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: