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1 Floods of June 28–29, 1998 in Ohio By G. F. Koltun ABSTRACT During a 96-hour period extending from June 26 to June 30, 1998, a stalled frontal system produced a series of storms that dumped 10 inches or more of rain on parts of Ohio. The storms occurred at the end of a relatively wet month, resulting in flooding and widespread damage throughout much of central, east-central, and southeastern Ohio. Twenty-three Ohio counties were declared Federal and State disaster areas as a result of the storms and flooding with an estimated economic impact of nearly $178 million. Twelve storm or flood-related fatalities were reported. Flooding was most severe in the Ohio counties of Guernsey, Noble, and Washington, which lie roughly along a north-south line coincident with the band of heaviest rainfall. Some streams in those counties had peak streamflows with estimated recurrence intervals in excess of 100 years. This report describes the meteorologic factors contributing to the floods and provides information on the resulting damages. Peakstreamflows, estimated recurrence intervals, and high-water elevation or stage data are reported for selected locations in the State. INTRODUCTION Storms during June 26–30, 1998, resulted in flooding and widespread damage throughout much of central, eastcentral, and southeastern Ohio. A total of 23 Ohio counties1 were declared Federal and State disaster areas, thousands of people were evacuated, more than 100 roads were closed because of flooding, and tens of thousands of people were without power. Twelve people died in Ohio as a result of the storms and flooding; six of the deaths occurred in Noble County alone. Several of the communities that were severely affected by the June 1998 floods had endured severe flooding only 18 months earlier (Jackson and others, 1997). Flood data are needed by Federal, State, and local agencies in order to make informed decisions about floodplain management and to provide information to assist in managing future flood emergencies. Given the extent and severity of the June 1998 floods, the U.S. Geological Survey (USGS), in cooperation with the Ohio Department of Natural Resources (ODNR), Division of Water, began a project to gather and describe pertinent flood information. Purpose and Scope This report describes the floods that occurred in Ohio in late June 1998. Peak-streamflow, estimated recurrence interval2 and (or) high-water elevation or stage data are presented for selected locations within the State. The meteorologic factors contributing to the floods are described, and information is provided on the resulting damages. Acknowledgments This report represents a compilation of information supplied by many agencies and individuals. The author acknowledges the general support and contributions of the ODNR, Division of Water. Special thanks are extended to David H. Cashell and Michael Schiefer, both of ODNR, for their contributions to this report describing damages and elevations of high-water marks. The author also thanks Julia Dian-Reed and Richard Kane of the National Weather Service (NWS) for their contributions and guidance on the meteorology associated with the floods. 1Two counties were declared disaster areas as a result of storms that occurred on June 24–25, 1998. 2Recurrence interval, in years, is equal to the reciprocal of the annual exceedance probability for a flood of the given magnitude. It represents a long-term average frequency with which one would expect to experience a flood of the given magnitude and, as such, does not preclude the occurrence of two or more large recurrenceinterval floods in any given shorter time span. 2 Floods of June 28–29, 1998 in Ohio METEOROLOGY ASSOCIATED WITH THE FLOOD The storms that produced the June 1998 floods were similar to the storms that produced severe flooding in southern Ohio 18 months earlier, in March 1997. Both storms involved frontal systems that stalled over Ohio with moisture-laden air being supplied from the Gulf of Mexico. Unlike the March 1997 floods, the June 1998 floods were immediately preceded by a fairly wet period that may have contributed to the severity of the floods. Antecedent Conditions May 1998. Precipitation for much of Ohio was below normal3 during May 1998. The National Oceanic and Atmospheric Administration (NOAA) divides the State into 10 climatological divisions. As shown in figure 1, the northern divisions experienced significantly below-normal rainfall, the central divisions experienced moderately to slightly below normal rainfall, and the