CPC: Flooding in the Central US: Special Climate Summary - 95/2

June 1995

1. Introduction

From April through early June, many areas of the central U.S. received more than twice their normal precipitation, including the eastern Dakotas, an area from central Kansas eastward to the western Ohio Valley, and the central Gulf Coast. This excessive precipitation, coupled with nearly-saturated soil moisture conditions 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. An abnormally strong and persistent southwesterly flow of air at jet stream level (200 mb) over the central United States pushed a series of major storm systems across the country, resulting in the repeated episodes of heavy precipitation. There are many similarities between the atmospheric circulation during April/May 1995 and that associated with the Midwest Floods of June/July 1993. These include: (1) a strong southward and eastward shift of the mean jet stream position over the North Pacific during the winter and early spring prior to the onset of heavy precipitation; (2) persistent upper-level troughs over both the western United States and the Canadian Maritime Provinces; (3) an unusually active storm track and abnormally strong southwesterly flow at jet stream level through the middle of the country; (4) an enhanced low-level flow of warm, moist air into the central states; and (5) abnormally wet long-term soil conditions across part of the affected areas for at least 6 months prior to the onset of flooding. However, there are also important differences between the two events. Among these are: (1) the extreme duration of excessive precipitation in 1993 compared to 1995; (2) the greater persistence and intensity of the abnormal atmospheric circulation pattern during 1993; (3) a southward shift and eastward elongation of the axis of heaviest precipitation in 1995 compared to 1993; (4) substantial flooding in the Ohio River Valley, along with an overall southward shift in the area of maximum flooding in the Mississippi Valley, during 1995 compared to 1993; and (5) the timing of the two flood events with respect to the annual maximum in solar heating and atmospheric instability in the Midwest. This Special Climate Summary describes the Spring 1995 precipitation and flooding event in the Midwest, and compares these conditions to those observed during the Midwest Flood of 1993 (section 2). In section 3, a more technical description of the atmospheric conditions during the Spring 1995 floods is presented [Note that this section is oriented toward individuals with a working background knowledge of meteorology.]. Short-range forecasts and the long-range precipitation outlook for the central United States is discussed in section 4, and conclusions are presented in section 5.

