Temperature forecasting is not as easy as it might look. The forecaster has to estimate where the air that will be over a location tomorrow is today, and then estimate how that air will be changed. Will it be heated by compression or cooled by clouds? Will it move over soil that is excessively wet or very dry, or will it move from a forested region to cropland? In winter, will it move over bare ground or deep snow? Will the air be mixed up vertically so conditions up in the atmosphere are mixed down to near the ground?
Of course, these things become harder and harder to estimate for the second, third, fourth day, and so on. Computer models are a big help, but they have their limitations. Despite the ability to do almost countless calculations in a short amount of time, these calculations are performed on a modeled, or simplified, system representing our world.