Snow Advance Index: A New Tool for Predicting Winter’s Severity

Researchers at AER have developed a new Snow Advance Index that could greatly improve the accuracy of winter seasonal forecasts. Based on research and testing, this index for the first time can accurately predict the severity of the upcoming winter. What makes this discovery so significant is that advances in seasonal forecasting have remained elusive, and for much of the United States seasonal forecasts are no more accurate today then they were decades ago.

Benefit to industry and government

Because weather impacts so many industries, this research and the inclusion of the Snow Advance Index in seasonal forecasts have direct benefit for businesses and government agencies that use long-range forecasts to optimize their operations and financial results and mitigate risk. Demonstrating this need, a recent report issued by the National Academy of Sciences on climate prediction concluded that more accurate climate forecasts would provide great benefit to society and would improve decision making from the agriculture to the energy sector.

Why has accurate seasonal forecasting been so difficult?

Significant improvements have been made in short term weather forecasting. People rely on the nightly broadcast of the seven day weather forecast, which is more often correct than not.

In contrast, making accurate seasonal forecasts continues to frustrate the large government forecast centers. Despite the use of ever more sophisticated climate models, the only demonstrable skill is that associated with the El Niño/Southern Oscillation (ENSO). Yet the temperature forecast accuracy related to ENSO is limited to western North America; and climate model forecasts for the Eastern US, Europe and East Asia remain no better than using past history or even a coin flip as a forecast tool.

Predicting the North Atlantic Oscillation or Arctic Oscillation (N/AO)

The scientific community acknowledges that a different climate phenomenon is strongly related to the weather in the Eastern US and Europe. It’s known as the North Atlantic Oscillation or Arctic Oscillation (N/AO) and is explained below.

Until now, the N/AO has been considered unpredictable; but this new Snow Advance Index developed at AER can, for the first time, predict the N/AO with a high degree of accuracy. Even more importantly, the Snow Advance Index predicts with unprecedented accuracy winter temperatures and thus the severity of winter weather in the Eastern US, Europe and East Asia. This is discussed below and shown in the Figure.

How the N/AO impacts temperatures in Eastern US, Europe and East Asia

The N/AO can be thought of as a winter severity index.

When the N/AO index is high, winters in the Eastern US, Europe and East Asia are milder or less severe.

In contrast, when the N/AO index is low, winters in the Eastern US, Europe and East Asia are more severe with increased bouts of cold weather and snowstorms.

The ability of forecasters to predict the N/AO, or winter severity index, would provide significantly improved temperature forecasts for the Eastern US, Europe and East Asia. Research at AER and other institutions has shown a statistical relationship between the Eurasian snow cover extent in October and the winter N/AO or the severity of winter weather.

When there is more Eurasian snow cover in October, the following winter N/AO is low and there is an increased frequency of severe weather in the mid-latitudes.

When there is less Eurasian snow cover in October, the following winter N/AO is high and there is a decrease in the frequency of severe weather in the mid-latitudes.

AER Researchers demonstrate how the Snow Advance Index predicts the N/AO

Until now only a moderate relationship has been demonstrated. However in a paper published in Geophysical Research Letters by myself and Justin Jones, a new Snow Advance Index is developed that is derived not from October mean snow cover extent, but rather as a function of how slowly or quickly the snow cover advances across Eurasia in October. This new Snow Advance Index is highly correlated with the N/AO or winter severity index (see Figure).

When snow cover advances rapidly across Eurasia in October, this is an indication that the upcoming winter will be more severe for the Eastern US, Europe and East Asia.

When snow cover advances slowly across Eurasia in October this is an indication that the upcoming winter will be milder for the Eastern US, Europe and East Asia.

Using the Snow Advance Index to improve Forecast Skill

Hindcasts or back testing of this new Snow Advance Index demonstrates great potential in both predicting the N/AO and improving winter temperature forecasts for much of the large population centers of the Northern Hemisphere.

This new Snow Advance Index will be used for the first time in the AER winter forecast for the winter season of 2011-2012.

The link between the N/AO and Winter Severity

The N/AO can be thought of as an index of high latitude blocking.

When the N/AO is high, high latitude blocking is much less frequent. The Jet Stream flows quickly from west to east carrying weather systems along quickly and acts as a divide between cold air to the north and warm air to the south. The Eastern US, Europe and East Asia, which lie in the mid-latitudes, experience long stretches of mild weather, as they remain cut off from cold, Arctic air masses.

When the N/AO is low, high latitude blocking is more frequent. The flow of air in the atmosphere is impeded or becomes blocked and the Jet Stream is diverted from its normal trajectory and meanders north and south around the high latitude blocking. Mixing of air masses occurs with warm air flowing north into the Arctic and cold air flowing south into the midlatitudes. The Eastern US, Europe and East Asia, which lie in the mid-latitudes experience more severe winter weather, as Arctic air masses repeatedly drive southward. Furthermore, the mixing of warm and cold air masses fuel storms and with more abundant cold air, the precipitation from these storms often falls as snow.