2. Spring 1995 Conditions

a. Precipitation (Figures 1.1, 1.2, 2.1 and 2.2) Since early April 1995, surplus precipitation has been observed over much of the Midwest, the Plains States, and the lower Ohio Valley. The eastern Dakotas, the central Gulf Coast, and an area from central Kansas eastward to the western Ohio Valley received 2 to 3 times their normal precipitation for the period (Fig. 1.1), with many sections receiving 18 to 30 inches of precipitation (Fig. 2.1). The largest totals were observed in eastern Oklahoma and Kansas, southwestern and central Missouri, southern Illinois and Indiana, and central Kentucky. May rainfall was particularly excessive, with Illinois and Kansas observing their wettest May since 1895, and Indiana, Missouri, and Colorado experiencing one of their five wettest Mays on record, according to the National Climatic Data Center. Despite some very large amounts, the areas receiving excessive precipitation were highly variable from one storm to the next and were stretched along an axis extending from the south-central Great Plains northeastward into the lower Ohio Valley. This axis was shifted southward and also extended well eastward from the area of maximum precipitation observed during the summer of 1993 (Figure 1.2 and 2.2). Most of the above areas also received surplus rains during the 1993 Midwest Flood (which peaked during July), as did northern Missouri, Iowa, southern Minnesota, and the Dakotas (Figure 1.1). In fact, 30 to 40 inches of rain fell on central Iowa during the summer of 1993 (Figure 2.2) as one major rainfall event after another affected the area. b. River Flooding (Figure 3) Much of both the lower and middle Missouri and Mississippi and lower Ohio Rivers and their tributaries have flooded at least briefly during the last 2 months (Figure 3). This area is primarily located south and east of the regions most severely affected by river flooding in 1993, namely the middle and upper Mississippi and the middle and lower Missouri Rivers and their tributaries. However, large portions of eastern Kansas, northern and central Missouri, and western Illinois were impacted by both flooding episodes. During the last month, water levels along the lower Missouri and middle Mississippi Rivers approached, but did not reach, those recorded during the Midwest Flood of 1993 (NOAA, 1994 and CAC, 1993). To the north and west of this region, flooding was significantly more serious in 1993, except through parts of the Dakotas (along the Red River of the North). In contrast, areas farther south along the lower Ohio, lower Mississippi, and Arkansas Rivers (which did not flood in 1993) briefly reached levels 8 to 16 feet over flood stage, but have since declined considerably. The NWS Office of Hydrology reported that as of June 14, 1995, river levels were still considerably above flood stage (or were expected to crest significantly over flood stage within the next few days) along the Mississippi River from south of Hannibal, MO southward to Reserve, LA, the Missouri River south of Brownsville, NE, the Arkansas River from Muskogee, OK eastward to near Conway, AR, the Illinois River below Havana, IL, and the Ohio River at Cairo, IL. c. Agricultural Impacts River flooding has prevented crop planting in the immediate flood plains along the middle and lower Missouri, middle Mississippi, and lower Ohio Rivers and their tributaries. The persistent wet weather has also delayed corn, soybean, and sorghum plantings from the central Great Plains northeastward into the Midwest by 2-3 weeks, according to the United States Department of Agriculture. As of June 11, the state of Missouri was most affected by the wet weather and flooding as only 44%, 17%, and 37% of their corn, soybeans, and sorghum were planted compared to the past 5-year average of 93%, 59%, and 68%, respectively. Other states with large sowing delays included Kansas with only 11% and 14% of their soybeans and sorghum planted (avg. 70% and 62%), and Illinois with only 5% of their sorghum planted (avg. 67%). In addition, unusually low temperatures in the north-central U.S. during May delayed spring wheat planting in the northern Plains and Red River Valley, and crop progress in the Dakotas is now approximately 2-3 weeks behind average. Fortunately, unseasonably hot and dry weather during mid-June greatly accelerated crop planting and early growth, with June 18 planting averages in the aforementioned states much closer to the 5-year mean. Crop progress, however, still lags several weeks behind average. d. Soil Moisture (Figures 4.1and 4.2) The Palmer Drought Index (PDI) as of June 10, 1995 indicates that soils are saturated over much of the Dakotas, Nebraska, southwestern Minnesota, Kansas, and most of Missouri (Figure 4.1). The northern portions of this area have reported long-term soil moisture surpluses since October 1994, in association with above-normal precipitation over the past 6-8 months (see Figure 6, bottom). Farther east, near normal long-term moisture conditions were reported over the lower Ohio Valley in early June 1995, although a few weeks of heavy precipitation during the past two months pushed the lower reaches of the Ohio River over flood stage at several locations. The current areal extent of "unusually moist" conditions is actually larger than was observed at the end of July 1993 during the peak of the Midwest Flood (Figure 4.2). However, the region of exceptionally wet soils was larger during the peak of the 1993 Flood, extending northeastward into the upper Mississippi Valley. Furthermore, the largest PDI values during 1993 (+9.6 in southwestern Minnesota on July 31) were almost two units higher than the largest values recorded through mid-June 1995 (+7.9 in central South Dakota on June 10). Farther to the east, near or below normal long-term soil moisture conditions covered the lower Ohio Valley in late July 1993, resulting in subnormal streamflows throughout the region. These conditions were in strong contrast to the periodic flooding observed in the Ohio Valley during the past two months. e. Atmospheric Circulation (Figures 5.1and 5.2) During April and May 1995, persistent upper-level troughs over both the western United States and the Canadian Maritime Provinces dominated the circulation. This pattern was associated with an abnormally strong and persistent southwesterly flow at the jet stream level throughout the Midwest (Figure 5.1), which produced large-scale upward motion and a favorable environment for above-normal precipitation in the region. The pattern was also associated with an enhanced storm track across the southwestern and central United States, resulting in a series of unusually strong storms that moved northeastward across the central United States. The heavy precipitation and flooding in the Midwest were directly linked to these intense storm systems. All of the atmospheric conditions described in this paragraph were similar to those observed during the flood of 1993, particularly during its onset in June. However, important differences in the atmospheric circulation between the 1993 and 1995 flood events are also evident. First, the moisture-laden southerly flow observed during June/July 1993 in the lower atmosphere (850 mb) over the Midwest was notably stronger and more persistent than during April/May 1995. In July 1993, this extremely strong moisture transport, coupled with a strong, nearly- stationary low-level frontal boundary, allowed for the repetitive formation of major thunderstorm complexes over the Midwest, resulting in a continuation of severe flooding throughout the region. Second, substantial variability in the paths of the individual storms during April/May 1995, and in the locations of their accompanying precipitation shields, produced relatively large regional variations in precipitation over the Midwest. In contrast, there was much less spatial variability to the storm track and rain shields in June/July 1993, resulting in inundating rain events repeatedly affecting the same areas. Lastly, the timing of the floods with respect to the annual maximum in solar heating was also different between the two events. In particular, the extremely heavy rainfall observed during July 1993 occurred during a period of maximum solar heating and atmospheric instability in the Midwest. During July 1993, individual storms systems moving through the Midwest were significantly weaker than those observed in either June 1993 or April/May 1995, yet the area continued to be inundated by major thunderstorm outbreaks and severe flooding. f. Antecedent Circulation and Precipitation (Figure 6) Since mid-December, extremely persistent upper-level circulation and temperature anomalies have dominated the extratropics of both hemispheres, as well as the global tropics and subtropics. In the extratropics, enhanced westerlies have prevailed in both hemispheres between 20o and 40o latitude, with the largest westerly anomalies observed over the Pacific basin (Figure 6, top). These anomalies reflected an equatorward shift of the jet stream in both hemispheres, along with a pronounced eastward extension of strong jet stream winds across the northern Pacific to the west coast of North America. In the tropics, easterly wind anomalies prevailed throughout the Pacific basin during the period. These circulation features are linked with above-normal 500-mb temperatures throughout the global tropics and subtropics (Figure 6, center). The aforementioned circulation and temperature patterns developed last Winter in association with the return of mature El Nio/Southern Oscillation (ENSO, or "warm episode") conditions to the tropical Pacific. This period of mature ENSO conditions was relatively short-lived (November-February), with sea- surface temperatures and thunderstorm activity in the equatorial Pacific dropping sharply toward near- normal levels during March/April 1995. However, despite this evolution, atmospheric conditions throughout the tropics and extratropics at upper-levels have remained in a highly disturbed, ENSO-like state. In the Northern Hemisphere, the area of abnormally strong jet stream winds during December 1994 - May 1995 extended from eastern Asia to the southwestern United States. This jet structure is similar to that noted during the four-month period prior to the onset of the Midwest Flood of 1993 when mature ENSO conditions also dominated the tropical Pacific (Bell and Janowiak, 1995). In both winters, this anomalous jet stream pattern brought very heavy precipitation to California, the Southwest, parts of the Rockies, and the northern Plains (Figure 6, bottom; see also CAC, 1995).

