YELLOWSTONE NATIONAL PARK, Wyo. – The historic Yellowstone flood this month occurred due to a combination of above-average snowfall and a large plume of subtropical moisture from the South Pacific known as the Atmospheric River , the latter occurring rarely in June.
This article explains how these factors came together to produce what is widely considered Yellowstone’s worst natural disaster since the 1988 fires.
A cool, wet spring results in an above-average snowpack
Northwest Wyoming experienced a below-average winter in terms of snowfall, as skiers and winter sports enthusiasts will be quick to point out. However, weather patterns have changed significantly this spring, with the strengthening La Niña likely playing a role.
Temperatures in the Greater Yellowstone area were well below average in April and May, as well as in early June, and precipitation was also above average. This trend has resulted in heavy snowfall late in the season and a delay in spring snowmelt.
So while snowpack was well below average at the end of March, by mid-May snowpack was above average in most elevation areas around Yellowstone. due to delayed snowmelt.
Additionally, a huge snowstorm affected the higher elevations of Yellowstone and the Beartooth Mountains just outside the park over Memorial Day weekend in late May, where up to three feet of fresh snow fell.
On June 11, a substantial snowpack remained over the higher elevations of Greater Yellowstone with snow water equivalent values (the amount of liquid contained in snow) about 150-200% of average.
As a result, there was plenty of water waiting to be melted and released from the snowpack by the time June 12 arrived.
An exceptionally strong and late atmospheric river event
An atmospheric river is a long, relatively narrow moisture plume containing significant amounts of water vapour, usually originating from the subtropics of the South Pacific. During the winter months, skiers often refer to this model as a “Pineapple Express” since the moisture often originates near Hawaii.
Atmospheric rivers are common in the cool season on 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 in many areas. Significant flooding in the Pacific Northwest and California is usually associated with atmospheric rivers.
Atmospheric river events quite frequently extend into northern Idaho and northwest Montana during the colder months, while northwest Wyoming typically sees a few atmospheric rivers hitting each year. Locally, these events are more common in the fall and winter and often result in heavy, wet snow on the higher elevations and can lead to winter rain events in the valley.
Atmospheric rivers in the western United States are most common from October through March, but occasionally occur in September, April, and May. Although not unprecedented, they are quite rare in June, July and August and it is even rarer for summer atmospheric river events to extend this far inland in the northwest. of Wyoming.
The atmospheric river that hit 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 classified this particular event as Category 5.
Many sites across the country launch weather balloons into the atmosphere to measure temperature, wind, humidity, and pressure at various altitudes up to 50,000 feet, and they also measure the amount of water vapor in the atmosphere, known as precipitable water.
The nearest balloon launch site, located roughly west of Yellowstone, is in Boise, and this particular site recorded its highest-ever precipitable water value for the first half of June in the days leading up to this event.
Atmospheric rivers typically ebb and flow with the jet stream, and in this particular case, the jet stream and moisture plume settled across Yellowstone on June 12 all day, resulting in a long period of heavy rain. .
Rainfall totals at various weather stations in Yellowstone National Park ranged from approximately 1.5 to 2.5 inches from Saturday evening June 11 through Monday morning June 13.
However, estimates outside of these areas indicate that higher elevation areas in the northern part of the park and surrounding regions received up to 3 to 5 inches of rain. For many, that 36-hour rain total was equivalent to a typical month of rainfall.
While a winter atmospheric fluvial event containing mild air can bring rainfall to the valley and low-lying areas, an event like this in the summer brought snow levels extending well above the mountain top levels. As a result, the heavy rains on the higher elevations resulted in substantial melting of the snowpack.
Remote snow measurement locations (known as SNOTELs) in the region lost over three inches of snowpack water during and after this event, which, combined with a few inches of rain over a period of 36 hours, may explain the severity of the flooding. and the runoff that occurred.
Is there a link with climate change?
An extreme weather event like the one that happened in Yellowstone will inevitably lead many to wonder if the event 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.
On the one hand, we needed a cooler than average spring and increased late season snowpack to create the previous conditions that contributed to that June flood. As the climate warms, snow accumulation will on average peak earlier and melt earlier in the year, but the reverse has happened this year. The cool, wet/snowy spring we experienced this year was largely due to the presence of a strong La Niña.
With respect to atmospheric fluvial events, in a warming climate there is evidence that they will become more intense over time, as warmer air is able to “hold” more moisture than warmer air. costs. However, there is no evidence that supports an increased frequency of end-of-season or summer fluvial atmospheric events like what we just experienced.
In all likelihood, this particular set of events leading up to the Yellowstone flood was just an anomaly that could have happened with or without climate change.
However, there is evidence that as the climate continues to warm in the future, the risk of flooding may increase in the Greater Yellowstone area. But not for the same reasons that this particular flood happened.
For example, stronger (and warmer) atmospheric fluvial events in the future could potentially lead to fall or even winter flooding in northwest Wyoming, especially if snow levels reach higher levels during these events than they have historically in previous years, resulting in winter rain-on-snow for more areas and at higher elevations than before.
Additionally, many climate change models project that winter precipitation (including snowfall) will increase over time, but spring temperatures will warm over time. A more frequent combination of snowy winters followed by warm springs could result in higher instances of spring flooding.
Summer precipitation is not expected to increase with climate change and, according to some projections, is expected to decrease. However, there is evidence that “extreme” summer rainfall events (i.e. the heaviest rain events) could become more significant in the future as the climate warms, which could mean a threat higher flash flooding due to heavy rainstorms.
Alan Smith, meteorologist