What Caused the Record Flooding on the Yellowstone River?

 FastCast—Wednesday, June 15 to Sunday, June 19:

A calm stretch of weather is ahead for Western Washington. On Wednesday, expect mostly cloudy conditions, with highs in the mid to upper 60s. Thursday will be partly sunny, with highs in the upper 60s. Friday will be overcast, with highs in the mid 60s and rain arriving by evening. Generally, expect 0.5 to 0.9 inches in the lowlands through Sunday, with most of the rain from late Friday through Saturday. More rain will fall south of Tacoma, on the coast, and in the mountains, where 1-2 inches are possible. There will be some showers on Sunday. Highs on Saturday and Sunday will be cooler, only reaching the low 60s. Expect lows throughout the entire period to be in the low 50s.

————————————————————

Continue reading the full blog below!

One of the biggest weather stories recently has been the complete closure of Yellowstone National Park due to record-breaking flooding on the Yellowstone River in NW Wyoming and Southern Montana. 

The photo below from a Yellowstone National Park Service helicopter via KBZK News (Bozeman, MT) shows the remnants of US Highway 89 (NE Entrance Road) along the Gardiner River, just north of Yellowstone National Park. The Gardiner River set an all time record at this location, which is about 20 miles south of where it flows into the Yellowstone River.

On this road alone, there are many scenes like this, where sections of highway are completely gone due to unprecedented flooding.

So…the big question is why did this flooding happen, and why on this scale? This blog will examine that.

The first factor is that there was significant rainfall over NW Wyoming and SW Montana, where the rivers in the Yellowstone River Basin originate. The map below shows rainfall in the region over the past week.

NW Wyoming and SW Montana received 3-5 inches of rain, a significant amount for that region. However, 3-5 inches of rain is usually not even close to enough to cause the massive flooding that occurred in Wyoming and Montana.

Enter in the snowpack situation…The snowpack in the mountains around the Yellowstone River’s source was significantly above average, a major contributor in the flooding. The graphic below from the USDA shows the percent of average snowpack in Wyoming.

Notice how much above average the snowpack is in NW Wyoming(100-800% of normal). Couple this with high temperatures as the flooding was occurring (NWS map below), and you can get a good picture of what caused this record-breaking flooding.

High temperatures in the headwaters region of the Yellowstone River were in the mid 40s to low 50s, much higher than necessary for significant snowmelt to occur. 

Unprecedented flooding was possible along the Yellowstone River and its tributaries because of the combination of heavy mountain rain, significant mountain snowmelt, and warm temperatures. 

So…what happened downriver?

Multiple all-time records were established on Monday, and some are still occurring…as seen in the hydrographs (river flood graphics) below.

The Yellowstone River shattered all-time records by 1-3 feet near Yellowstone National Park and near Livingston, MT. 

As this blog was being written on Tuesday night, the flood was moving downriver, and an all-time record is being set in Billings, Montana’s largest city.

The river will crest very near major flood stage. The Montana National Guard landed in Billings this evening to assist citizens as the flooding continues.

Some of the images of the flooding, from Yellowstone National Park all the way downriver into Southern Montana, are quite dramatic and devastating, as homes are washed away, water rescues occur, and towns are flooded. The images below from The Weather Channel show some of the impacts of the flooding on the Yellowstone River.

The flooding has receded in Yellowstone National Park and SW Montana, and the flooding is moving down the Yellowstone River toward Billings and other cities in Eastern Montana. However, the impacts of washed out roads and major water damage in Yellowstone National Park will likely bring closures for some time.