Prolonged December dry spell decreases early season snowpack totals

Posted at 11:54 AM, Jan 07, 2021
and last updated 2021-01-07 13:54:01-05

Entering the 2021 Water Year on October 1 there was a great deal of excitement about the possible “La Nina” winter forecasted for the fall and winter seasons. What would this mean for Montana’s winter air temperatures and one of Montana’s most precious resources, the snowpack?

The Natural Resources Conservation Service in Bozeman said in a news release that October delivered the predicted outcome, well below normal temperatures statewide, and above average precipitation for most areas of Montana. Only the eastern border and southwest corner of the state would be missed by passing storms. Ending the month, snowpack was off to a strong start in most mountain locations.

Weather patterns changed in November. The northern half of the state was favored for precipitation while the southern half of the state experienced below normal monthly totals. November air temperature was reported as near to slightly above average for many locations in the western half of Montana, but well above average in the southeast corner of the state.

“Unfortunately, as we have seen in previous La Nina years in Montana, a forecasted La Nina winter isn’t a guarantee of cold and wet conditions during every month of the snow season, it’s only an increased probability of that occurring over a given period of time,” reported Lucas Zukiewicz, NRCS water supply specialist in Montana.

The last week of November marked the beginning of a prolonged dry period for almost all mountain locations, with many mountain Snowpack Telemetry (SNOTEL) sites receiving little snowfall between November 20 and December 12. This prolonged dry period caused snowpack percentages to decline across the state, especially at low- to mid-elevation mountain locations. “The weather over the final two weeks of December was more active, and storms before the new year began helped to build the mountain snowpack and stop the decline in percentages,” said Zukiewicz.

Snowpack on January 1 varies widely across Montana. Snowpack in some river basins along the Rocky Mountain Front is above normal for this date while snowpack is near to slightly below normal in other western Montana river basins. “Southwest Montana, which didn’t receive the early boost in snowpack totals during October, is the only region in the state with snowpack that is well below normal for this date,” stated Zukiewicz. Snowpack in the Gallatin, Madison and Jefferson River basins ranges from 66 to 87 percent of normal.

On January 1, around 35 to 45 percent of the seasonal peak snowpack has typically accumulated at mountain locations across the state, leaving plenty of time for snow totals to recover before runoff begins this spring and summer. “You don’t have to look far back in time to find a winter where early season snowpack totals weren’t looking good in certain parts of the state,” said Zukiewicz. “Just last winter, snowpack totals in many river basins in western Montana along the Idaho border were below normal, only to have the weather patterns change and improve conditions before we got to runoff.”

According to forecasts published January 6 by NOAA’s Climate Prediction Center, the next two weeks are predicted to bring higher probabilities of above normal precipitation to many parts on Montana and the warmer than normal temperatures are likely to persist. “At this point we’ll take what precipitation we can get, especially in southwestern Montana,” said Zukiewicz.

The NRCS will continue to monitor conditions across the state, and a full report of conditions for January 1 can be found in the monthly Water Supply Outlook Report available on the Montana Snow Survey website on Friday, January 8.

Monthly Water Supply Outlook Reports can be found on the NRCS website after the fifth business day of the month.