A European windstorm is a name given to the strongest extratropical cyclones which occur across the North Atlantic Ocean and continental Europe. They form as areas of low atmospheric pressure, sometimes starting as nor'easters off the New England coastline, most windstorms strengthen when they encounter the Jet Stream this also causes them to track across the North Atlantic Ocean towards western Europe. They are most common in the autumn and winter months. On average, the month when most windstorms form is January.
Extratropical cyclones are generally driven, or "steered", by deep westerly winds in a general west to east motion across both the Northern and Southern hemispheres of the Earth.
Changes in direction of this nature are most commonly observed as a result of a cyclone's interaction with other low pressure systems, troughs, ridges, or with anticyclones. A strong and stationary anticyclone can effectively block the path of an extratropical cyclone.
Such blocking patterns are quite normal, and will generally result in a weakening of the cyclone, the weakening of the anticyclone, a diversion of the cyclone towards the anticyclone's periphery, or a combination of all three to some extent depending on the precise conditions. It is also common for an extratropical cyclone to strengthen as the blocking anticyclone or ridge weakens in these circumstances.
The state of the North Atlantic Oscillation relates strongly to the frequency, intensity, and tracks of European windstorms. An enhanced number of storms have been noted over the North Atlantic/European region during positive NAO phases (compared to negative NAO phases) and is due to larger areas of suitable growth conditions. The occurrence of extreme North Atlantic cyclones is aligned with the NAO state during the cyclones' development