Omega blocks are a type of upper-level weather pattern. They are so-named because the isobars or geopotential height contours with which they are associated in the Northern Hemisphere resemble an Ω, the uppercase Greek letter omega. They typically have a low-high-low pattern, arranged in the west–east direction. The lows are referred to as "closed" or "cut-off" lows which are on either side of the omega high.
The atmosphere in midlatitudes is dominated by prevailing west to east winds that usually move weather systems consistently in that direction. However, at times the flow takes on a more north/south component with waves in the upper atmosphere. Blocks in meteorology are large-scale patterns in the mid to supper atmospheric pressure field that are nearly stationary, effectively "blocking" or redirecting migratory cyclones and their associated fronts. They are also known as blocking highs or blocking anticyclones. These blocks can remain in place for several days or even weeks, causing the areas affected by them to have the same kind of weather for an extended period of time (e.g. precipitation for some areas around the lows, clear skies for others with the high). In the Northern Hemisphere, extended blocking occurs most frequently in the spring over the eastern Pacific and Atlantic Oceans. Whilst these events are linked to the occurrence of extreme weather events such as heat domes, the onset and decay of these events is still not well understood.
500mb geopotential height forecast by US numerical weather prediction model NAM for 14 May 2006, showing omega block over western North America. Base image source NOAA.