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2 Characterization In general, there is no unique definition of an extreme weather event, and the word "extreme" refers either to rareness (a statistical extreme) or to the severity of the resulting damages (a physical extreme). Thus, what is considered extreme de- pends invariably on the local context. A rare event need not be damaging, and damaging events can occur regularly in certain regions at certain times of year—for example, hurricanes in the Caribbean during late summer and early autumn, and heavy precipitation in northern Britain during winter. Often the defini- tion of an extreme event extends beyond weather to other aspects of the environment that do the actual weather-related damage—for example, flooding, dry soils, dry reservoirs, or wildfires. Accordingly, the built environment can play a large role in the extent of the damage. For example, the Jeddah, Saudi Arabia, flash floods of November 25, 2009, led to hun- dreds of deaths, with many people drowning in their cars. The rainfall was excessive by local standards (72 mm within 4 hours, equal to twice the annual average), but would not have caused difficulties in most other places around the world. Some extremes develop rapidly from unstable atmospheric conditions, and are spatially localized. One class of such events is associated with mesoscale weather phenomena (time scales of a few hours, spatial scales of several kilometers), and includes tornadoes, thunderstorms (including hail storms), and rapidly growing wildfires. Another class of such events is associated with synoptic-scale weather phenomena (time scales of a day or longer, spatial scales of 100 km or so), and includes tropical cyclones, extratropical cyclones, and their related effects (heavy precipitation, including heavy snow, windstorms, and storm surges). Other extremes are associated with persistent weather regimes related to the atmospheric circulation, especially to blocking anticyclones, which lead to cold snaps in winter and to heat waves in summer. On still longer time scales, droughts are associated with seasonal variations in weather, generally associated with effects of sea-surface temperature variations. Weather Forecasting One of the main motivations for weather forecasting is to predict extreme weather events before they occur, so as to issue warnings and take precautions to minimize the damage. Prediction of mesoscale weather phenomena is extremely challenging, and generally the predictions are only of meteo- rological conditions considered to be conducive to an event. Prediction of synoptic-scale weather phenomena is quite successful, and normally there will be a warning of such events several days in advance. There is still a probabilistic element to the forecasts, in terms of the exact location and intensity of the most damaging aspects of the extreme weather event, but overall the weather forecasts are physically realistic and accu- rate enough to base action on. Prediction of persistent weather regimes—both their onset and their cessation—has long been a challenge for weather forecasters, although forecast systems are slowly improving in this respect. On the seasonal time scale, most of the demonstrable forecast skill is associated with predictable patterns of sea-surface temperature, such as the El Niño Southern Oscillation phenomenon, but there is a large random element to the forecasts and no single forecast can be considered as right or wrong. Climate Change Climate change can be expected to alter the magnitude and frequency of extreme weather events. Understanding these changes is important for all kinds of reasons, ranging from reinsurance to urban and rural planning. However, the level of scientific understanding varies greatly among the differ- ent event types. This variance largely reflects the difference in relative importance between the thermodynamic (tempera- ture and moisture content, and sea level for storm surges) and dynamic (wind) aspects of the event type. Thermodynamic aspects are related directly to the increase in globally averaged surface temperature, which is understood theoretically, seen in observations, and consistently simulated in climate models. Dynamic aspects are related to changes in the atmospheric circulation and the dynamics of storms, and are much more uncertain, with no consensus on a theoretical understanding, no clear signal in observations (because of the high level of chaotic internal variability, which obscures any climate signal), and disagreement among climate models. The highest level of scientific understanding therefore con- cerns changes in extreme weather event types that are strongly linked to thermodynamic aspects of climate change. These include heat extremes (expected to increase in magnitude and frequency), cold extremes (expected to decrease), extreme precipitation (expected to increase from the higher mois- ture content of warmer air), and high sea levels (expected to increase from the thermal expansion of a warming ocean and the melting of glaciers and ice sheets). Although there could be regional exceptions because of changes in atmospheric circula- tion and the dynamics of weather events—an active topic of research concerns whether cold extremes in northern mid- latitudes might become more frequent—changes in the overall magnitude or frequency of these extreme event types have been observed, attributed to anthropogenic climate change, and are expected to continue. Extreme Weather Event (continued) + ward ' s science