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Spring snow storms bring increased moisture to an already wet Wyoming

by Wyoming Livestock Roundup

Warmer temperatures have been juxtaposed against increased precipitation over the last week across Wyoming.

Lee Hackleman, Natural Resources Conservation Service (NRCS) water supply specialist, commented, “Quite a bit of the state added another three inches of snow water equivalent (SWE) to their snowpack during the first week of May. Statewide, the SWE went up 20 percent from 117 percent to 137 percent of median.”

He continued, “That had to be one of our biggest storms for quite a while statewide. This next week shows showers for a couple of days then warming, so I would guess that we should stay around 135 percent for the next week at least.”

Hackleman noted that the Wind River snowpack is still of great concern in regards to flooding.

The May 1 Wyoming Water Supply Outlook Report showed that year-to-date precipitation in Wyoming averaged 139 percent for all basins across the state, varying from 78 to 188 percent of average. Monthly precipitation for individual basins across the state ranged from 59 to 194 percent of average, for an overall average of 150 percent.

“Basin reservoir levels for Wyoming vary from 59 to 194 percent of average, for an overall average of 126 percent,” Hackleman continued in the report.


Reservoirs across the state are, overall, above average for storage. However, in the Snake River Basin, reservoirs are only at 59 percent of their average May 1 capacity. The Wind River Basin’s reservoirs are at 88 percent and Big Horn River Basin reservoirs are currently at 92 percent of capacity.

The remainder of the reservoirs in the state are above average capacity, with Buffalo Bill at 106 percent, the Madison-Gallatin River Basin at 108 percent, the Tongue River Basin reservoir at 194 percent, Belle Fourche at 113 percent and Cheyenne River Basin at 111 percent.

Additionally, the Upper and Lower North Platte River basins are at 150 and 133 percent, respectively, and the Upper Green River Basin is at 105 percent. The Lower Green River Basin is slightly above average, at 101 percent, and the Upper Bear River Basin reservoirs are currently at 112 percent capacity.


With reservoirs at above-average capacity right now, streamflow yields are also forecasted to be at 175 percent of average.

“The Snake, Madison and Upper Yellowstone River basins should yield about 170, 115 and 147 percent of average, respectively,” said the report. “Yields from the Wind and Big Horn River basins should be about 263 and 267 percent of average.”

The Shoshone is expected to flow at 187 percent of normal, and the Clarks Fork River Basin is anticipated to be 169 percent of average.

“Yields from the Powder and Tongue River basins should be about 222 and 141 percent, respectively, of average,” Hackleman said, noting that the Upper North Platte, Sweetwater, Lower North Platte and Laramie River basins are forecasted at 108, 262, 121 and 103 percent of average, respectively. Additionally, the Green River and Smith’s Fork basins are predicted at 225 and 193 percent of average.

Only the Cheyenne and Little Snake River basins were below normal, at 59 and 83 percent of average May to September streamflow yield.

Making predictions

Hackleman explained in the report, “Most annual streamflow in the western United States originates as snowfall that has accumulated in the mountains during the winter and early spring.”

Hydrologists estimate runoff using measurements of SWE and computerized simulation models.

“Forecasts of any kind, of course, are not perfect,” he commented. “Streamflow forecast uncertainty arises from uncertain knowledge of future weather conditions, uncertainty in the forecasting procedure and errors in the data.”

As the season progresses, forecasts are more certain, but Hackleman emphasized that users should be cognizant of the uncertainty when making operation decisions.

Saige Albert is managing editor of the Wyoming Livestock Roundup. Send comments on this article to

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