Block (meteorology): Difference between revisions

When an upper-level [[Ridge (meteorology)|high]]- or [[Trough (meteorology)|low]]-pressure system becomes stuck in place due to a lack of [[steering current]]s, it is known as being "cut off". The usual pattern which leads to this is the [[jet stream]] retreating poleward, leaving the then cut-off system behind.<ref name="Atmospheric Blocking">[http://www.theweatherprediction.com/blocking/ Atmospheric Blocking<!-- Bot generated title -->]</ref> Whether or not the system is of high- or low-pressure variety dictates the weather that the block causes. Precisely this situation occurred over the southern [[United States]] during late spring and early summer of 2007, when a cut-off-low system hovering over the region brought unusually cool temperatures and an extraordinary amount of rain to [[Texas]] and [[Oklahoma]] (see [[June 2007 Texas flooding]]), and a cut-off-high near the coast of Georgia that caused a drought in the Southeast that same year. Rainy, cooler weather results if the block is a low in the US. [[Hurricane Ian]] in the last week of September 2022 drifted northward and its remnants became detached from the jet stream, resulting in a stationary low pressure system spinning off the Northeastern US and bringing several days of precipitation until a front finally moved through on October 6.<ref name="Atmospheric Blocking"/>

Such cold winters over the contiguous United States and southern Canada as [[1912 United States cold wave|1911/12]], [[1936 North American cold wave|1935/36]], 1949/50, [[cold wave of 1978|1977/78]] and 1978/79, 1993/94, and 2017/18 resulted from blocks in the [[Gulf of Alaska]] or to the east of the [[Mackenzie Mountains]] directing very cold Arctic air with a long trajectory as far as the [[southern United States|American South]],<ref>Carrera, M.L.; Higgins, R.W. and Kousky, V.E.; ‘[https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-3237.1 Downstream Weather Impacts Associated with Atmospheric Blocking over the Northeast Pacific]’; ''Journal of Climate'', 17 (2004), pp. 4823–4839</ref> as did the Western cold waves of 1889/90 and January 1950. In [[northern Europe|Northern]] and [[Western Europe]], cold winters such as 1683/84, 1739/40, 1794/95, 1829/30, [[winter of 1894–95 in the United Kingdom|1894/95]], 1916/17, 1941/42, February 1947 and [[winter of 1962–63 in the United Kingdom|1962/63]] are almost always associated with high- latitude Atlantic blocking and an equatorward shift of the polar jet stream to [[Portugal]] and even [[Morocco]].<ref name="Seesaw"/> Over [[Central Asia]], unusually cold winters like 1899/1900, 1929/30 and 1930/31, 1944/45, 1954/55 and [[winter of 1968-69 in Central Asia|1968/69]]<ref>Hirschi, Joël J.-M. and Sinha, Bablu; ‘Negative NAO and cold Eurasian winters: how exceptional was the winter of 1962/1963?’; ''[[Weather (journal)|Weather]]'' 62 (2007); pp. 43–48</ref> are associated with blocking near the [[Ural Mountains]] extending the [[Siberian High]] westwards to push the very cold air from the Siberian “[[cold pole]]” outward towards the [[Aral Sea|Aral]] and [[Caspian Sea]]s. Unlike other midlatitude regions of the Northern Hemisphere, however, cold winters in Europe (e.g. 1916/17, 1962/63) are often very mild over Central Asia, which can gain warm air advection from subtropical cyclones pushed to the south under negative NAO conditions.