YELLOWSTONE NATIONAL PARK, Wyo. — The historic flooding in Yellowstone this month occurred due to a combination of above-average snowpack along with a significant plume of subtropical moisture originating in the South Pacific known as an atmospheric river, the latter of which rarely occurs in June.

This article will explain how these factors came together to produce what is widely being viewed as Yellowstone’s worst natural disaster since the 1988 fires.

Cool, Wet Spring Leads to an Above-Average Snowpack

Northwest Wyoming experienced a below-average winter in terms of snowfall, as skiers and snow sports enthusiasts will be quick to point out. However, the weather pattern shifted substantially this spring, with a strengthening La Niña likely playing a role.

Temperatures across the Greater Yellowstone Region were well below average during April and May, as well as early June, and precipitation was also above average. This pattern resulted in substantial late-season snowfall and a delay in spring snowmelt.

So even though snowpack was well below average at the end of March, by the middle of May, snowpack was above average across most higher elevation areas around Yellowstone due to the delay in snowmelt.

In addition, a huge snowstorm impacted the higher elevations of Yellowstone and the Beartooth Mountains just outside of the park over Memorial Day weekend in late May, where up to three feet of new snow fell.

By June 11, substantial snowpack remained over the higher elevations of Greater Yellowstone with snow water equivalent values (the amount of liquid contained in the snow) running about 150-200% of average.

June 11, 2022 snow water equivalent compared to historical averages for the date.

As a result, there was a lot of water waiting to be melted and released from the snowpack by the time June 12 arrived.

An Unusually Strong and Late Atmospheric River Event

An atmospheric river is a long and relatively narrow plume of moisture containing significant amounts of water vapor, typically originating in the subtropical regions of the South Pacific. In the winter months, skiers often refer to this pattern as a “Pineapple Express” since the moisture often originates near Hawaii.

North America from space. Elements of this image furnished by NASA

Atmospheric rivers are common during the cool season across the West Coast, including the Cascade and Sierra Nevada Ranges, and tend to produce heavy precipitation. Mild air often accompanies these events resulting in high snow levels and transitions from snow to rain for many areas. Significant flood events across the Pacific Northwest and California are usually associated with atmospheric rivers.

Atmospheric river events extend into Northern Idaho and Northwest Montana somewhat frequently during the cooler months as well, while Northwest Wyoming typically sees a few impactful atmospheric rivers each year. Locally, these events are most common during the fall and winter and often result in heavy, wet snow across the higher elevations and can lead to winter rain events in the valley.

Atmospheric rivers in the Western U.S. are most common from October to March but occasionally occur in September, April and May. While not unprecedented, they are quite rare in June, July and August and it’s even more rare for summer atmospheric river events to extend this far inland into Northwest Wyoming.

The atmospheric river that impacted Yellowstone on June 12 was also much stronger than usual, even by winter standards. The Center for Western Weather and Water Extremes began categorizing atmospheric river events from one to five (similar to hurricanes, but of course much less destructive) and labeled this particular event a Category 5.

Numerous sites across the country launch weather balloons into the atmosphere to measure temperature, wind, humidity, and pressure at different altitudes up to 50,000 feet, and they also measure the amount of water vapor in the atmosphere which is known as precipitable water.

The nearest balloon launch site that is located roughly due west of Yellowstone is in Boise, and this particular site recorded its highest precipitable water value on record for the first half of June in the days leading up to this event.

Atmospheric rivers usually ebb and flow with the jet stream, and in this particular case, the jet stream and the moisture plume set up across Yellowstone on June 12 for the entire day, leading to an extended period of heavy rainfall.

Rain totals at various weather stations across Yellowstone National Park ranged from about 1.5 to 2.5 inches from Saturday night, June 11 through Monday morning, June 13.

However, estimates outside of these areas indicate that higher elevation areas across the northern section of the park and surrounding regions received as much as 3 to 5 inches of rain. For many, this 36-hour rain total was equivalent to a typical month’s worth of precipitation.

Estimated rain totals from June 10-13, nearly all of which fell between Saturday night and Monday morning. Source: pivotalweather.com.

While a winter atmospheric river event containing mild air may result in rain across valley and lower elevation areas, an event like this in the summer resulted in snow levels extending well above mountain-top levels. As a result, the heavy rain across the higher elevations led to substantial melting of the snowpack.

Remote snow measurement locations (known as SNOTEL) across the area lost more than three inches of water from the snowpack during and after this event, which combined with a few inches of rainfall within a 36-hour span, can explain the severity of the flooding and runoff that occurred.

Is there a Climate Change Connection?

An extreme weather event such as what happened in Yellowstone will inevitably lead many to wonder if the occurrence is related to climate change. In this particular case, climate change may have been a minor factor, but there is little evidence to support that climate change was a significant factor.

For one, we needed a cooler than average spring and a late-season boost in snowpack to set up the preceding conditions that contributed to this June flooding event. As the climate warms, snowpack will, on average, peak earlier and melt earlier in the year, but the opposite occurred this year. The cool and wet/snowy spring we experienced this year can largely be explained by the presence of a strong La Niña.

As far as atmospheric river events go, under a warming climate, there is evidence supporting that they will become more intense over time since warmer air is able to “hold” more moisture compared to cooler air. However, there is no evidence that supports an increased frequency in late season or summer atmospheric river events such as what we just experienced.

In all likelihood, this particular set of events leading to the Yellowstone flood was just an anomaly that could have occurred with or without climate change.

However, there is evidence that as the climate continues to become warmer in the future, the risk of flood events could increase across the Greater Yellowstone Region. Just not for the same reasons that this particular flood event occurred.

For example, stronger (and warmer) atmospheric river events in the future could potentially lead to autumn or even winter flood events in Northwest Wyoming, especially if snow levels spike to higher levels during these events than they historically have in years prior, resulting in mid-winter rain-on-snow for more areas and higher elevations than before.

Also, many climate change models are projecting that winter precipitation (including snowfall) will actually increase over time, but that spring temperatures will warm up over time. A more frequent combination of snowy winters followed by warm springs could lead to higher instances of spring river flooding.

Summer rainfall is not expected to increase with climate change, and in some projections is expected to decrease. However, there is evidence that summer rainfall “extremes” (meaning the heaviest rain events) could become heavier in the future as the climate warms, which could mean a higher threat of flash flooding from heavy rain-producing thunderstorms.

Alan Smith, Meteorologist

Buckrail Meteorologist Alan Smith

Alan is a professional meteorologist who holds a degree from MSU Denver and writes weather forecasts for Buckrail. He has worked in the private sector of weather forecasting since 2013 and has lived in Jackson since 2015. Alan specializes in mountain weather and forecasts for ski areas across North America.