Excerpts from:
Precipitation in the Upper Mississippi River Basin,
January 1 Through July 31, 1993
K.L. Wahl, K.C. Vining, and G.J. Wiche.
U.S. Geological Survey Circular 1120-B
Abstract
Excessive precipitation produced severe flooding in a nine-State area
in the upper Mississippi River Basin during spring and summer 1993.
Following a spring that was wetter than average, weather patterns that
persisted from early June through July caused the Upper Midwest to be
deluged with an unusually large amount of rainfall. Monthly precicitation
data were examined at 10 weather-station locations in the flood-affected region
to illustrate precipitation patterns and amounts in the flood-affected
area. In 1993, all 10 of the selected locations received greater than
the normal rainfall for January through June 1961-90, 8 of the 10 locations
receiced more than 200 percent of the normal rainfall for July 1961-90,
and 3 received more than 400 percent of the normal rainfall for July.
(The average rainfall for any given 30-year period is termed the "normal"
rainfall for the given period.) May through July 1993 was the wettest or
nearly the wettest such period on record at many locations in the flooded
area. Of the 10 locations, 6 received more rainfall in the first 7 months
of 1993 than generally is received in a year.
INTRODUCTION
This report, one of a series that documents the effects of the 1993 flooding in
the upper Mississippi River Basin, covers precipitation in the upper
Mississippi River Basin from January 1 through July 31, 1993. As of August
1993, large storms continued to pass through the affected area; however,
weather conditions indicate that the widespread flooding might be coming to an
end. The floodwaters in the Mississippi river crested at St. Louis, Missouri,
on August 1, while the river stages along the mainstem and on the principal
tributary streams upstream from St. Louis were receding.
This report provides a general overview of the usually excessive precipitation
that produced severe flooding in a nine-State area in the upper Mississippi
river Basin (fig. 1) during spring and summer
1993. A general overview of precipitation during the first 7 months of 1993 is
presented.
Preparation of this report required cooperation among many persons in many
organizations. The authors are particularly appreciative of the cooperations
of State Climatologists and of persons in various State agencies and in the
offices of the National Weather Service, National Oceanic and Atmospheric
Administration, for making provisional data and maps available on a near
real-time basis.
PRECIPITATION, JANUARY 1-JULY 31, 1993
General precipitation conditions throughout the nine-State area of the upper
Midwest for the 7-month period ending in July are best summarized by the
following news item from the Associated Press, July 17, 1993:
The rain, of course, was the problem. Although much of the spring downpour
fell on vast undammed tracts and could not be contained, the real culprit was
the volume dumped by stalled weather systems. With the ground already
saturated by spring rainfall, June was the wettest month in the regions since
record keeping began. July was just as soggy.
The Associated Press statement was accurate when written in mid-July but proved
to be conservative. July ended up being much soggier than June.
Monthly Precipitation
Annual precipitation over the nine-State area affected by the flooding (fig. 1) generally averages more than 30 inches but
ranges from about 16 inches in south-central North Dakota to about 40 inches in
southern Missouri. Although precipitation is about evenly divided between the
first and last halves of the year. Normally, 45 percent of the annual
precipitation falls between April 1 and July 31; June precipitation represents
about 15 percent (2-5 inches) of the average annual precipitation total.
Figure 1: The Mississippi River Basin and
general area of flood streams, June to August 1993.
Precipitation amounts recorded throughout the upper Mississippi River Basin
during the first 7 months of 1993 generally were substantially greater than
normal (January-July 1961-90). Although April-through-July precipitation was
much greater than normal. little evidence early in the year indicated that
precipitation amounts in 1993 would be above normal. January-through-March
precipitation in the States of the upper Mississippi was near normal to
slightly above normal. Because precipitation for those 3 months is often in
the form of snow and generally total less than 6 inches of moisture, potential
flooding caused little concern. However, that situation began to change in
April.
Precipitation in April and May over the area ranged from near normal to much
greater than normal, but the areas of greatest precipitation differed for the 2
months. April rainfall was nearly twice the normal amounts in parts of
Wisconsin and in Missouri but was only moderately above normal in much of the
remainder of the flood-affected area. By contrast, May rainfall was more than
twice the normal amounts for the month over an area that extended from
southeastern South Dakota across Iowa to eastern Kansas. The largest storms,
though, were still to come.
In early June, a weather pattern (fig. 2)
developed that was characterized by a strong low-pressure system over the
Western United States and a corresponding large high-pressure system positioned
over the Southeastern United States. The jetstream dipped south over the
Western United States and flowed northeasterly across the upper Midwest. A
southeastern high blocked the eastward movement of storms, thus creating a
convergence zone between the warm, moist flow from the Gulf of Mexico and the
much cooler nd drier air from Canada, which resulted in thunderstorms. This
pattern persisted through most of June and July (National Weather Service,
1993). As a result, the upper Midwest with this convergence zone was deluged
with rain while the Southeastern and the Eastern United States from Alabama to
Vermont, under the influence of the high-pressure system, was very hot and
dry. Slight movements in the atmospheric pattern determined the timing and
location of the excessive rainfall throughout the upper Midwest.
Figure 2: Dominant weather patterns over the
United States for June through July 1993.
The persistence of this weather pattern caused unusually large amounts of rain
to fall over the upper Midwest. These large amounts and the wetter-than-normal
spring produced flooding throughout the upper Mississippi River Basin,
including the lower Mississippi River Basin. The rains were extraordinary in
the areal extent and in the amounts accumulated in the first 7 months of the
year. Precipitation for the first 7 months totaled more than 20 inches over
most of the flood-affected area and was more than 40 inches in areas of
northeast Kansas and east-central Iowa (fig.
3a). Most of the area received from 150 to 200 percent of the 1961-91
normal amounts for January through July (fig.
3b).
Figure 3. Areal distribution of total precipitation in the area of flooding in
the upper Mississippi River Basin, January to July 1993.
(Left) A: In inches
(Right) B: In percentage of 30-year normal
for January through July 1961-90.
