How early is too early to trust a snow forecast?

DES MOINES, Iowa – It's that time of year again — viral social media posts showing apocalyptic forecasts of multiple *feet* of snow across central Iowa.

Those forecasts are almost always wrong.

So when can you trust a snow forecast? And better yet, who can you trust a forecast from?

First, let's get some context. The latest first trace of snow on record was set on December 2, 1939 in Des Moines. On average, the first trace of snow falls around the first of November. So it's abnormal to not have seen snow yet this year. Normally, the first tenth of an inch of snow falls around the middle of November, and the first inch of snowfall is in late November to early December. Of course, these are averages that are bound to vary from year to year.

Here's where forecasting snowfall more specifically can get complicated. Weather models can forecast 10 days (or more) in advance. However, the accuracy of weather models decreases dramatically past the five-day mark and especially past the seven-day mark. For that reason, model forecasts over a week out can change dramatically within just a few hours — showing feet of snow at one moment and then no snow just a few hours later.

Weather models are made up of math equations using approximations. Because of that, errors multiply over time. A range of outcomes are possible given a set of current conditions.

By nature, weather models have to choose one outcome. Naturally, the farther out in time that outcome is predicted, the less accurate it has the potential to be.

For that reason, trained meteorologists often wait until closer to a weather event to create a forecast. This forecast is more likely to be accurate — closer to the event, the range of possible outcomes is much smaller.

Therefore, it's important to get your weather forecast from a reliable source. Meteorologists in local media, like here at WHO 13, and at the National Weather Service weigh the possible outcomes and the potential a weather model has to be accurate.

Usually, that means waiting until about three to four days from a snow event to create a forecast with numbers. Up until that point, forecasters are able to say there is the potential for snow, but it's difficult to get precise with numbers and impacts.

So if you see a snow map making a prediction more than five days out, take it with a grain of salt. Raw model output is often inaccurate, so trained forecasters are able to balance model output with what is likely to actually happen.

Stay with WHO 13 this winter — we'll keep you up to date with the latest the winter weather brings us!