Yellowstone floods reveal how climate change has made forecasting difficult

The Yellowstone National Park area's weather forecast the morning of June 12 seemed fairly tame: warmer temperatures and rain showers would accelerate mountain snow melt and could produce "minor flooding."

A National Weather Service bulletin recommended moving livestock from low-lying areas but made no mention of danger to people.

By nightfall, after several centimetres of rain fell on a deep spring snowpack, there were record-shattering floods.

Torrents of water poured off the mountains. Swollen rivers carrying boulders and trees smashed through Montana towns over the next several days. The flooding swept away houses, wiped out bridges and forced the evacuation of more than 10,000 tourists, park employees and residents near the park.

As a cleanup expected to last months grinds on, climate experts and meteorologists say the gap between the destruction and what was forecast underscores a troublesome aspect of climate change: Models used to predict storm impacts do not always keep up with increasingly devastating rainstorms, hurricanes, heat waves and other events.

"Those rivers had never reached those levels. We literally were flying blind not even knowing what the impacts would be," said Arin Peters, a senior hydrologist with the National Weather Service.

Hydrologic models used to predict flooding are based on long-term, historical records. But they do not reflect changes to the climate that emerged over the past decade, said meteorologist and Weather Underground founder Jeff Masters.

"Those models are going to be inadequate to deal with a new climate," Masters said.

Scientists said the Yellowstone flooding was consistent with changes already documented around the park as temperatures warm.

Those changes include less snowfall in mid-winter and more spring precipitation — setting the stage for flash floods when rains fall on the snow, said Montana State University climate scientist Cathy Whitlock.

Warming trends mean spring floods will increase in frequency — even as the region suffers from long-term drought that keeps much of the rest of the year dry, she said.

Masters and other experts noted that computer modelling of storms has become more sophisticated and is generally more accurate than ever. But extreme weather by its nature is hard to predict, and as such events happen more frequently there will be many more chances for forecasters to get it wrong.

The rate of the most extreme rainstorms has increased by a factor of five, Masters said. So an event with a one per cent chance of happening in any given year — commonly referred to as a "one in 100-year" event — now has a five per cent chance of happening, he said.

"We are literally re-writing our weather history book," said University of Oklahoma meteorology professor Jason Furtado.