FROM NRCS: After a winter and spring that dropped seemingly non-stop snowfall across most of Montana, spring runoff is finally here. April started just like many of the other months so far this snow season, with abundant precipitation falling and continuing to build the mountain snowpack, according to snow survey data collected by the USDA Natural Resources Conservation Service.
Some areas in southwest Montana set new records for April precipitation (Gallatin and Madison valleys), and almost all areas except the Rocky Mountain Front and Hi-Line received near to above normal precipitation. “The crazy fact about this April was that average to well-above average precipitation fell during the first 18 days of the month; the latter half of the month was dominated by high pressure with abundant sunshine and well-above-average temperatures,” said Lucas Zukiewicz, NRCS water supply specialist for Montana.
Early April precipitation only continued to add to a snowpack that was well-above normal across the state, and the snowpack was record-setting in the Upper Clark Fork, Blackfoot and Clark’s Fork (Yellowstone) River basins. Peak snow water equivalent in these basins exceeded prior record years of 1975, 1997 and 2011, and occurred during the third week of the month. Many SNOTEL sites and snowcourses in these basins remain record for May 1 even though melt has already started at low to mid-elevations.
“Snowpack percentages are incredibly high for this date across the state, and in almost all river basins,” Zukiewicz said. “At some point in the spring, I stopped getting excited by the continued snowfall, and started worrying about it.”
The NRCS forecasts long-duration streamflows for water users across the state, and the May 1 – July 31 forecasts reflect the well-above normal snowpack across the state and are above to well-above average in almost all river basins. Even though the agency forecasts long-term volumes, it works closely with cooperators in the National Weather Service, Bureau of Reclamation, and Montana Department of Natural Resources and Conservation to provide information to them on current snowpack conditions and forecasted volumes in order to manage reservoirs and issue information to the public regarding the coming runoff.
Water managers in the state have been actively managing reservoirs in anticipation of the well-above-average runoff in some basins, and reservoirs in some systems had been dropped to near historic levels before runoff began in mid-to-late April. Since late January the Bureau of Reclamation has dropped Hungry Horse Reservoir in the Flathead River basin nearly 70 feet, and in the Sun River basin, Gibson Reservoir was dropped to close to a record low on April 1.
“Active management in the river systems with reservoirs this year has been great to see with the huge snowpack we have, but water users on non-controlled streams and rivers will be at the mercy of the weather this year with regards to snowmelt runoff,” Zukiewicz said.
Active snowmelt runoff began mid-month across the state, with low-elevation melt causing substantial increases in river and stream flows. “In many cases, the large increases in flows were from elevations below what we measure with our current monitoring network,” he said. “Valley and plains snow this year was abundant due to the below normal temperatures. When it and the low elevation mountain snowpack started to melt, it resulted in quick increases in flows in many rivers and streams.” Snowpack remains well-above normal for this date at almost all water yielding elevations, meaning that the bulk of the snow water remains to enter the rivers and streams.
The official National Weather Service flood potential forecasts across the state indicate there is significant potential for flooding along some rivers and streams this spring and summer, something that is already occurring. “Ultimately, we’ll have to wait and see how fast the snowpack comes out this year, but the potential is there for big flows to occur with the amount of snow still left in the mountains,” Zukiewicz said. “A long period of sunny days with above average temperatures or a rain-on-snow event would be a game changer. A close eye should be kept on the weather forecasts this spring and summer, especially if you live in a low-lying area near a river or stream.”
Current snowpack conditions and long-term streamflow forecasts can be found in this month’s NRCS Montana Water Supply Outlook Report, which can be found at the website below after the 5th business day of the month: