**The Midwest Flood of 2008**

*A preliminary assessement*

*NOAA CSI Team*

Supported by NOAA's Climate Program Office Project

*"Explaining Climate Conditions to Improve Predictions"*

**What is the Current Climate Situation?**

Historical record high flows are occurring in major Midwest rivers,
including the DesMoines, Cedar, and Wisconsin Rivers. These feed
directly into the Mississippi River, which now is also expected to
sustain near record flows. The 14 June USGS monitor of river gages
indicates historic high flows occurring across most of Iowa, southern
Wisconsin and northern Illinois (denoted by black circles in ***Figure.
1***)

![](media/image1.png){width="5.123611111111111in"
height="2.970833333333333in"}

**Figure 1.** *Map of real-time streamflow compared to historical
streamflow for the day of the year (United States). The map depicts
streamflow conditions as computed at USGS streamgages. The colors
represent real-time streamflow compared to
[**percentiles**](javascript:openInfoWin('http://water.usgs.gov/waterwatch/ptile.html', 'Percentile', 350, 600))
of historical daily streamflow for this day of the year. Above 90
percentile denoted by blue, and record high floe denoted by black
circles*.

The immediate cause for the flooding streams and rivers has been
frequent heavy rain events concentrated in the catchments and headwaters
of these Midwest rivers. Rainfall has exceeded the norms by +5" to +10"
since June 1 (***Figure 2a***). This focus zone for rains sits in a
boundary between persistent abnormally low pressure over the North
Plains and abnormally high pressure over the Southern Plains/Ohio
Valley.

![](media/image2.png){width="5.379166666666666in"
height="3.207638888888889in"}

**Figure 2a.** *Map of accumulated rainfall depatures sduring the 13-day
period 1 June thru 13 June 2008 (United States). Areas of above normal
rainfall denoted in green, and blue shades. Areas receiving more than
+10" above their normals for this period are shown in bright magenta.*

The time history of daily precipitation at Cedar Rapids, Iowa reveals
the sustained high levels of moisture since late summer 2007, and then
the recent deluge atop the already wet conditions (***Figure 2b***).
Daily rainfall often exceeding 1", has been measured almost every day
for the past 3 weeks in the area.

![](media/image3.png){width="4.373611111111111in"
height="3.495833333333333in"}

**Figure 2b**. *Daily observed rainfall meaured at he observaing
station, Cedar Rapids, Iowa.*

*The time series spans the 1-year period from 10 June 2007 thru 9 June
2008.*

**What were the Antecedent Cimate Conditions?**

The Upper Midwest has experienced wet conditions for many months *[prior
to]{.underline}* the current additional heavy June rains. For the
12-month period 1 June 2007 thru 31 May 2008, some areas in the current
flood zone already received +15" to +20" of excess precipitation
(***Figure 3***). The June rains, therefore, have fallen upon saturated
soils resulting in the vast majority of recent rains running directly
into streams. These antecedent conditions are thus also an important
factor responsible for the current floods.

![](media/image4.png){width="4.622222222222222in"
height="2.5729166666666665in"}

**Figure 3.** *Map of accumulated rainfall depatures during the 12-month
period 1 June 2007 thru 31 May 2008 (United States). Areas of above
normal rainfall denoted in green, and blue shades. Areas receiving more
than +20" above their normals for this period are shown in bright
magenta.*

The existence of antecedent wet soils in spring 2008, resulting from
prior heavy winter percipitation including unusually deep winter snows
in the Upper Midwest permitted NOAA to provide early warning for floods
(**Figure 4**)**.** The March 20 spring outlook by NOAA's Advanced
Hydrologic Prediction Service (AHPS) stated that, "Major floods striking
America's heartland this week offer a preview of the spring seasonal
outlook... We expect rains and melting snow to bring more flooding this
spring... Americans should be on high alert to flood conditions...
Above-normal flood potential is evident in much of the Mississippi River
basin, the Ohio River basin, the lower Missouri River basin..."