3. Technical Discussion of the Atmospheric Circulation Associated with the Spring 1995 Floods

Note that this section is oriented toward individuals with a working background knowledge of meteorology. a. Circulation Dynamics (Figures 7 - 8) During April and May 1995, a persistent anomalous upper-level trough developed over the western United States, a weak ridge became established over the East, and another trough formed over the Canadian Maritimes (Figure 7, top). Enhanced jet stream winds of 8 to 12 m/s above normal extended northeastward from southern California to the northeastern United States (Figure 7, bottom). Thus, the Midwest was located downstream of the mean trough axis, and also within the left-front quadrant of the largest southwesterly wind anomalies (Figure 7, bottom) These conditions are similar to those observed during the peak of the Midwest Flood in June/July 1993 (Bell and Janowiak, 1995). The importance of these large-scale circulation features in promoting above-normal precipitation over the Midwest during Spring 1995 is illustrated in Figure 8. During April-May the Midwest was dominated by strong cyclonic vorticity advection at jet stream level (Fig. 8, top left) which results in large- scale upper-level divergence (Figure 8, top right) and ascending motion in lower and middle levels of the atmosphere. All of these factors contributed to a favorable environment for above-normal precipitation throughout the Midwest. Additionally, this large-scale circulation pattern was associated with a pronounced southward shift of the storm track from the eastern North Pacific to the central United States (Figure 8, bottom), and with substantial storm amplification and significantly above-normal storm activity over the southwestern Plains. This area was located just downstream of the mean trough axis over the southwestern United States. These major storms were then directed northeastward toward the Midwest along the axis of the anomalous jet stream, where they tapped warm, moist air from over the Gulf of Mexico and produced the extremely heavy precipitation totals which initiated the floods. These conditions were reminiscent of those observed in June 1993. At that time, enhanced westerly flow at jet stream level served as a "duct" for a series of major cyclones that propagated across the central North Pacific and into the central United States. These storms triggered a series of intense thunderstorm complexes over the Midwest, resulting in the onset of major flooding. b. Teleconnection Pattern (Figure 9) The circulation pattern described above reflects a pronounced negative phase of the Pacific Transition (PT) teleconnection pattern (Figure 9, left). The PT pattern is one prominent pattern of low- frequency variability observed over North America between May and August. The May 1995 value of the PT pattern (Figure 9, right) was similar to those observed in May and June 1993 during the onset of the 1993 floods, but was markedly less than the record negative PT value observed in July 1993 during the peak of that flood. Current observations indicate that the negative phase of the PT pattern began to dissipate rapidly in late May, and that the long-term characteristics of the circulation no longer resemble this pattern. Both the atmospheric circulation and rainfall totals throughout the central United States have become highly variable since late May.

