An omega block shields the low pressure systems with its bad weather. The name comes from the shape of the isobars, that look like the greek letter Ω.
The figure shows a section of the weather map of the Deutscher Wetterdienst (DWD) on July 1, 2015 00 UTC. The omega shape is illustrated.
The omega block at the beginning of July, 2015 provided a heat wave with temperatures up to 40°C in Germany. In this warm air mass heat thunderstorms developed and a slow shift of the omega block to the east creates strong thunderstorms in Germany at the border to the cooler, moist air mass.
Another prominent omega block provided a very summerly soccer world championship 2006 in Germany. In winter a stable high involve coldness.