Acts like the loss of a lane on a freeway. Traffic backs up before the constriction, speeds up after it. Northwesterly flow is often dry. Upper-level winds tend to slow down and converge upwind of a trough. This piling up of air in the upper atmosphere favors subsiding air and surface high pressure.
Southwesterly flow is often wet.
Upper-level winds tend to speed up and diverge downwind of a trough. This induces divergence aloft and reinforces cyclone strength. H. Examples of linkage between upper level flow and surface cyclones. Note surface low embedded in the lee of upper-level trough. Note that cyclones are supported and steered by the jet stream configuration.