a Winter Storm Warning will be met. As the event gets closer and things start to become more clear, the watch is either upgraded to a warning or downgraded to an advisory. This brings us to the Winter Storm Warning, which is issued when a combination of hazardous winter weather is occurring, imminent, or is expected. The criteria for a Winter Storm Warning is two inches or more of snow during a 12 hour period, four inches or more of snow during a 24 hour period, and/or a quarter of an inch of ice or greater.
Types Of Wintry Precipitation
We can see any type of precipitation across the Tennessee Valley during the winter. As precipitation falls to the ground it goes through different levels of the atmosphere and sometimes the temperatures at these levels are different. This will lead to different forms of precipitation we see at the surface.
Two types of precipitation that are similar and can cause hazardous conditions are freezing rain and sleet. Both types begin as ice pellets and as they fall from the clouds they encounter a layer of warm air, causing the ice pellets to melt. As it continues its way to the surface it then encounters a layer of cold air. The main difference is that with freezing rain, the precipitation freezes on surfaces at or below freezing, unlike sleet, which refreezes before hitting the ground. With sleet there is more cold air above the surface, allowing the precipitation more time to freeze before hitting the ground.
Snow is the other type of wintry precipitation we see in the Tennessee Valley. Depending on ground temperatures, snow will first accumulate on grassy surfaces before roadways. The snowiest winter season on record here in Huntsville was 1963-1964 where two feet of snow fell. The majority of this fell on New Year’s Eve where 15.7 inches fell. This is also the daily record snowfall for Huntsville. Huntsville normally sees 2 days of measurable snowfall each winter which is an inch or more of snow.
