Although it is a rare phenomenon, hail on the Colorado Plateau can be a significant hazard. This study identifies threshold values of specific atmospheric variables associated with observed severe hail occurrences for northern Arizona during the North American monsoon seasons associated with the 1996–2009 timeframe. The threshold constraints on hail are found by constructing long-term means of the overall, non-severe hail, and severe hail atmospheric environments over northern Arizona, based on Flagstaff, Arizona, 1200 UTC radiosonde data. Results indicate that days experiencing severe hail possess a significantly different morning atmospheric profile. Key variations between days with severe hail versus days without severe hail are elevated sub-300-hPa moisture, a warmer atmosphere, lighter above surface wind speeds, more southerly to southeasterly oriented winds throughout the atmospheric column (except at the 700-hPa pressure level), and noticeably higher geopotential heights at all mandatory pressure levels. Large-scale synoptic patterns associated with the North American Monsoon are the driving force behind whether a severe hail or non-severe hail environment exists over northern Arizona.
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