Plot shows a time-height cross-section of maximum reflectivity over the location in southwest Fort Collins where it rained the most. Radar data were collected by the WSR-88D in Cheyenne, WY over the full storm period from 17:25–22:25 MDT on 28 July 1997. Maximum reflectivities were calculated over a cylindrical volume with dimensions of 14 km in height and 10 km in diameter. The O's on the x-axis indicate cloud-to-ground (CG) lightning strikes associated with the storms over Fort Collins. The 0°C and –10°C isotherms are plotted and are estimated from the Denver sounding launched at 00:00 UTC on 29 July (shown earlier). Two black lines are also plotted. One traces the 40 dBZ echo and the other traces rain mass flux. The red arrows indicate four periods of heavy precipitation.

The following points are discussed: 1) the CSU-CHILL dual-polarization radar, located 40 miles east-southeast of Fort Collins, was scanning during the time period plotted. Dual-polarization data was used to diagnose the fraction of ice versus water above the freezing level; 2) during the first two periods of heavy precipitation, the storms were shallow and produced no CG lightning. The CHILL radar indicated that the fraction of ice was less than 25%; 3) during the second two periods of heavy precipitation, the storms were slightly deeper and produced CG lightning. The CHILL radar indicated that the fraction of ice was greater than 75%; 4) a radar reflectivity threshold of 45 dBZ at –10°C is chosen to distinguish the first two periods of heavy precipitation from the second two periods. This threshold appears to identify ice particle sizes and concentrations necessary to produce CG lightning (i.e., high concentrations of millimeter-sized graupel). Note that the 45 dBZ at –10°C threshold does not guarantee CG lightning: no lightning is produced between 200 and 240 minutes after 17:25 MDT, even though the threshold is met.