CPC: Flooding in the Central US: Special Climate Summary - 95/2
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
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
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 Ni¤o/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
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
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.
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.
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.