Hotter-than-normal May spurs rapid snowmelt

Springtime conditions across Montana felt almost summerlike with warmer than normal May temperatures spurring rapid snowmelt.

Snowpack across Montana is around 50%-70% of median, well below normal for June 1. This is a drastic drop from May 1 where snowpack was largely 75%-110% of median.

The Flathead basin dropped from 86% of median on May 1 to 59% on June 1, with the month of May delivering 73% of the 20-year median precipitation.

Snowpack in the Bitterroot basin melted out especially rapidly, decreasing from around 75% of median on May 1 to around 25% of median on June 1. The Rocky Mountain Front snowpack remains well below normal, with all SNOTEL sites in the Sun-Teton-Maris basin melted out.

SNOTEL sites across Montana reached peak Snow Water Equivalent (SWE) accumulation from late March to early May.

“The timing of peak SWE varied across the state from near normal to a few weeks early,” said Florence Miller, USDA Natural Resources Conservation Service (NRCS) Hydrologist. “Most SNOTELs accumulated between 70%-120% of normal peak snowpack. Warmer temperatures in May caused the snowpack to melt quicker than normal, which subsequently reduced snowpack percentages drastically.”

May precipitation varied across the state and fell mostly as rain. Only higher elevation SNOTELs recorded measurable snowfall. Northwest Montana had below normal May precipitation of around 55%-75% of median, and below normal year-to-date precipitation, recording 70%-90% of normal conditions for this time of year.

Currently 82% of the state is experiencing drought, with much of the Rocky Mountain Front in category D2 Severe Drought, with very low soil moisture levels in the 0-2 percentile.

May streamflow matched snowpack and precipitation patterns.

“Fast snowmelt and earlier than normal runoff in the month of May rapidly decreased snowpack statewide. This potentially corresponds to less available water later in the summer,” said Miller. She noted that water supply forecasts decreased from 70%-110% of normal streamflow for most of the state on May 1, to 50%-100% of streamflow forecasted on June 1.

The Mission and Swan Ranges are the most likely to experience near normal stream flows with forecasts ranging from 89%-95% of normal. However, the Flathead River itself, near Polson, is only expected to have 80% of median streamflow due to low precipitation and snowpack totals along the headwaters of the North and Middle Forks of the Flathead.

Warmer than normal temperatures in May meant faster than normal melting. The month began with a warm spell, and average temperatures at Montana SNOTELs peaking around 12 °F above normal. The strongest heat wave of the month arrived around May 10, when SNOTEL sites averaged around 17 °F above normal, setting a daily record by 5 °F.

From May 13-23 temperatures hovered 5-8 °F degrees below normal while an extended period of active weather swept through the region, allowing for some snow accumulation at many higher elevation SNOTEL sites.

Temperatures slowly climbed the last week of May across all basins. SNOTEL sites in the Kootenai and Flathead basins edged above daily records on May 29 and May 31, with many lower elevation SNOTEL sites like Kraft Creek reaching above 80 °F degrees for the first time this season.

Above normal temperatures during June are likely across the state, according to the NOAA Climate Prediction Center. There is also a strong probability that total June precipitation will be below normal across Montana, with the western border having the highest chance of below normal precipitation.

A full report of conditions on June 1 can be found in the monthly Water Supply Outlook Report available on the Montana Snow Survey website. In addition, real-time snow survey data can be found at nrcs.usda.gov/montana/snow-survey.