6 National Weather Digest SEVERE WEATHER A RADAR-BASED THUNDERSTORM AND INTENSE THUNDERSTORM DAY CLIMATOLOGY FOR NEW JERSEY

A 5-year radar climatology of summer season (May-Sept.) thunderstorm and intense thunderstorm daysfor New Jersey was developed using manually digitized radar (MDR) data from four local radar sites. Echo intensity levels equal to 3 or more were determined to be associated with sll/face reports of thunder in the majority of cases (75 percent). Intense thunderstorm days were arbitrarily defined to occur when echo intensity levels reached 5 or 6. A network of 63 grid boxes located in and around New Jersey was used to tabulate thunderstorm and intense thunderstorm days frequencies over a 5-season period (1978-1982). Frequencies and averages of thunderstorm days were plotted by grid box and isoplethed by month and season. Thunderstorm and intense thunder-storm day frequencies by month were generally highest over interior sections of southeastern Pennsylvania and south-western New Jersey. When thunderstorm and intense thun-derstorm days were averaged over 4 seasons the maximum frequencies were found over southwestern and northern New Jersey and southeastern Pennsylvania. Comparisons to con-ventional c1imatologies indicated that despite some similar-ities, the radar-based climatology provided much finer res-olution and indicated frequencies over urban areas to be higher than conventional values