Despite the “horoscope” forecasts on Accuweather, most meteorologists will tell you that 7 days is about the extent of a reliable weather forecast. We can give you (at times, especially in the warm season) an idea of above or below normal out to maybe ten days but that is really pushing it. Beyond that, your best bet is to rely on the climatological averages (or add a degree due to the strong climate warming in higher latitudes). This is now been confirmed by Falko Judt at the National Center for Atmospheric Research in Boulder. Judt used their MPAS model and performed a unique experiment.

David Hosansky at NCAR describes it:

“To examine the limits of predictability, Judt ran two nearly identical simulations with the MPAS model. The first was a 20-day control simulation that began with actual observations on a day in late October. For the purpose of the study, Judt assumed that this simulation perfectly described the weather for that 20-day period.

Then he ran an alternative 20-day simulation that began with starting conditions that differed from the control simulation by only the tiniest amount — on the order of a thousandth of a degree — a change too small to be detected by a standard weather station. This second simulation played the role of a forecast for the idealized experiment.

Judt was interested in how closely the “forecast” with nearly perfect starting conditions (the second simulation) could track the “actual weather” (the first simulation).

Both simulations, run on the NCAR-Wyoming Supercomputing Center, had a highly detailed resolution of 4 kilometers (about 2.5 miles).

For the first six days, the simulations tracked closely. After that, the “forecast” simulation began to deviate increasingly from the control simulation. Driving the deviation were small-scale events, such as thunderstorms, which over time began influencing larger-scale, regional atmospheric patterns until, after 17 days, the weather patterns in the two model simulations were no longer linked in a predictive way.

The paper was published last year in Journal of the Atmospheric Sciences. Judt described it as a thought experiment.

“This helps us make decisions about where to focus our research,” he said. “If we had perfect observations, we could make almost perfect forecasts of the day-to-day weather out to six or seven days. If we want to develop models that will accurately forecast the weather a month into the future, that’s not a realistic goal.”