Contact: Stephanie Kenitzer FOR IMMEDIATE RELEASE
(301) 713-0622 6/23/95
A significant short-term change in atmospheric circulation patterns has given the midwestern United States a much-needed respite from the excessive rainfall of April and May. But global patterns remain in an unusual, El Nino-like state, and the potential for renewed heavy rains will persist until this pattern breaks, the Commerce Department said today in releasing a Special Climate Summary by the National Weather Service's Climate Prediction Center.
El Nino conditions are characterized by the warming of sea surface temperatures in the Pacific Ocean that can drastically change weather in parts of the globe.
The current El Nino-like conditions include persistent upper-level circulation and temperature anomalies throughout the global tropics. The circulation and temperature patterns developed last winter in association with the return of a mature El Nino/Southern Oscillation (ENSO). Although the period of mature ENSO ended in late February, the atmosphere had yet to respond in late May, and remained in a highly disturbed ENSO-like state.
"We're accustomed to abnormal temperature and circulation patterns during an El Nino phase, but these conditions are staying long after the El Nino ended," said David Rodenhuis, director of the Climate Prediction Center. "It is these El Nino- like atmospheric conditions that fueled the southward shift and strenghtening of the jet streams, which resulted in a series of strong storm tracks across the southwestern and central U.S."
More than twice the normal rainfall during the last few months in the Midwest, combined with nearly-saturated soils over much of the region, resulted in flooding along the lower and middle Missouri, middle and lower Mississippi, and lower Ohio Rivers and many of their tributaries.
From April through early June, many areas of the central United States received over 200% of their normal precipitation, including the eastern Dakotas, an area from central Kansas eastward to the western Ohio Valley, and the central Gulf Coast.
May rainfall was particularly excessive, with Illinois and Kansas observing their wettest May since records began in 1895, and Indiana, Missouri and Colorado experiencing one of their five wettest Mays on record.
The report also notes that the current atmospheric circulations are similar to those associated with the 1993 Midwest floods. The common factors include a strong south and eastward shift of the jet stream, persistent upper level troughs over the western U.S. and Canadian Maritime Provinces, unusually active storm systems and abnormally wet long-term soil conditions. There are also important differences between the two events, including the extreme duration of the rainfall in 1993, the greater persistence of the atmospheric pattern during 1993, an overall southward shift of the maximum flooding area during 1995, and the earlier onset of this year's flooding than the 1993 mid-summer flood.
A resurgence or lack of Midwest floods depends on the evolution of the atmospheric circulation during the upcoming weeks. The report notes that with limited climate signals present, the current long-range forecast is indeterminate. However, if large-scale atmospheric patterns similar to those noted above redevelop and persist, there will be an above-normal chance of surplus rainfall in the central United States.
The Special Climate Summary is available through the Internet on the Climate Prediction Center's World Wide Web Home Page, http://nic.fb4.noaa.gov. It is also available over National Weather Service media, including Automation of Field Operations and Family of Services.
The National Weather Service is an agency of the Commerce Department's National Oceanic and Atmospheric Administration.
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Editor's Note: For a hard copy of the report, contact the NWS Office of Public Affairs at (301) 713-0622 or Dave Miskus at the Climate Prediction Center at (301) 763-4670.