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First Water Supply Forecast for 2021

We've all been paying attention to the lagging snowpack levels in the upper Madison basin for this winter. After a record setting February for snow, the upper Madison basin is now sitting slightly more comfortably at 90% of normal. Here's the new snow water equivalent graph which puts things in perspective compared to previous years.

2021 is the black line. Green line is "normal." At this point, we've got about 50 days left until average date "peak" snow water equivalent is reached.


March is the first month of each water year that NRCS offers their water supply forecast. It can be found HERE.


While the current level of the mountain snowpack “reservoir” is known at this time, the next few months will be huge in dictating the upcoming spring and summer runoff (future snowfall, summer precipitation, temperatures, etc.). Because of this, the forecasts are presented as a range of outcomes from the 10% exceedance (wet outcome- occurs 10% of the time) through the 90% exceedance (dry outcome - occurs 90% of the time). Looking at the range of forecasts for the areas where median (50% exceedance) forecasts are below average on March 1st, the range of outcomes indicates a chance that if wet patterns continue, near to above-average flows are still possible, though less likely.


Sometimes these graphs are confusing and a little difficult to grasp what exactly they're trying to tell us. So hopefully this helps. First, here's the reference for forecast ranges which provides a visual alternative to a table format. The forecast range is represented by a colored bar. Vertical lines on the bar signify the five forecast exceedances.


Here's an example of how to interpret them:



The numbers above the forecast bars are the five exceedance probability volumes in thousand acre-feet (KAF). I like to toggle the units from KAF to Percent of Normal, which you'll see in the image below. You can play around with the various units HERE. Each exceedance forecast’s percent of average can be estimated by looking at the horizontal axis. The gray line centered above 100% on the horizontal axis represents the 1981-2010 historical average streamflow for the forecast period. In this example, almost all the forecast bars in the basin are shifted right of the gray vertical line indicating forecasts of above average streamflow. The 50% exceedance is represented by the black line in the green portion of the colored bar. For the top most line, this represents a forecast volume of 490KAF, which is ~123% of average. If drier than normal future conditions occur the 70% exceedance forecast may be more likely (455KAF or ~114% of average). If future conditions turn wetter than normal, the 30% exceedance forecast may be more likely (525KAF or ~132% of average).


So, with that in mind, here's where the report is forecasting water supply for the Madison this season:

As you can see, the forecast is currently calling for below normal water supply, but we're nowhere near the bad spot we were back in January, thanks to the God of Snow, Ullr, giving us a big February. To illustrate just how important all that snow in February was to our rivers, in some basins February snow accounts for nearly 70% of total snowpack! Crazy.


So, although reaching 100% of normal in the Madison basin is unlikely, there's still time for gains to be made (about 50 days, according to median peak date). The report says that medium and long-range forecasts issued by NOAA's Climate Prediction Center beyond the first week of March indicate a return to more seasonal weather from March 7 -11, with increased chances of precipitation in southwest Montana. Longer range forecasts, issued through mid-month, indicate increased possibilities of below normal temperatures and increased chances for above-normal precipitation across the state's central and southern portions.


Here's NOAA's most recent 3 month forecast for temperature and precipitation. Southwest Montana is sitting comfortably in the "normal" range for both.

Anyway, that's a lot of graphs and numbers to basically tell us we're below normal for water supply right now, but it could be worse and there's still time to keep that supply trending in the right direction.