[lllr_finger]

As a follow-up, if you're wondering why, 500mb is roughly 5700m/18kft above ground level in the US - not quite jet stream level (200-300mb) - but high enough to give insight into large scale patterns that drive surface patterns.

Where surface features are more localized and near-term, 500mb analysis gives you insight into whether there is broad zonal flow, a deepening upper trough, and all sorts of important information about how things on a larger scale can play out in the longer term. They're sometimes called "steering winds", because they're the primary component that dictates direction and speed of storms.