Two 500-year floods in 15 years
The U.S. Geological Survey has preliminary data showing that this month's floods on four of Iowa's rivers--the Cedar, Iowa, Shell Rock, and Wapsipinicon--were 500-year floods. Back in 1993, many rivers in the Midwest also experienced 500-year floods, so the region has endured two 500-year floods in the past 15 years. How can this be? First of all a definition--a 500-year flood is an event that has only a 0.2% chance of occurring in a given year, based on available river flow data. Of course, reliable data only goes back a century at most, so designation of a 500-year flood event is somewhat subjective. Still, it seems rather improbable that two such huge floods should occur within such a short time span, raising the question of whether the floods were, in part, human-caused.
In a provocative story in the Washington Post today, it was pointed out that part of the flooding is due to the draining of wetlands for farming purposes. As nature's natural buffers against flooding are drained and filled to provide room for more farmland, run-off and flooding are bound to increase. Furthermore, as more levees are built to protect more valuable farmland and new developments, flood waters are pushed out of the former areas they were allowed to spread out in and forced into river channels behind the new levees. Even higher levees must then be constructed to hold back the increased volume of water they are asked to contain.
Climate change contributing to flooding?
The heaviest types of rains--those likely to cause flooding--have increased in recent years (see my February blog, "The future of flooding", for more detail). According to the United Nations' Intergovernmental Panel on Climate Change (IPCC) 2007 report, "The frequency of heavy precipitation events has increased over most land areas". Indeed, global warming theory has long predicted an increase in heavy precipitation events. As the climate warms, evaporation of moisture from the oceans increases, resulting in more water vapor in the air. According to the 2007 IPCC report, water vapor in the global atmosphere has increased by about 5% over the 20th century, and 4% since 1970.
Over the U.S., where we have very good precipitation records, annual average precipitation has increased 7% over the past century (Groisman et al., 2004). The same study also found a 14% increase in heavy (top 5%) and 20% increase in very heavy (top 1%) precipitation events over the U.S. in the past century. Kunkel et al. (2003) also found an increase in heavy precipitation events over the U.S. in recent decades, but noted that heavy precipitation events were nearly as frequent at the end of the 19th century and beginning of the 20th century, though the data is not as reliable back then. Thus, climate change is likely partly to blame for increased flooding in the U.S., although we cannot rule out long-term natural variations in precipitation.
Figure 1. Forecast change in precipitation and runoff for the period 2080 to 2099 compared to 1980 to 1999. The forecasts come from the A1B scenario from multiple climate models used for the Intergovernmental Panel on Climate Change (IPCC) 2007 report.
The forecast
According to a multi-model consensus of the climate models run for the Intergovernmental Panel on Climate Change (IPCC) 2007 report, precipitation and river runoff for the Mississippi River drainage basin are expected to increase only slightly by the end of this century (Figure 1). However, more of this rain is expected to fall in heavy precipitation events, the ones most likely to cause flooding. As a result, the U.S. needs to prepare for an increase in the number and severity of 100-year and 500-year flooding events in the coming century.
References
Kunkel, K. E., D. R. Easterling, K. Redmond, and K. Hubbard, 2003, "Temporal variations of extreme precipitation events in the United States: 1895.2000", Geophys. Res. Lett., 30(17), 1900, doi:10.1029/2003GL018052.
Groisman, P.Y., R.W. Knight, T.R. Karl, D.R. Easterling, B. Sun, and J.H. Lawrimore, 2004, "Contemporary Changes of the Hydrological Cycle over the Contiguous United States: Trends Derived from In Situ Observations," J. Hydrometeor., 5, 64.85.
Tropics
It's quiet in the tropics. There are no threat areas to discuss, and none of the models are forecasting tropical storm formation in the next seven days.
Jeff Masters
Reader Comments
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18Z NAM
Pressure is approx 1012 mb and falling but that is relatively high.Link
Finally the grass is getting some decent water.
and yes, if i could I would send it everyone else's way too. Gotta love it - hopefully the storms will come on a daily basis and we'll get back to regular summer weather...
So does this NAM forecast something small developing or has it changed its mind?
TampaSpin might want to see that ;)
Eh... the media just likes to drege up acid in everyone's stomach more often for nothing. If you stick by the TV like this guy did, it will make you manic depressive, and you'll rob yourself the experience of your local reality. I'd rather watch 5 minutes of raw in-the-storm b-roll video with no blabber mouth reporter annoying me on the audio. Just my 2 cents.
Get through that weather ok press?
Sheri
sheri
I just hope it will STAY that way - unfortunately I think I will be wrong....
Thanks to all for your insights - back to lurking...
Tropical Cyclone Outlook (0000z 22Jun)
=============================
A small area of low pressure located about 875 miles southwest of the southern tip of Baja California has continued to produce thunderstorms intermittently and has changed little in organization during the past several hours. The system still has a small window of opportunity to become a tropical depression during the next 12-24 hours before moving west-northwest into an area less conductive for tropical cyclone development.
Tropical Cyclone Potential Potential
=====================================
MODERATE (25-50%)
I asked for rain and boy did I get it lol
CA
where is it going and dos it have a ch at 92L??
CA
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