Some types can stop large parts of the country, with chances of darkening, impassable roads, frost, power outages and roof collapses.
It is critical that predictors do their best to do well.
There are three basic types of winter rainfall: snow, sleet, and icy rain. The type of precipitation you get is based on the temperature of the air at different levels of the atmosphere.
Think of the atmosphere as a tall column. If this air column is below freezing from top to bottom, the result will be snow.
If there is warm air at the top and temperatures below those that freeze towards the bottom of the column, you will have sleet. This is because the warm air in the atmosphere will produce a drop of rain. When the raindrop falls and enters the air below the freezing temperature, it will freeze before it touches the surface. Because the sleet is basically a drop of icy rain, it can actually be heard when it comes to the surface.
Finally, if the entire air column is above the freezing temperature to the surface, the raindrop will freeze on contact. This is called icy rain and is the most dangerous type of precipitation.
“Some of the most disastrous winter storms are mainly due to icy rain,” the National Meteorological Service says.
Freezing rain freezes everything that comes in contact and causes a dangerous glaze of ice on roads, trees, power lines and the vehicle. The weight of the ice can cause trees and power lines to fall, causing power outages during extremely cold temperatures.
Road ice turns lanes into ice tracks and can cause major traffic accidents and interstate buildup.
It takes less than a quarter of an inch of ice to make travel dangerous. The ends of the trees begin to break with a quarter or a half inch of ice buildup. And when more than half a centimeter of ice accumulates, there will be widespread damage to trees and extensive power cuts.
The importance of being right
Deciding where rain ends and snow begins can mean the difference between where to use defrosting machines, aircraft defrosting, school cancellations and more. It is also incredibly important to know which areas will receive sleet or icy rain.
The rain-snow line is a term we often hear when a winter storm is forecast. The term simply defines the point at which rain travels to the snow. In mountainous regions, it takes on a vertical connotation and is defined by the elevation at which rain travels through the snow.
If the forecast for the rain and snow line is incorrect, this could mean that a city expects to receive all the rain by a storm and end up receiving all the snow. This could have devastating consequences if a city does not plan to snow and has not treated roads, canceled schools or warned its residents.
The same could also be true if a city hopes to get all the snow and ends up getting all the rain. They would have wasted countless dollars from taxpayers dealing with roads and closing companies in preparation, and then nothing happens.
Most people living in the south can remember a “snow day” where no snow fell. Key decisions, such as school closures, must be made at night, when winter rainfall is expected to begin during the morning hours. If the temperature is a little warmer than expected, winter rainfall will fall like rain and you have a reduced forecast.
For example, it is essential to know the altitude of the rain and snow line in Colorado to see if any of the mountain passes on Interstate 70 will be able to remain open. In New York, it is crucial to know if the rain-snow line will reach the I-95 corridor or remain west.
Why are bridges the first to freeze?
During the winter weather, bridges and overpasses are the first to freeze. This is because they have air on all sides, so they will cool down faster than surface roads. Surface roads are naturally more isolated because cold air is only above the surface, while the ground below keeps the roadway warmer.
Then you have what’s called a winter mix. It’s basically a bit of everything. When you are close to the rain-snow line, rain can pass into the snow over time and pass into the snow. Or the snow will pass in the rain.
Meteorologists have the important task not only of identifying what type of precipitation will fall, but also of timing when one type of winter precipitation will pass to another type.
It is crucial to know what time the rain will change for snow. Will it happen before or after the night shift?
It is certainly a challenge and not always perfect. But knowing what will fall and when is the key to staying ready for winter.
CNN meteorologist Taylor Ward contributed to this report.