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Extremes of wet-bulb temperature (WBT)jointly reflecting temperature and specific humidityhave seen relatively little study in terms of climatology, despite their demonstrated relevance for health and economic impacts. In this study, we uncover and characterize distinct spatiotemporal patterns of WBT extremes in the contiguous United States for the 1981-2015 period, focusing on identifying and making a first pass at understanding regional differences. We find that anomalies of specific humidity are of greater importance than those of temperature in controlling extreme WBT in most of the contiguous U.S., particularly for southern and arid regions. Composites of extreme-WBT days for each region reveal coherent sea-surface temperature anomalies and midlevel and upper -level geopotential-height anomalies that differ considerably between regions, particularly in terms of the resulting low-level temperature and moisture fields. These findings suggest that the primary factors controlling the timing and intensity of WBT extremes, while ultimately forced by synoptic-scale weather patterns, vary spatially according to both local geography and baseline climate. We demonstrate this conclusion by showing how regional features such as late-summer WBT extremes in the Southwest and southern Great Plains derive primarily from spatial and temporal variations in moist low-level flows.

Plain Language Summary Wet-bulb temperature (WBT), as a combination of temperature and humidity, is a good proxy for the health and economic impacts of heat. However, WBT extremes have rarely been studied. We assemble a new 35 -year hourly-resolution dataset from contiguous -U.S. weather stations. In the western U.S., we find that when the wind blows from a direction favorable for high temperature, it is simultaneously unfavorable for high humidity, but in the eastern U.S. extreme temperature and moisture are tightly linked. We devise a new approach to quantify the moistness of heat and find that WBT extremes often result from hot dry areas having unusual moisture spikes, or cool moist areas having temperature spikes. We show the first maps of the average time of year of WBT extremesmost interesting is the late-summer extreme WBT in the southern Great Plains and Southwest. We explain this in terms of moisture advection connected with the North American Monsoon (Southwest) and the strength of southerly winds from the Gulf of Mexico (Great Plains). Lastly, we find distinct atmospheric and sea surface temperature patterns associated with WBT extremes that differ between regions and that are present at least 10 days before the extremes, indicating the possibility of more accurate predictions.