Utilizing standardized anomalies to assess synoptic scale weather events in the central United States

Anomalous synoptic weather events produce memorable and even historic weather impacts. Because of the potential for high impact weather as well as the visibility of these events, it is crucial for forecasters to be able to anticipate significant or rare events. Hart and Grumm (2001) focused on the eastern United States, while Graham and Grumm (2006) investigated the western U.S.; this study will apply an objective classification method for synoptic systems occurring in the central U.S. between the Rocky and Appalachian Mountains.

Following the methodology presented by Hart and Grumm (2001) and Graham and Grumm (2006), synoptic scale events in the central United States will be ranked objectively based on standardized anomalies. The study will rank anomalies of wind, geopotential height, temperature, and moisture utilizing a standardized anomalies method of pressure weighted anomalies for each variable. The results will allow the ranking of past events, with a summary of the top 10 events for each variable as well as for all variables combined. By providing a guideline of anomalies for previous events, forecasters will be able to place forecasted events in a historical context.

Standardized anomalies were computed from NCAR/NCEP global reanalysis data for the period of 1 January 1948 through 31 December 2008, focused on a domain from 82°W to 110°W and from 26°N to 55°N. The computations were done in 6-hour increments with anomalies computed for heights, temperatures, winds, specific humidity at mandatory levels, as well as surface variables such as mean sea level pressure and precipitable water. The base climatology for the means and standard deviations was computed for the 30 year period of 1971-2000. Results will be sorted by anomalies in order to objectively rank past events by their departure from climatology, providing a context for forecasted events based on standardized anomalies.