Developing an Accumulating Hail Climatology for the National Weather Service in Hanford, California

Several studies have focused on thunderstorms and/or showers that produce sub-severe accumulating hail across the Western United States (Garner et. al. 2019; Kalina et. al. 2016). These studies highlighted impacts to transportation when roads quickly became covered with hail, sometimes leading to traffic accidents and highway closures.

The purpose of this study (February 2000 – May 2019) was to focus on accumulating hail events and their associated radar and environmental characteristics, within the National Weather Service (NWS) Hanford County Warning Area (CWA), to help operational forecasters increase lead time and mitigate transportation risk. Increased situational awareness and confidence from this familiarization can enhance the Interactive Decision Support Services (IDSS) provided to core partners such as the California Department of Transportation (CalTrans) and the California Highway Patrol (CHP). In addition, IDSS messaging can be targeted at highly frequented National Parks (Yosemite, Kings Canyon, and Sequoia) in the Hanford CWA, since accumulating hail events are not solely a cool season phenomenon, especially at high elevation.

For this study, we created a composite sounding by combining data from nearby upper-air locations closest to the thunderstorms and/or showers which were producing accumulating hail, according to the National Centers for Environmental Information database. We then compared this data to the base reflectivity and dual-pol radar products in order to increase forecasters’ confidence during these events. The composite soundings showed that the average freezing level was 7300 ft above mean sea level (MSL) and had a corresponding reflectivity of 50.8 dBZ. Additionally, the wet-bulb zero level (WBZ), which can help determine the melting potential, was 3937 ft above MSL. Although the amount of dual-pol data is limited, the thunderstorms and/or showers analyzed had characteristics most common with the production of small, dry hail. The average dual-pol Correlation Coefficient (CC) was 99.5, Differential Reflectivity (ZDR) of 0.31 dB, and Specific Differential Phase (KDP) of 0.62 deg/km. The average height of the -20 Celsius level and reflectivity was 14453 ft and 38.8 dBZ, respectively. Reflectivity at the -20 Celsius level for accumulating hail had a wide range of values, from 16 dBZ to 59 dBZ, indicating that thunderstorms and/or showers could produce accumulating hail during any time of the year, not just the cool season. In conclusion, our research necessitated a Call-To-Action statement in our Significant Weather Advisories and enhanced social media graphics, to alert the public about the risks associated with accumulating small hail from thunderstorms and/or showers.