STEAMBOAT SPRINGS, Colo. — For all the attention on epic flooding in the Mississippi Valley, a quiet threat has been growing here in the West where winter snows have piled up on mountain ranges throughout the region.

Thanks to a blizzard-filled winter and an unusually cold and wet spring, more than 90 measuring sites from Montana to New Mexico and California to Colorado have record snowpack totals on the ground for late May, according to a federal report released last week.

Those giant and spectacularly beautiful snowpacks will now melt under the hotter, sunnier skies of June — mildly if weather conditions are just right, wildly and perhaps catastrophically if they are not.

Fear of a sudden thaw, releasing millions of gallons of water through river channels and narrow canyons, has disaster experts on edge.