![](media/image5.jpeg){width="5.997916666666667in"
height="4.401388888888889in"}

Figure 4. ***NOAA's spring flood outlook, issued 20 March 2008. Owing to
heavy winter snows and above normal precipitation over parts of
Wisconsin and Illinois, as much as a 20 to 30 percent chance of major
flooding on some rivers in southern Wisconsin and northern Illinois was
predicted.***

**Is the wet Upper Midwest Climate Condition the Result of La Niña?**

Colder than normal sea surface temperatures developed in the equatorial
central Pacific in mid-2007, developing into a strong La Niña during
winter. According to NOAA's most recent ENSO Diagnostic Discussion on 5
June 2008, a transition from La Niña to ENSO-neutral conditions is
expected during June-July 2008.

Yet, historical observations (***Figure 5***) of U.S. precipitation
anomalies during past La Niñas indicate wet conditions mainly over the
Ohio Valley, somewhat south and east of the flood region. In fact, the
severe Midwest [drought]{.underline} of 1988 coincided with a recent
strong La Niña event. It thus appears that canonical La Niña forcing
alone was unlikely a principal factor in rendering conducive climate
conditions for Midwest flooding. There is some indication, however, that
other sea surface temperature conditions may have been contributing
factors, as discussed further below.

![](media/image6.jpeg){width="6.0in" height="4.5in"}

**Figure 5.** *United States map of the observed January- March
seasonally averaged precipitation anomalies (mm/month) for the composite
of 20^th^ Century La Niña events. Dry conditions are shown in tan and
yellow shades, wet conditions in green shades.*

**Were wet Upper Midwest Conditions during Winter/Spring '08
Anticipated?**

As indicated above, the antecedent soil conditions were a useful
predictor at the end of March for subsequent flooding risks in
April-June. The question is whether the meteorological conditions that
led to such soil saturation were themselves expected based on climate
predictors available at the beginning of the winter season.

The principal predictors that determined the winter and spring 2008
precipitation outlooks issued by NOAA were the historical l La Niña
footprint , and the numerical simulations of precipitiation based on
climate models influenced by the *global* sea surface temperature
conditions. The official NOAA seasonal precipitation outlook for the
January-February-March season, issued on 20 December 2007 (**Figure 6**)
mostly reflected the historical La Niña effect. It departed slightly
from that, however, by shifting the expectation for increased
precipitation to Indiana and eastern Illinois, north and west of the La
Niña signal.

![](media/image7.png){width="5.9944444444444445in"
height="5.329861111111111in"}**Figure 6.** United States map of the
3-month precipitation probability outlook for the January-march 2008
season. Forecast is for the change in tercile categories of seasonal
precipitation. Increased risk of lower dry terciles shown in tan shades.
Increased risk of upper wet terciles shown in green shades.

As part of a NOAA research effort to explain climate variations and to
improve their predictions, climate model experiments have been completed
in which the actual observed global sea surface temperatures were used
to drive the models. Four different models were used to generate 60
simulations for the January-March 2008 period. The comparison in Figure
7 indicates that several features of the winter (JFM 2008) precipitation
pattern were consistent with the signal resulting from global ocean
forcing, including a dry southern tier, wet over the Interior West, and
wet over the Great Lakes. The simulation results suggest that the
intensity of observed wet Upper Midwest conditions was unlikely due to
global ocean conditions alone, although the simulations do indicate that
the Upper Midwest was an epicenter of increased wet risk, somewhat
displaced from the La Niña composite location. Additional assessment and
supportive modeling is required to determine the features of global sea
surface temperatures during the past year 2007-08 that may offered some
early warning for wet conditions in the Midwest.

![](media/image8.jpeg){width="3.3520833333333333in"
height="5.622916666666667in"}

**Figure 7**. *The January-March seasonally avergaed precipitation
anomailes (mm/month) observed (top), and simulated by climate models
forced with the observed global sea surface temperatures (bottom).
Observations based on the CAMS-OPI belnd of insitu and satellite
rainfall estimates. Simulations based on 4 models for which a 60-member
ensemble was performed. Wet (dry) conditions indicated in green (yellow)
colors. Reference is 1979-2000.*

