LONG RANGE FORECASTING: A seven-day forecast can accurately predict the weather about 80 percent of the time and a five-day forecast can accurately predict the weather approximately 90 percent of the time. However, a 10-day—or longer—forecast is only right about half the time. Meteorologists use computer programs called weather models to make forecasts.
Since we can’t collect data from the future, models have to use estimates and assumptions to predict future weather. The atmosphere is changing all the time, so those estimates are less reliable the further you get into the future.
Meteorologist Brittney Merlot explains exactly why 10 days out or further, forecasts can change drastically. Especially when it comes to forecasting a snow storm, it's like the game of Plinko.