Trends in synoptic heat events in four Minnesota urban areas through the 21st century

Extreme heat is often overlooked as a public health concern in Minnesota, where intraseasonal summer variability limits acclimatization to oppressive heat conditions. Specific categories of synoptic-scale air masses are linked to summer excess mortality and elevated health risk in the Midwestern United States, particularly within urban areas. Between 1948 and 2019, Minnesota's four largest urban areas have experienced decreased nighttime cooling, while warmer and more humid air masses have increased in frequency at the expense of cooler and drier ones. We used downscaled CMIP5 climate projections for 21st-century Minnesota, under RCP4.5 and RCP8.5 emissions scenarios, to generate daily synoptic classifications and evaluate projected frequency and character trends in the highest-risk air masses. Projections show dramatic increases in both the frequency and temperature of days within the Dry Tropical category, neither of which have changed significantly thus far across Minnesota's historical record. Frequency and duration of consecutive-day episodes of excess heat, as identified either by synoptic classifications or by the Excess Heat Factor, are likewise expected to increase more substantially in the future than they have in the past. Other projected trends, such as rising dew point temperatures and nighttime air temperatures, represent continuations of already existing historical trends.