Abstract: Two fundamental questions for atmospheric scientists are how predictable our day-to-day weather is and what determines the limit of predictability. Here we first derived a simple analytical error model for an idealized atmosphere that has a -3 kinetic energy spectrum slope at the synoptic scales and a -5/3 slope at the smaller scales. This model is demonstrated to explain well the error growth characteristics of mid-latitude weather simulated by a state-of-the-art operational global prediction model that has mean energy spectra slopes at respective scales similar to the idealized atmosphere. This theoretical framework elucidates the joint effects of large-scale baroclinicity and small-scale convective instability in dictating the error growth and saturation and ultimately limiting multiscale atmospheric predictability.