Recent Trends in Convective Forecast Parameters in North America (1979- 2007)

This paper explores recent temporal and spatial trends in environmental conditions that are known to influence convective storm morphology. The analysis uses the NCEP North American Regional Reanalysis (NARR) dataset of monthly means for the month of June, 1979--2007, valid at 00 UTC. Trends in convective available potential energy (CAPE) and the height of the lifting condensation level (LCL) are examined. The time series trend is calculated at each grid point using simple linear regression, and only the "statistically significant" trends are retained. The results are also compared with the radiosonde archive maintained by the Forecast Systems Laboratory (FSL). Substantial increases in CAPE are found in portions of south Texas, and in proximity to the Great Lakes and eastern United States coast. Notable decreases occur in the U.S. High Plains, east of the Rocky Mountains. There are also trends in the height of the LCL, especially along the U.S.--Canada border. However, some of the results are discovered to be data artifacts, rather than persistent climatological trends. For the trends that are statistically robust, hypotheses for their existence are discussed. These findings could have important implications for understanding the long-term evolution of thunderstorm climatologies and aid in interpreting climate studies that consider convective storms.