Force fields are "made up" explanations for why a certain meteorological event does not occur in a particular region. This force field can be as small as a city to as large as a U.S. state or bigger. Examples include regions: within tornado alley that have never seen a tornado, the lack of precipitation over a region during a rain event when all surrounding regions did get rain, the rapid dying of a supercell as it approaches a region, a prog which shows light precipitation over a region surrounded with heavier precipitation, and a lack or only a small amount of snow over a region when surrounding regions got much more.
A bullseye of a minimum value on a weather prog or precipitation chart is sometimes referred to as a force field. Of course a logical meteorological explanation can be given in most of these circumstances. Some explanations include: the law of probability, a lack of low level convergence over a region compared to surrounding regions, the robbing of moisture from an upstream MCS, local ridging, a relative minimum in UVV mechanisms, locally strong cap, a dry slot, a locally significant negative UVV mechanism (NVA, CAA, downslope flow, convergence aloft, low level divergence).
Jokingly, these relative minimums in precipitation, snowfall, etc. are attributed to force fields. Stories include aliens setting up a force field in the atmosphere and a secret government precipitation diminishment project.