EPEC: A Tool for Anticipating Excessive Precipitation with Elevated Thunderstorms

The Excessive Precipitation with Elevated Convection (EPEC) tool is a parameter that was created in order to assist in the prediction of heavy rainfall associated with elevated convection.  Borne of mean and interquartile range fields from composites of elevated heavy rain events, EPEC uses basic metrics for instability, moisture, and lift to help refine an area where excessive rainfall from elevated thunderstorms will occur.  The purpose for this research is to verify EPEC performance as a predictive parameter.  This verification was done on 15 heavy rainfall events from elevated convection from the warm seasons of 2014 and 2015.  Comparison was made between the EPEC at, say 00 UTC, and the 6-hour Stage IV accumulated precipitation for the ensuing period between 00 and 06 UTC (in this example).  Statistical analysis reveals that all but 1 of the 15 events had a positive linear relationship between the EPEC index and the 6-hour Stage IV precipitation data. 9 events had a probability of detection value greater than 0.5 (p<0.05), while only 2 events had a false alarm ratio less than 0.5. We may thus conclude that even though EPEC is able to correctly forecast observed events, the parameter also over-forecasts the events. This is confirmed in the high bias numbers.  

It must be stressed that EPEC has been evaluated only on the cold side of thermal boundaries.  Use of EPEC is predicated on the presence of 1) a thermal boundary (often a stationary front); 2) a jet streak northeast of the heavy rainfall location; 3) significant moisture in the deep troposphere to the north of the surface front; 4) potential instability. 

The EPEC tool has shown that there is a positive relationship with heavy precipitation. When EPEC is employed on the cold side of the θ_e gradient, it can help predict heavy rainfall associated with elevated convection. This parameter is not meant to be used on its own, but is meant to aide the forecaster in pinpointing an area of possible heavy rainfall with elevated convection that could lead to flash flooding.