DPWX/Street flooding and hail in Ft. Collins: 22 May 2018


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Authors: P. C. Kennedy, H. Reges, R. Schumacher

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CSU-CHILL S-band RHI scan through a severe thunderstorm that affected Fort Collins on 22 May 2018. The spatial organization of the radial velocities implies the existence of strong updrafts. The storm produced heavy rain, local flooding, and significant hail accumulations in the city. Radar plots showing some of the storm's dual polarization and airflow characteristics have been assembled.


A thunderstorm produced a combination of very high rain rates and ground-covering hail accumulations over portions of Fort Collins Colorado around 6 PM local time on 22 May 2018. This intense precipitation resulted in traffic disruptions due to accumulations of standing water in a number of low-lying streets. The hailstone concentrations were also high enough deposit a considerable amount of leaf debris below the trees in many neighborhoods. Since the Collaborative Community Rain, Hail and Snow (CoCoRaHS) network was originally developed in the Fort Collins area, a number of observing sites have been established in the area affected by this storm. The following plot shows the precipitation totals filed by CoCoRaHS volunteer observers in the Fort Collins area; these precipitation totals are for the 24 hour period ending around 7 AM local time on the morning following the storm. (Most, but not all, of the precipitation fell during the early evening thunderstorm.) Several reports of rain totals in excess of 2.5 inches were received from the west-central portion of the city. (For reference, the red dot marks the general location of the deepest reflectivity core seen in the CHILL radar RHI scan shown at the top of this article.)

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Radar observations of the Fort Collins storm

Test operations of the S-band (11 cm wavelength) portion of the CSU-CHILL radar were being conducted as the storm affected Fort Collins. The following plot shows the reflectivity data collected in a 1.9 degree elevation scan over the city area at 0011 UTC (locally, 1811 MDT on the 22nd). A 10 dB contour interval was used in generating the plot; red shading indicates reflectivity levels at or above 60 dBZ.

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To better associate the CHILL reflectivity data with the CoCoRaHS observations, the following loop shows a series of low elevation angle PPI sweeps plotted as a semi-transparent overlay on the rain fall reports. The evolving high reflectivity echo core moved over the city in a 30 - 40 minute period.

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The following plot shows a snapshot of the dual polarization data as seen in a 0.5 degree elevation scan at 0011 UTC / 1811 MDT when the storm was in its intense phase. The color-filled overlay is rain rate (mm/hr) as estimated using the procedures outlined in Cifelli et al (JAOT 2011, p352-364). The rain rates were quite high at this time. For example, the 130 mm/hr rate indicated by the start of the bright yellow shading corresponds to 5.1 inches per hour. This value is consistent with the CoCoRaHS rain fall totals of ~2.5 inches and the radar-indicated storm duration of 30 - 40 minutes. Dual polarization signatures of hail are indicated in the region enclosed by the solid blue contour line. Within this area, the Hail Differential Reflectivity parameter (HDR) is greater than +10 dB. As formulated by Aydin et al. (JCAM 1986, p. 1475-1484), positive HDR values are associated with a high probability of hail in the near-surface (above freezing) heights in thunderstorm precipitation.

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Confirmation of high rain rates was provided by the tipping bucket rain gauge that is a component in the instrumentation suite installed at the Fort Collins weather station located on the CSU main campus. (Approximately 2 km west of the CSU Oval location in the following dual-Doppler plot.)The next plot shows the rain accumulation as a function of time during the one hour period starting at 00 UTC on 23 May (locally, 1800 MDT on 22 May 2018). As indicated by the two reference lines plotted in gray, the rain rates during the first ~10 minutes of the event were on the order of 130 mm / hr.

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The starting time of the CSU-CHILL 0011 UTC volume occurred within 45 seconds of the starting time of a volume scan made by the NWS WSR-88D radar in Cheyenne, Wyoming. This reasonably good scan synchronization allowed the synthesis of the horizontal wind field from the radial velocities recorded by the two Doppler systems. The vectors in the following plot show the dual-Doppler horizontal wind field at the 2.9 km MSL (~1.5 km AGL) height level. Well-organized inflow was present on the eastern side of the storm. The flow field also contained convergence and cyclonic rotation in the vicinity of the reflectivity core that was located over the CSU campus area at this time.

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The final plot presents CHILL radar data that was collected in an RHI scan done on an azimuth of 288 degrees at 0004 UTC. (The location of this RHI plane is marked in the preceding dual-Doppler plot.) 50 dBZ and greater reflectivity values extended to heights of ~ 10 km AGL (upper left). The radial velocity pattern (upper right) contained both surface-level convergence and storm-top divergence. This configuration implied the existence of updrafts that supported active precipitation growth. The lower two panels highlight some of the dual polarization characteristics of the precipitation near the ground. The differential reflectivity (Zdr) in the lower left panel exhibits a sharp transition from positive values at ranges less than ~38 km to near 0 dB levels in the main reflectivity core (~40 km range). The positive Zdr regime signifies the the precipitation is primarily composed of oblate raindrops. The preferential horizontal orientation of these flattened drop shapes increases the strength of the horizontally-polarized radar return relative to that of the vertically-polarized return. Since Zdr is based on the logarithm of the H/V received signal strength ratio, the rain-dominated region has characteristically positive Zdr. In the the high reflectivity core, the presence of tumbling, quasi-spherical hailstones tends to equalize the H and V received powers. The lower right panel shows the differential propagation phase (phidp) field. A color change from blue towards yellow with increasing range indicates that the phase of the H return is increasingly lagging that of the V return. This increasing H wave retardation arises when a high concentration of oblate drops exits along the radar beam path; thus the region of rapidly changing phidp in the ~35 to 45 km range interval is consistent with the high rain rates that generated local flooding in Fort Collins.

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The CoCoRaHS project supported this usage the network's rainfall data. The dedicated reporting efforts of the individual CoCoRaHS observers are of critical importance to many research activities.