ATMOSPHERIC PROCESSES ASSOCIATED WITH SUMMER FLOODS AND DROUGHTS IN THE CENTRAL UNITED-STATES

Persistent wet and dry events over the central United States are exami ned during summer. Composites. based on selected persistent wet and dr y events reveal common atmospheric processes and circulation features, During summer, heavy precipitation in the central United States is ac companied by less precipitation to the south, in a band that extends f rom the tropical eastern Pacific through the Gulf of Mexico into the w estern Atlantic. Dry conditions also occur along the western coasts of Canada and Mexico during persistent wet episodes in the central Unite d States. This rainfall pattern is supported by an inverse temperature -rainfall relationship over North America. During dry events, high pre ssure extends throughout a vertical column in a pattern that covers No rth America from 30 degrees to 60 degrees N. In contrast, during wet e vents, the high pressure is confined to the eastern half of North Amer ica, with low pressure prevailing in the western half. Increased north ward meridional winds are found between this cyclonic-anticyclonic dip ole, leading to increased moisture nux from the Gulf of Mexico at low levels. A significant precursor to wet events is the enhancement of we sterlies over the eastern Pacific and western North America from 30 de grees to 40 degrees N. Synoptic-scale eddies intensify prior to onset and accelerate this westerly flow as revealed by Eliassen-Palm Bur dia gnostics. One pentad before onset, rainfall begins in Texas, and the l ow level jet (LLJ) in the Great Plains strengthens, The intensified LL J transports moisture into the central United States and the moisture convergence downwind from the LW maintains rainfall. For dry events, h eating occurs in the tropical eastern Pacific associated with the nort hward shift of the ITCZ roughly one pentad prior to onset, The prevail ing easterly flow over subtropical portions of North America is not co nducive to moisture transport into the United States, and without the support of moisture influx from the Gulf of Mexico, dry conditions pre vail.