Wide swaths of the West and Midwest are, or are threatened with, flooding driven by snowmelt. A heatwave set to descend on the West Coast this week is expected to rapidly accelerate snowmelt, potentially overflowing dreams and exacerbating extant valley flooding. “The temperatures really matter up where the snow is — those are also going to be anomalously warm,” Alan Haynes, a hydrologist at the California Nevada River Forecast Center, told the Washington Post.

Daytime highs in the 50s and (especially) nighttime lows above freezing as high as 8,000 ft. — exacerbated in places by recent burn scars that heat up faster than forested terrain — are expected to set off substantial melting. “That’s when you can get a lot of flow,” Haynes said. Large portions of Yosemite National Park will close Friday in anticipation of the dangerous melt-driven streams. More than 1,200 miles to the east, the Mississippi River is also overflowing its banks, flooding roads in Minnesota, Wisconsin, and Iowa. (California: Washington Post $, AP; Yosemite: AP; Midwest: Gizmodo)