Record warm temperatures in April when the last of winter’s snow should have been falling mean Montana’s streamflows will drop early again this year, according to the Natural Resources Conservation Service.
The May Water Supply Outlook is usually the best predictor of how Montana’s snow pack looks heading into the summer, and despite hopes in early April, the outlook doesn’t look good.
Heading into April, few snowstorms had added to the snowpack, but rains had helped to increase the water content of the existing snow, which was a plus. The moisture in the snowpack was close to average in many mountain ranges. But then April hit.
Only one mid-month storm added to the snowpack, although it dropped less snow west of the Continental Divide.
Meanwhile, days of record warmth where the nights barely reached freezing caused substantial snow melt over the month, reducing snowpack in some areas by as much as a third.
With the melt and associated runoff coming earlier than normal this year, a number of sites west of the Divide measured record snowmelt for the month of April. On average, Montana lost a quarter of its snow moisture during April.
As a result, only the Lower Yellowstone basin still has average moisture remaining in the snowpack of the surrounding mountains. But average in eastern Montana is still not that much. The Smith-Judith-Musselshell and the Gallatin basins have close to 90 percent of average, and the rest of Montana drops off from there.
Most of the Columbia River basin has only three-fourths of its average moisture with the Kootenai River basin the worst off with only 68 percent.
But the worst basin by far is the Sun-Teton-Marias along the northern Rocky Mountain Front, which retains only a third of its average moisture. That’s why the National Interagency Fire Center has identified the area as a potential hot spot for wildfires this year.
Low snowpack and soils and vegetation that have become dried out over successive years create a potential tinderbox for any lightning strike or smoldering campfire.
With low snow in the mountains, little water will remain to trickle down over time to replenish Montana’s streams as the summer heat kicks in. Some rivers in northwestern Montana have already had their peak flows due to snowmelt. That means streams will depend mainly on groundwater and what few rainstorms pass through.
So it could be a tough year for trout and irrigation needs. It’s likely that Montana Fish, Wildlife & Parks will have to put fishing restrictions in place early as low streams heat up and senior water rights holders may have to make some water calls.
The NRCS is predicting that streamflows will be about 80 percent of average statewide through July. The lower Yellowstone and Gallatin should be the least affected, running at around 90 percent of average. But streams along the Rocky Mountain Front are predicted to have only half of their normal flows.
The catch is that predictions are based upon average weather trends and this year has already been anything but average. Early snowmelt runoff will occur if temperatures continue to be above average, and spring and summer precipitation will be key unless there is a major pattern change, according to the NRCS.
However, the National Weather Service long-range forecast shows a change isn’t likely. For the next three months, Montana is predicted to have above-normal temperatures, with the likelihood increasing toward the northwest corner of the state. Precipitation is predicted to be normal with a chance that the most southwestern slice of the state around Dillon might have slightly above normal precipitation.
Fortunately, most of the reservoirs contain an average amount or better of water, but all are below full capacity. Reservoirs will continue to be depleted if Montana continues to experience warm, dry winters.