4. Precipitation Outlooks

a. 6 to 10 Day Outlook (Figure 10, top) The 6-10 day outlook (Figure 10, top) indicates near to above-normal rainfall (greater than 0.5 inch, with amounts up to 1.5 inches) over most of the southern and central Plains and the lower Missouri, middle Mississippi, and lower Ohio Valleys. The forecast indicates a destabilized atmosphere and localized convection due to weak cyclonic circulation aloft and ample surface moisture; however, the outlook also suggests relatively weak southwesterly flow at jet stream level across the Midwest. This indicates a substantial short-term change in the circulation from that which initiated the recent flooding in the central United States. This forecast is consistent with the highly variable nature of the atmospheric circulation over the Midwest since late May, when the extremely persistent negative phase of the PT pattern began to dissipate. b. 30- and 90-Day Outlooks (Figure 10, bottom) The outlook for the month of July (Figure 10, bottom) indicates enhanced chances for above-normal precipitation across the northern tier of states from the Pacific Northwest to the Great Lakes, with the greatest likelihood for above-normal precipitation existing over South Dakota. Farther south and east, across the lower Missouri, the lower and middle Mississippi, and the lower Ohio Valleys, forecast skill was deemed to be insufficient for the issuance of any prediction beyond climatology. The seasonal (90-day) outlook (July-September) depicts no significant precipitation trends beyond climatology over most of the country, except for slightly wetter than normal conditions in the upper Midwest centered over Wisconsin.

5. Conclusions

Several factors contributed to the recent flooding in the Midwest and Plains States. First, high antecedent soil moisture levels throughout the northern Plains, along with surplus precipitation in these regions and parts of the Rockies during the past several months, produced abnormally high river levels in the upper and middle Missouri Valley. Second, a series of major storms battered the Midwest in April and May, producing excessive precipitation from central Oklahoma northeastward to central Kentucky. This rainfall induced unusually high river levels in the middle and lower Mississippi and lower Ohio Valleys, and aggravated already-elevated river levels along the Missouri River and some of its tributaries. The atmospheric flow patterns prior to both the onset of the 1995 and 1993 floods were remarkably similar, and reflected ENSO-influenced atmospheric conditions in both cases. These conditions were highlighted by an equatorward shift of the jet stream over the North Pacific and a pronounced eastward extension of strong jet stream winds into the western United States. This circulation brought above-normal precipitation and elevated soil moisture levels to large sections of the central United States prior to the onset of both episodes. During April and May 1995, pronounced troughs became established over the western United States and the Canadian Maritimes. These features were indicative of a strong negative phase of the Pacific Transition (PT) teleconnection pattern which also dominated the atmospheric circulation during the Flood of 1993. This circulation was important dynamically to the onset of heavy precipitation in the Midwest during April 1995 for three reasons. First, the pattern brought large-scale upper-level divergence, which induced ascending motion and precipitation throughout the Plains and Midwest. Second, the pattern was associated with a pronounced intensification of the storm track over the western and central United States. Third, the strong southwesterly flow at jet stream level directed these storms into the Midwest, where they subsequently triggered heavy precipitation and flooding. A similar sequence of events occurred during June 1993 in conjunction with the onset of the Flood of 1993 (Bell and Janowiak, 1995). Conditions in the tropics remain unusually warm at mid-levels of the atmosphere. These conditions were associated with enhanced westerlies across the northern Pacific Ocean and ultimately contributed to the development of the negative phase of the PT pattern during April and May. However, as the Northern Hemisphere transitions into summer, the jet stream over the northern Pacific typically becomes less coupled to conditions in the tropics, and is more heavily dominated by internal atmospheric dynamics and random climatic variability. The strong seasonal dependence between conditions in the tropics and the extratropical jet stream makes it very difficult to predict how long enhanced westerlies might persist across the North Pacific. Additionally, the PT pattern has become highly variable since late May, indicating an end to the extremely persistent circulation features and sustained periods of above-normal rainfall that dominated the central United States during most of April and May. A resurgence of Midwest floods depends on the evolution of the atmospheric circulation during the upcoming weeks. With no strong climate signal present, the long-range forecast is indeterminate, and a prediction beyond climatology cannot be issued. However, if large-scale flow conditions similar to those noted above redevelop and persist, then there will be an above-normal chance of surplus rainfall in the central United States. The Climate Prediction Center will continue to monitor this situation, and issue further information as conditions warrant.

6. References

Bell, G. D. and J. E. Janowiak, 1994: Atmospheric circulation during the Midwest floods of 1993. Bull. Amer. Met. Soc., 76, 681-696. CAC, 1993: Special Climate Summary #93/3, Growing Season Update. United States Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service, National Meteorological Center, Climate Analysis Center, 24 pp. CAC, 1995: Special Climate Summary #95/1, ENSO Winter Impacts, California Flooding, Mild to the East. United States Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service, National Meteorological Center, Climate Analysis Center, 16 pp. NOAA, 1994: The Great Flood of 1993. United States Department of Commerce, National Oceanic and Atmospheric Administration Natural Disaster Survey Report, 335 pp. The Great Flood of 1993 is available from the NWS Office of Hydrology by calling (301) 713-0006. Special Climate Summaries are available from the Climate Prediction Center by calling (301) 763-4670.