A: When April 1 falls on a Monday.

That’s the conclusion of a clever bit of work by Tom Pagano, former NRCS forecaster who used to do the Rio Grande forecasts before he went on to bigger and better things. “Bigger and better” has included a stint in Australia and a current world tour of the water forecasting world that has made for some delightful blogging. Tom’s a clever forecaster always looking for ways to improve the product (he makes an appearance in my book if you want to know more about how snow is measured and how that then translates into a river forecast) and his peripatetic existence apparently also includes time for some clever science.

For a new paper, he analyzed the day of the week for some 300,000 snow measurements and found that in recent years, the snow measurement folks have been increasingly likely to go out early and take their readings if the normal measurement date falls on a weekend. That could mean that the “April 1 forecast” is based on an underestimate of the amount of snow on the ground because of a bias toward missing end-of-the-month storms if they happen to hit on a weekend. He’s got a nice blog post explaining the result:

Some years it doesn’t matter, it all just depends on the weather around the first of the month. When you average it over many stations and many years, collecting the data 3 days early means that February 1st measurements are roughly biased 5% of normal too low, and May 1st measurements are about 5% too high.