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US. GIoball CI °Harm^ eseairm::[° IPirogirarn 44 along the downwind coasts of the Great Lakes have increased. Factors contributing to this increase include reduced ice cover due to warming, which lengthens the period of open water. In addition, cold air moving over rela- tively warm, open lake water induces strong evaporation, often causing heavy lake - effect snow. Heavy snowfall and snowstorm fre- quency have increased in many northern parts of the United States. In the South however, where temperatures are already marginal for heavy snowfall, climate warming has led to a reduction in heavy snowfall and snowstorm frequency. These trends suggest a northward shift in snowstorm occurrence .61 Global Climate Change Impacts in the United States Increases in the Number of Days with Very Heavy Precipitation (1958 to 2007) iinurea,ses in Annual Number of Day III °�IIIoo(1111 II�II�1111 ( "111111ro��111�'llhit� �u�llr(n Illlillllr��nlll to �I coIllmrm Illmrmo11 coII "mrmlllmrmollrll airld1 Illmrmo11 0 10% 11-20% 21-30% 31 .40,x, 41 - %)'% 51-60% firlt(:NIIIS(r 'as rf ^Ir)('IIIoIIII "III�1 airld crasoirlai U..)lsclatecl furorrr Guroir.•,rrrari e�t crl,. 1''' rll�'fIkrf��llll '� Ilit�� t��lll�� IIII "III att(l�Ilrlllrlll (�:111allll"III)((� , �Illrllld" The map shows the percentage increases in the average number r�lllllll "III'�� fP��rf��l��IIP " "rifP��r IIP " "ri��IlrfP��r f��l��IIII "Illf��lf�P��rllll "III ��IIr��f�P��rr "a of days with very heavy precipitation (defined as the heaviest I percent of all events) from 1958 to 2007 for each region. There IIIII"IIIto I'7iitlr tlr tlNrlllt IIItlii loIllrlll)('t'rlr, are clear trends toward more days with very heavy precipitation l'fott(PNlr fair p(PNlrlllor "ally IIIIIII "Ill (PA (PNPNlll"Ill ). for the nation as a whole, and particularly in the Northeast and Midwest. While it sounds counterintuitive, a warmer world produces both wetter and drier conditions. Even though total global precipitation increases, the regional and seasonal distribution of precipitation changes, and more precipitation comes in heavier rains (which can cause flooding) rather than light events. In the past century, averaged over the United States, total precipitation has increased by about 7 percent, while the heaviest 1 percent of rain events increased by nearly 20 percent.68 This has been especially noteworthy in the Northeast, where the annual number of days with very heavy precipi- tation has increased most in the past 50 years, as shown in the adjacent figure. Flooding often occurs when heavy precipitation persists for weeks to months in large river basins. Such extended periods of heavy precipitation have also been increasing over the past century, most notably in the past two to three decades in the United States .112 Observations also show that over the past several decades, extended dry periods have become more frequent in parts of the United States, especially the Southwest and the eastern United States .146,147 Longer periods between rainfalls, combined with higher air temperatures, dry out soils and vegeta- tion, causing drought. For the future, precipitation intensity is projected to increase everywhere, with the largest increases occurring in areas in which average precipitation increases the most. For example, the Midwest and Northeast, where total precipitation is expected to increase the most, would also experience the largest increases in heavy precipitation events. The number of dry days between precipitation events is also projected to increase, especially in the more and areas. Mid - continental areas and the Southwest are particularly threatened by future drought. The magnitude of the projected changes in extremes is expected to be greater than changes in averages, and hence detectable sooner .49,61,90,142,148