Temperatures near normal. Meridional flow - ridges and troughs are well developed. Pronounced temperature anomalies. G. Upper-level pattern and surface precipitation forecast. 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.