A Review of the Warning Methodology for the Active 2022 North American Monsoon by NWS Phoenix, Arizona

The 2022 North American Monsoon (NAM) was one of the more active in recent history. The Desert Southwest experienced its wettest NAM since 2006. The severe thunderstorm warning (SVR) count, issued by National Weather Service (NWS) Weather Forecast Offices (WFO), was quite high. A total of 577 SVRs were issued in Arizona, with over 700 across the Desert Southwest (3rd highest since 2008), during the 2022 NAM, which runs from June 15 through September 30. WFOs Phoenix (PSR) and Tucson issued over 200 SVRs each in their respective county warning areas (CWA) (highest counts since 2008). The 239 SVRs issued by WFO Phoenix was 129 higher than the second highest count of 110 SVRs in 2021. Considering the warning count was more than double the previous highest count, this led to the initial question of whether PSR overwarned during the 2022 season. There are certainly factors like changing philosophies, turnover in warning forecaster experience, and new scientific methodology and technology, independent of how active the season is, that can contribute to varying counts of SVRs at an office. One of the primary methods PSR meteorologists are trained to utilize for SVR decision making is to assess mass-loading in cellular convection. A key predictor PSR uses are reflectivity values to specified heights. In particular, 50 dBZ to 20K feet or higher and 60 dBZ to 15K feet or higher. The 50 dBZ and 60 dBZ height predictor was found to be one of the best predictive tools for SVR decision making based on a recent 2019 PSR Hollings study by LeBel et al., which was presented during the American Meteorological Society’s 100th national conference in Boston, MA. This study developed a probabilistic predictive tool for severe winds from cellular convection in the Phoenix metro area.

The predictors developed from the Hollings study were referenced to review each storm warned by PSR within their CWA during the 2022 NAM. The review found that roughly 95% of the storms warned by PSR in 2022 had a 50% probability or higher of producing severe winds based on the 50 dBZ and 60 dBZ height predictive tool from the Hollings study. Although roughly 70% of the warnings went unverified, which is common during the monsoon each year for PSR verification, it is believed the unverified warned storms were still capable of producing severe wind impacts based on the mass loading storm characteristics. The majority of verified warnings were issued around the Greater Phoenix area, thus factors such as warnings in remote areas contributed to the lower verification scores. Additionally, of the verified warned storms, all of them met the 50 dBZ and 60 dBZ base predictor heights for severe winds and roughly 95% of the unverified warned storms also met the predictor. It is encouraging to know that PSR meteorologists used a consistent warning methodology during the active 2022 NAM and to know that despite the high SVR count, nearly all SVRs were warranted based on the predictors for severe cellular storms from the 2019 Hollings study.