Road pavement temperatures and their impact on travel during snow storms

The National Weather Service issues a variety of products such as snow advisories and heavy snow warnings to raise public awareness of upcoming events. Except for instances of blowing snow, the issuance of these products is totally based on snowfall amount. For example, in eastern Washington a heavy snow warning is issued if four or more inches of snow are expected in a 12-hour period, with a snow advisory issued for lesser amounts. And while it's generally true that the greater the snowfall the higher the impact to the public, there are situations where additional factors can modify the effects of a given storm. One of these factors is often wind, and this is typically well addressed by NWS forecasts and warnings (e.g. Blizzard Warnings, Snow and Blowing Snow Advisories). Another modifying factor is the road pavement temperatures which can exasperate or minimize the impact snowfall have on traveling conditions.

In recent years, the state's Departments of Transportation (DOT) have deployed roadside weather sensors to assist their operations which are often provided on the Internet for real-time use. These DOT sensors measure various aspects of the road surface itself, in addition to air temperature, dew point, and wind. At some sites, this may be no more than a sensor that measures the temperature of the pavement surface. Other sites may have more sophisticated equipment that can measure road moisture state (i.e. dry, wet, icy, etc) and sub surface temperature, in addition to pavement temperature. While forecasters rely on “standard” observations from the DOT (e.g. air temperature, dew point, and wind), the road surface sensors typically receive less attention from NWS forecasters.

NWS meteorologists have used basic knowledge of pavement temperatures in the past and provided forecast detail such as “snow accumulation mainly on grassy surfaces.” This was typically employed during fall or spring events when the sun angle is high and relatively warm ground (including road) temperatures would melt any snowfall. By doing this, the forecasters were attempting to downplay the public impact of the snowfall. However, there are instances where the road temperatures can actually heighten the impact that a snowfall event has on the traveling public and this has often not been incorporated in NWS products.

In the current study, snowfall events from eastern Montana and eastern Washington are reviewed with a focus on the pavement temperature and impact on traveling conditions. Some cases are identified where above-freezing pavement temperatures at the start of a storm initially melt the falling snow. As the event continues, the pavement cools below 0°C, which freezes the melted snow resulting in hazardous ice-covered roads with a significant impact to travel. In other cases, the entire snowfall event takes place with below freezing pavement. Thus, no melting or re-freezing occurs, resulting in packed snow instead of ice and less hazardous traveling conditions.

The cases reviewed show that snowfall events where melt/re-freeze occurs resulted in considerably higher impact on travel than events where snowfall was heavier but did not create icy conditions. In other words, an NWS snow advisory event (determined by snowfall amount alone) can have a much larger and more dangerous impact than an NWS heavy snow warning event due to the different types of road conditions that can be created. While forecasters are trained to focus on the meteorology (e.g. how much will it snow?), an additional awareness of the impact of winter weather events on traveling conditions, as well as a better method to relay that information, will improve the service to the public.