Urban Warming Challenges Verification of Frost Advisories and Freeze Warnings in Madison, Wisconsin

Urban heat islands (UHIs) may increase the likelihood that frost sensitive plants will escape freezing night-time temperatures in late spring and early fall. Using data from 151 temperature sensors in the Madison, Wisconsin, region during March 2012-October 2016, we found that during time periods when the National Weather Service (NWS) issued freeze warnings (threshold of 0.0 & DEG;C) or frost advisories (threshold of 2.22 & DEG;C) were valid, temperatures in Madison's most densely populated, built-up areas often did not fall below the respective temperature thresholds. Urban locations had a mean minimum temperature of 0.72 & DEG; and 1.39 & DEG;C for spring and fall freeze warnings, respectively, compared to-0.52 & DEG; and-0.53 & DEG;C for rural locations. On average, 31% of the region's land area experienced minimum temperatures above the re-spective temperature thresholds during freeze warnings and frost advisories, and the likelihood of temperatures falling be-low critical temperature thresholds increased as the distance away from core urban centers increased. The urban-rural temperature differences were greatest in fall compared to spring, and when sensor temperatures did drop below thresholds, the maximum time spent at or below thresholds was highest for rural locations during fall freeze warnings (6.2 h) compared to urban locations (4.8 h). These findings potentially have widely varying implications for the general public and industry. UHIs create localized, positive perturbations to nighttime temperatures that are difficult to account for in forecasts; there-fore, freeze warnings and frost advisories may have varying degrees of verification in medium-sized cities like Madison, Wisconsin, that are surrounded by cropland and natural vegetation.