World Resources Institute explains that recent record warm temperatures, combined with low precipitation have combined to make the situation even worse than either factor alone:
From the Southwest to the Great Lakes, temperatures have been so high and rainfall so low that the drying effect of warmer air temperatures far exceeded what little precipitation there’s been, resulting in moisture being drawn out of soils. This situation has occurred at a critical time in the country’s growing season, leaving the ground sapped of moisture and, in turn, wreaking havoc on crops and livestock. According to the National Oceanic and Atmospheric Administration (NOAA), even if a normal amount of rainfall occurred this year, it could not have offset the drying effect of the record-breaking heat.Furthermore, lower than normal snowfall across those areas of the nation dependent on spring melting for water combined with more rapid melting than usual, brought on by the warmest winter in the US on record, resulting in lower than usual reservoir levels.
Read more: WRI Insights
What's the Science Behind the US Drought?
World Resources Institute explains why the current drought is so bad.