southern divisions experienced slightly above-normal rainfall. June 1998. Rain fell at some place in Ohio on every day in June. Light, scattered showers fell during the first week of the month, with maximum total accumulations of less than 2 in. Most NOAA precipitation monitoring sites reported less than 1 in. of accumulation for the same period. Storms were common throughout June 9–17, during which many of the NOAA precipitation monitoring sites reported one or more single-day accumulations of rainfall in excess of 1 in. More rain fell around the State during the period June 18–25, with total accumulations for the period generally being much less than for June 9–17. As figure 1 shows, precipitation for June was above normal in all 10 NOAA divisions and in excess of 230 percent of normal in the three southern divisions. According to Cashell (1998), June 1998 ranks as the second wettest June for the State as a whole since 1883. Storms of June 26–30, 1998 On Thursday, June 25, 1998, a weak low-pressure system was positioned over the western Great Lakes and a trailing cold front was positioned across Wisconsin, Iowa and eastern Nebraska. Warm, moist air was being carried from the Gulf of Mexico to Ohio by strong winds from the southwest. By 8:00 a.m. on Friday, June 26, 1998, the lowpressure system had moved east, and the trailing cold front was slowing down and dipping farther south across central Michigan to northern Illinois. The first heavy rainfall occurred in east-central Ohio between late Friday afternoon the 26th and early Saturday the 27th. By Saturday morning, the cold front had progressed farther to the south-southeast and became nearly stationary. At that point, the frontal boundary ran from northwestern Ohio to southeastern Ohio. This nearly stationary frontal boundary acted as a corridor or focus for a series of showers and thunderstorms, a phenomenon known as “training”. When training occurs, rain falls repeatedly over the same general areas, frequently aggravating flooding due to highly saturated ground conditions. Depending on the size and conditions at the frontal boundary, training can last for 24 hours or more and cover several hundred square miles, or it can occur on a much smaller scale and last only a few hours. A frontal boundary is an area where warm and cool air collide; such collision can result in lifting of the warm air mass and instability along the front, creating thunderstorms. In some cases, secondary, smaller scale frontal boundaries (called surface boundaries) form, resulting in scattered areas of even greater instability. During June 26–29, 1998, several surface boundaries formed in southeastern Ohio, as well as in some sections of central Ohio, resulting in several training episodes. A surface boundary formed south of the front by late Saturday (the 27th) and moved southeast, causing more rainfall. The largest rainfall amounts occurred between June 27 and June 29, with more rainfall occurring earlier in the period for areas in the northwest and later in the period for areas in the southeast. Storms continued through June 30, however, most areas received 0.5 in. of rainfall or less from these final storms. Isolines4 of total accumulated precipitation for the 96-hour period from 8 a.m. eastern daylight time (EDT) on June 26, 1998, to 8 a.m. EDT on June 30, 1998 (fig. 2) show that Guernsy, Morgan, Noble, and Washington Counties received the most rainfall, with accumulations in excess of 10 in. By comparison, the 100-year, 5-day (120-hour) rainfall for this area is 6.98 in. (Huff and Angel, 1992). A fairly isolated pocket of heavy rainfall also occurred in southwestern Franklin County and northwestern Pickaway County, with accumulations in excess of 9 in. The heaviest rainfall occurred in the Franklin-Pickaway County area during the early morning hours of June 29, 1998, when very unstable air along a surface boundary combined with very high moisture within the lowest layers of the atmosphere. The prevailing conditions set up a training pattern that resulted in 5 or more inches of rainfall over the area in less than 6 hours, an amount in excess of the 100-year, 6-hour rainfall for that area (Huff and Angel, 1992). 3“Normal” refers to the average value for the period 1961–90. 4Isolines are lines on a map (or chart) with each line representing a constant value of some feature. Meteorology Associated With the Flood 3 Figure 1. Regionally averaged monthly total precipitation and percentage of normal precipitation by National Weather Service Division, May and June 1998. 84 83 82

Floods of June 28-29, 1998 in Ohio