"Within the sector lying between 120° W and 60° E longitude and at the 500 mb level, the normal circulation pattern may be described as a relatively broad westerly current (jet) […] A particular longitudinal variation in this circulation pattern frequently appears on the synoptic chart irrespective of season. This anomaly expresses itself as follows: the flow over the North American continent and western Atlantic, which is an essentially zonal motion at a relatively high energy level, subsequently “breaks down” over the European continent or eastern Atlantic into a more cellular motion at a lower energy level. […]
A usual feature […] is the formation of a quasi-stationary warm ridge or anti-cyclone just downstream from the point of “breakdown”. […] Situations, showing the pronounced development of such a quasi-stationary anticyclone were first noted by GARRIOTT (1904), and are now commonly referred to as examples of “blocking action” […]" - classical definition by Rex (1950)
Here in Europe a lot of our synoptic-scale (large-scale, ca. 1000 km) weather is influenced by westerly (from west to east) flow. Alongside the so called storm tracks, cyclones (low-pressure systems) move into Europe and bring weather fronts with them. During a blocking event in the classical sense the cyclones are blocked from moving eastwards by a persistent anti-cyclone (high-pressure system). In Europe blocking is preferentially located west of the British Isles over the north-eastern Atlantic or over Scandinavia. Different subtypes of blocks can be distinguished depending on the relation of the high and low pressure fields. A blocking high just to the north of a low pressure system is termed Rex block. So called Omega blocks consist of a high pressure system in the north and two low pressures systems on the southern flanks. In the pressure surface they look similar to the Greek letter Ω.
These arrangements of highs and lows influence the weather in Europe quite significantly by blocking the climatological flow for up to several weeks. These stationary conditions are very favorable for the development of all kinds of extremes which can severely affect weather and thus society. Pfahl and Wernli (2012) showed that up to 80% of summer hot temperature extremes in northern Europe are associated with blocking. Buehler et al. (2011) connected winter cold spells with blocking. More specifically single extreme events like the Russian heatwave (Matsueda 2011), the severe flooding in Pakistan (Galarneau at al. 2010), and the cold European winter all in 2010 (Cattiaux et al. 2010) have been connected to blocking.
Rex, D. F., 1950: Blocking Action in the Middle Troposphere and its Effect upon Regional Climate I: An aerological study of blocking action, Tellus, 2, 196–211, doi:10.1111/j.2153-3490.1950.tb00331.x.
Pfahl, S., and H. Wernli, 2012: Quantifying the relevance of atmospheric blocking for co-located temperature extremes in the Northern Hemisphere on (sub-)daily time scales. Geophys. Res. Lett., 39 (12), doi:10.1029/2012GL052261.
Buehler, T., C. C. Raible, and T. F. Stocker, 2011: The relationship of winter season North Atlantic blocking frequencies to extreme cold or dry spells in the ERA-40. Tellus A, 63 (2), 212–222, doi:10.1111/j.1600-0870.2010.00492.x.
Matsueda, M., 2011: Predictability of Euro-Russian blocking in summer of 2010. Geophys. Res. Lett., 38 (6), L06801, doi:10.1029/2010GL046557.
Galarneau Jr., T. J., Hamill, T. M., Dole, R. M., and Perlwitz, J., 2012: A Multiscale Analysis of the Extreme Weather Events over Western Russia and Northern Pakistan during July 2010, Mon. Wea. Rev., 140, 1639–1664, doi:10.1175/MWR-D-11-00191.1.
Cattiaux, J., R. Vautard, C. Cassou, P. Yiou, V. Masson-Delmotte, and F. Codron, 2010: Winter 2010 in Europe: A cold extreme in a warming climate. Geophys. Res. Lett., 37 (20), L20704, doi:10.1029/2010GL044613.
Pfahl, S., C. Schwierz, M. Croci-Maspoli, C. M. Grams, and H. Wernli, 2015: Importance of latent heat release in ascending air streams for atmospheric blocking. Nature Geoscience, 8, 610–614, doi:10.1038/ngeo2487.
Cassuo, C., and J. Cattiaux, 2016: Disruption of the European climate seasonal clock in a warming world. Nature Climate Change, doi:10.1038/nclimate2969.