BLOCKED PATTERN

Large scale obstruction of the normal west to east progression of surface cyclones in the mid-latitudes. The upper flow changes from predominantly zonal (q.v.) to meridional (q.v.). In a 'meridional' block, the upper flow divides upwind of the block and flows around quasi-stationary vortices - one anticyclonic and the other cyclonic. In the 'omega' block case the strongest flow is diverted to lower latitudes, leaving a slow-moving anticyclonic vortex on the poleward flank of the displaced zonal flow.