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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I certainly dont want anything like this:
315K PV forecasts, a tropical wave will be situated over the EATL in 84 hrs time while it also showed a vigorous tropical wave currently over EAFRICA. The latter is similar to an August African Wave...Ouch.
I dont see any reason for atleast one named storm in July. And if that doesnt happens, then I fear an August similar to 2004 and 2007, when all conditions and the warming water (due to lack of storms in June & July) release to produce few but powerful storms.
lol...the nightmares return...lol
why is the EATL so active so early
Normally, cooled upwelled waters of the EATL keep tropical waves in check as they exit, they barely get pass 20W with significant convection.
This year is different...during the winter/spring of 2007-2008, the High shifted westward than normal. This reduced the effects of the cold Canary current and ehanced warm southerly flow and downwelling leading to a significant increase in SSTs in that region.
The result is, the tropical waves are able to maintain convection further west than normal for this time of year due to these warm SSTs.
Another factor is above average rainfall of the ITCZ and Sahel.
456....now is that High forecast to remain in position the next couple of weeks or months
The exact position cannot be accurately forecast but we have been observing a Negative NAO as expected and this is to continue into atleast July.
A Negative NAO favors a Azores high that is weaker than normal in the Central Subtropical Atlantic.
One thing I notice, there has not been vigorous frontal actvity for June.
There are two routes they can use, but normally they sail almost due south and hang a right toward Hawaii around San Diego or Baha.
The other route is the Northern Circle (Seldom used but much shorter)
Question: Which is the route to take this year, and what will the weather be like on the southern route... and will the present west coast invest have any effect on them?
I am watching to see if the high overhead expands Westward in tandem with the wave.
It has lifted a bit to the North today so this one could stand a chance. First we would need to see a surface low develop and at the moment there is no evidence at all of that.
The deep convection has hung around for over 12 hrs during the diurnal minimum so let's see what happens tonight.
That was an excellent analysis 456 - thanks!!
Also satellite imgagery showed some level of turning.
And this is what the NHC said at 8pm:
...TROPICAL WAVES...
TROPICAL WAVE IS ALONG 36W S OF 16N MOVING W NEAR 20 KT. BROAD
LOW-MID LEVEL CYCLONIC TURNING IS EVIDENT ON SATELLITE IMAGERY.
UPPER LEVEL FEATURES CONTINUE TO KEEP MOST OF THE CONVECTION W
OF THE AXIS. SCATTERED MODERATE TO STRONG CONVECTION IS FROM
4N-10N BETWEEN 35W-42W.
BBL
Whatever vorticity there is does not bear mentioning IMO.
Nearly every TWave has " broad low to mid level turning ". That's why they are called TWaves !
Hi guys, can anyone help with all of the Spanish post? Anyone speaks Spanish?
I think they should rewrite the post that was put in spanish into english......for translations purposes......or stop posting in spanish.......
Action: |
Um... ok. I'll stop writing in Spanish... but I am willing to bet that at least 50% of the people on this blog speak Spanish.. and I'm sure there are a few living in the Caribe that don't speak English at all. Plus, the blog is really slow so it doesn't matter TOO much
i am from Puerto Rico too =)
Well pueden hablar español =)
solo den traducciones para que nadie se nos pierda =)
---------------
Well you can speak Spanish =)
just give transduction so no one gets lost =)
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