Sounding Characteristics and Dual-Pol Signatures of Severe Hail Events across Central California

The dearth of severe weather events across the National Weather Service (NWS) Hanford/San Joaquin Valley’s County Warning Area (CWA) can sometimes muddle the situational awareness of the radar operator at our office. Many weeks during the summer are completely dry, with nary a cloud to be seen, save for the uncommon influx of monsoonal moisture along the Sierra Nevada or Mojave Desert. Despite practice exercises or simulations, forecasters need a reference of environmental characteristics that have produced severe hail in the past. Not only will this information give the radar operator more confidence with regards to the issuance of severe thunderstorm warnings for hail, but also will serve as a primer for the utilization of dual-pol data during storm assessment. The goal of our research was to create a clearer picture of what constitutes a favorable environment for severe hail throughout the challenging topography of NWS Hanford.

The paucity of severe hail events which correlated with reliable dual-pol signatures was the main challenge in our research. With dual-pol data only available back to 2013 and the change of severe hail criteria from 0.75” to 1” in diameter, our sample size was limited. Despite this, our climatology goes back to 2004 and contains 42 occurrences of thunderstorms which produced severe hail (1” or greater), from the National Centers for Environmental Information’s Storm Events Database. Reflectivity (Z), Correlation Coefficient (CC), Specific Differential Phase (KDP) and Differential Reflectivity (ZDR) images and values were collected. Also, the freezing level and -20 Celsius height were recorded from the closest upper-air site, along with the Z values.

From a dual-pol perspective, April 1 2013 was a notable event because not only did it feature KDP and ZDR values very close to zero (an indicator of dry, frozen hailstones) but also had reliable sounding characteristics. More specifically, both the 50 dBZ and 60 dBZ heights for April 1 2013 were in the top third of the data recorded. The most visually impressive dual-pol signature was the 1.25” diameter hail recorded on April 25 2016, and this example can be used to familiarize forecasters also. For all severe hail dual-pol events, the Average CC was 97, KDP was 0.52 deg/km and ZDP was 0.37 dBZ.

The second part of the study focused on the height of the 50 dBZ and 60 dBZ reflectivity of the storm, 700mb-500mb lapse rates, the freezing level and the -20 Celsius (C) level. It is suggested in this study that NWS Hanford forecasters determine the freezing level and -20 C level to use as a first guess when estimating updraft strength; or the ability to suspend larger hailstones. The mean freezing level for severe hail was 8720 ft mean sea level (MSL) and the -20 C level was 18484 ft MSL. The 50 dBZ and 60 dBZ heights gathered for this study occurred 15 minutes or more before the hail was reported on the ground. This should help NWS Hanford warning forecasters anticipate the occurrence of severe hail and increase lead times. For hail between 1” and 1.25” in diameter, the average height of 60 dBZ was 18612 ft MSL and the average height of the 50 dBZ level was 23866 ft MSL. A few severe thunderstorms produced hail greater than 1.25”, those 60 dBZ and 50 dBZ average heights were 21778 ft MSL and 27372 ft MSL, respectively. The average lapse rate in the 700mb-500mb layer was -6.76 degrees (deg) C/km, with a standard deviation of 1 deg C/km for all events.

Overall, April was the most frequent month for severe hail, with 18. The most intense thunderstorm analyzed in the study occurred near Orange Cove, CA on May 29 2009 with 1.5” diameter hail, a 60 dBZ level of 32900 ft MSL, a 50 dBZ level of 40700 ft MSL and 700mb-500mb lapse rate of -7.8 degrees C/km. To conclude, our presentation will show the composite sounding created from all severe hail events that have occurred in the NWS Hanford County Warning Area and the best dual-pol signatures from the study.