Early season weather patterns hit or miss across Montana

After an early-season storm dropped snow in Montana’s high country at the end of September, it seemed like the new 2020 water year was off to a strong start. The wet and cold weather patterns that dominated October boosted snowpack and precipitation totals in many basins east of the Divide by early November, with some mountain SNOTEL (SNOwpack TELemetry) sites reporting the first or second most snow water equivalent contained within the snowpack for that date.

“While we got off to a great start early in the year, the major change in weather patterns during November weren’t kind to the western half of the state with regards to precipitation or snowfall. While the eastern half of the state received above normal precipitation, the mountains were largely left high and dry,” said Lucas Zukiewicz, USDA Natural Resources Conservation Service (NRCS) water supply specialist for Montana. Precipitation totals west of the Divide were well below normal, ranging from 29 to 56 percent of normal at mountain locations while basins east of the Divide ranged from 46 to 76 percent of normal. Precipitation totals along the Idaho border were particularly low, with many mountain SNOTEL sites reporting their first or second lowest November precipitation on record.

The large-scale weather patterns over western Montana remained the same throughout most of December, but unlike the previous month, December resulted in a lack of precipitation statewide. “The first three weeks of the month were very dry in most locations across the state, but more favorable weather patterns began to emerge during final week,” said Zukiewicz.

The last ten days of December featured an “atmospheric river” pointed squarely at the northwest corner of the state that provided abundant moisture at the SNOTEL sites along the Canadian border in northwest Montana and interior British Columbia. The weather patterns through the end of the month remained active, dropping significant amounts of snowfall at mountain locations across western Montana.

Water year precipitation totals, which began on October 1, reflect the lack of November and early December precipitation. Water year deficits are the largest west of the Divide, where water year totals range from 66 to 84 percent of average. East of the Divide, totals are slightly better with water year precipitation ranging from 71 to 100 percent of normal.

Snowpack on January 1, which looks at the accumulated snowpack on the ground and not the total amount of precipitation, ranges widely across the state. Snowpack in river basins west of the Divide ranges from well below normal in the Lower Clark Fork to near to above normal at high elevations in the Flathead River basin. Mountain snowpack east of the Divide received an early boost in September and October and now ranges from below normal in southwest Montana to above normal as you move north along the Divide.

While snowpack totals may look grim in some regions on January 1st, there is time for improvement, and seven- to ten-day weather forecasts indicate continued wet weather across the state.

“The January 1 snowpack totals are more like a progress report than a report card. Only 35 to 45 percent of the seasonal snowpack has typically accumulated at this time, meaning there is plenty of time for snowpack to make improvements by the time the final report card comes out in April,” according to Zukiewicz. “That said, as winter progresses, a close eye will be kept of the basins west of the Divide where snowpack and water year deficits are the largest.”