**What is the attributable impact of greenhouse gas forcing?**

In considering possible attribution of changes in the risk of such
flooding events to anthropogenic forcing, the findings of several recent
assessment reports are relevant, including the Intergovernmental Panel
on Climate Change Fourth Assessment Report (IPCC 2007), and the U.S.
Climate Change Science Plans Synthesis and Assessment Products SAP1.3
(*[Attribution of U.S. Cimate Variations and Change, hereafter CCSP
1.3]{.underline}*), and SAP 3.3 (*[Analysis of the Observed Variations
and Changes in Climate Extremes, hereafter CCSP 3.3]{.underline}*). It
should be noted that no single event can be attributed to any specific
cause, including anthropogenic forcing; understanding the change in the
probability of such events awaits further attribution studies.
Nonetheless, whereas no such studies have been attempted yet for the
Cedar River, it is useful to place this extreme local event into what is
known about larger scale changes now occurring in the water cycle.

The current Upper Midwest extreme rain event fits into an emerging
pattern of change in the atmospheric water cycle. There is evidence that
the character of precipitation events over many land areas has changed,
regardless of whether a region's mean precipitation has changed and
despite indications that yearly and decadal flucuations in precipitation
totals are unlikely related to anthropogenic forcing (CCSP 1.3). IPCC
(2007) indicates that it is likely that the number of heavy
precipitation events (for instance, above the 95^th^ percentile) have
increased over many land areas since 1950. **Figure 8** illustrates the
observed increase in the % of annual precipitation resulting from very
wet days. Likewise, CCSP 3.3 notes that heavy precipitation events
averaged over North America have increased over the past 50 years. This
increase is consistent with observed increases in atmospheric water
vapor, which in turn have been associated with human-caused increases in
greenhouse gases.

![](media/image9.jpeg){width="5.993055555555555in"
height="5.135416666666667in"}

**Figure. 8** *Observed trends (% per decade) over the priod 1951 to
2003 in the contribution to total annual precipitation from very wet
days (top panel). Anomalies (%) of global (regions with data in top
panel) annual time series of very wet days defined as the % change from
the 1961-1990 base period average (22.5%). Figure from IPCC (2007).*

.

Analysis of regional trends in very heavy precipitation events over the
past century are well reflected in the Upper Midwest region currently
affected by flooding (Groisman et al., 2005). The US National Asessement
(2000) and additional work by Groisman has shown that more frequent
heavy precipitation events are related to amplified positive trends in
the highest riverflows of undisturbed streams across North America. The
Midwest flood appears not to be an isolated event, but may be part of
larger scale water cycle changes over the extratorpical land areas. .
For example, the IPCC (2007) comments that "The observed increase in
precipitation variability at a majority of German precipitation stations
during the last century...is indicative of an enhancement of the
probability of both floods and droughts." IPCC (2007) notes one study
(Milly et al. 2002) in which a significant increasing trend in the
frequency of great floods (discharge exceeding 100-year levels) was
found, based on very large river basins (drainage areas greater than
200,000 km^2^). A second study (Kundzewicz et al. 2005) focused on
annual extreme flows for a larger sample of rivers and found a mixture
of significant increases, decreases, and non-significant changes.

References:

**Climate Change Science Program** Synthesis and Assessement Product
1.3, 2008: *Reanalysis of Historical Climate Data for Key Atmospheric
Features; Implications for Attribution of Causes of Observed Change*. In
press.

**Climate Change Science Program** Synthesis and Assessement Product
3.3, 2008: Weather and Climate Extremes in a Changing Climate. In press.

**IPCC**, 2007: Summary for Policymakers. In: *Climate Change 2007: The
Physical Science Basis. Contribution of Working Group I to the Fourth
Assessment Report of the Intergovernmental Panel on Climate Change*
\[Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt,
M.Tignor and H.L. Miller (eds.)\]. Cambridge University Press,
Cambridge, United Kingdom and New York, NY, USA.

**Milly,** P.C.D., et al., 2002 : Increasing risk of great floods in a
changing climate.

*Nature,* **415**, 514-517.

**Kundzewicz,** Z. W., et al., 2005: Trend detection in river flow: 1.
Annual maximum

flow. *Hydrolog. Sci.,* **50,** 797-810.