Winter Precipitation: More Rain, Less Snow

Even in a warming world, snow will fall. However, the amount of snow and when it falls will likely change as greenhouse gases build up in the atmosphere.

This week, we examined cold season precipitation at stations across the country, specifically looking at how much snow is falling compared to rain. Our analysis is consistent with earlier EPA findings. The Northwest and the Upper Midwest are the climate regions seeing the largest decreases in precipitation falling as snow over the past 66 years.

Although specific trends differ, for most parts of the country, the decrease in snow is happening primarily during the shoulder months (October-November and March-April), when warming temperatures are causing more precipitation to fall as rain rather than snow. In effect, this shortens the snow season for most of the country.

While less snow sounds good for some, it does have effects on business and the environment. Less snow will make it more difficult to operate ski resorts and reduce some winter sports opportunities. Additionally, with rising temperatures, an earlier end to the snow season usually means an earlier annual snow melt. As a result, places that depend on melting snow for water may find those resources stretched by the summer. Snow on the ground is also a good insulator, helping keep soils moist during the winter, rather than exposing them to the bitter Arctic winds that can dry them out.

Methodology: To determine which months to include for each station’s snowfall season, we collected precipitation data for each station since 1949, including the amount of snowfall per month each year. If there were more than 10 occurrences of any month (i.e. 10 Decembers) with more than 1” of snow, that month was included in the station’s analysis. In an effort to exclude very rare events, stations that had fewer than 3 months that met the above criterion were dropped from the analysis. To determine the ratio of snow to rain, if any snow was observed on a specific day, then all precipitation that day was classified as snow water equivalent. If no snow was observed, then all precipitation was classified as rain. The ratio for each month is the monthly snow water equivalent divided by total precipitation for the month. This method is based on several research papers conducted on this topic.