How do meteorologists determine how much snow will fall?

We look at several things when determining how much snow will fall. First, we look at how much liquid precipitation the storm will produce then convert that to inches of snow. To do this we need to determine the snow to liquid equivalent.

The snow to liquid equivalent is the amount of liquid precipitation that is produced after melting snow. Temperature and the track of the storm are important factors that determine this value. The “average” snow to liquid ratio is 10:1 which means that if 10 inches of snow fell and that snow was melted it would produce 1 inch of liquid precipitation in the rain gauge.

This average does not always hold true and depends on the atmospheric conditions. A heavy, wet snow usually occurs when temperatures are near or just below freezing. This type of snow has a snow to liquid equivalent less than 10 to 1. So, a 5 to 1 ratio means that an inch of liquid precipitation would produce 5 inches of snow. On the other hand, dry snow tends to have a higher ratio and usually occurs with colder temperatures. So a 15 to 1 ratio means we would get 15 inches of snow with an inch of liquid precipitation.

Dry air fed into winter storm systems can also cause snow amounts to vary greatly. The “dry slot” occurs in storm systems as dry air is pulled into the storm system, and sinks to the surface causing the precipitation to end or turn to drizzle. Areas within the dry slot will end up with much lower snow amounts.

Another thing we take into consideration is snow banding which is a concentration of falling snow in a 20 to 50 mile band. The convergence of winds within the storm system causes the precipitation to spread into a band, producing a narrow corridor of heavy snow. Locating these bands is more difficult until the storm is about two hours away and the banding is detected by radar.