Block (meteorology)

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.

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 6 October.

However, when a blocking high is situated in the Tasman Sea it can cause torrential rains in eastern Australia, as in the cases of the 2021 and 2022 flood events.

[15] In Australia, blocking highs generally occur in the Great Australian Bight and the Tasman Sea, which are powerful high-pressure systems that usually develop further south than normal.

They stay virtually unmoving for a lengthy period (i.e. several days to weeks) and thus block the regular easterly motion of weather systems across southern Australia.

In Northern and Western Europe, cold winters such as 1683/84, 1739/40, 1794/95, 1829/30, 1894/95, 1916/17, 1941/42, February 1947 and 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.

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.

Heat waves in summer are the result of similar blocking patterns, typically involving the placement of the semi-permanent subtropical ridge.

In many cases such as the 1999 US drought, the heat wave was preceded by prior months of below normal precipitation that prevented temperatures from cooling.