Using Anomalies to Forecast High Impact Events

Forecasting high impact weather events continues to be one of the greatest challenges and one of the most valuable roles of weather forecasting. The use of climatological data and internal model climatological data with Numerical Weather Prediction (NWP) provides the ability to identify forecasts of both meteorologically and climatologically significant weather events. Significant departures from normal in the model and climate space can help identify a range of high impact weather events.

This paper will show how the use of climate data with numerical guidance aided in identifying a range of high impact weather events. The focus is on weather events in the Great Lakes region. Climate forecast system and NCEP model data is compared to the 21-day centered means and standard deviations using standardized anomalies.

Two historic winter storms from the winter of 2011, the events of 31 January 2011 and 2 February are examined. These data show that there were clear signals in the low-level easterly wind and pressure anomalies indicating a potential high impact winter storm. This approach is also applied to the 17-23 July 2010 flooding event in the western Great Lakes. During this event, a large subtropical ridge brought a plume of anomalously high moisture into the region during the flood episode. The anomalies in the precipitable water field showed the strong surge of moisture within the well-recognized “ring-of-fire” return flow around the subtropical ridge.