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Two 500-year floods in 15 years

By: Dr. Jeff Masters , 2:48 PM GMT on June 19, 2008

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.

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.

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

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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1104. moonlightcowboy
7:59 PM CDT on June 21, 2008
Stormdude, that sounds reasonable. Unless, there's a sneeker (and there could be), I'm not looking for any activity until about mid-July.
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1103. yamil20
12:55 AM GMT on Junio 22, 2008
good evening everyone.can some one explain what causes the shear.thanks in advance
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1102. stormdude77
8:57 PM AST on June 21, 2008
I suspect shear values will start to drop in the next two weeks or so, when the positive phrase of the MJO reaches the Atlantic...JMO
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1100. JLPR
12:53 AM GMT on June 22, 2008
yup for the moment it looks interesting =)
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1098. moonlightcowboy
7:50 PM CDT on June 21, 2008
The twave has too much shear north of it, and shear is expected to increase west of it, too. Not much chance of development I don't think. But, convection does seem to be building, spreading outwards.
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1097. JLPR
12:49 AM GMT on June 22, 2008
well I must say that tropical wave looks interesting =P
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1096. Weather456
8:44 PM AST on June 21, 2008
Overcoming wind shear to its north, which is expected to remain unfavourable over the next 72 hrs, is the main problem with this wave. If it does enter the Caribbean, wind shear is expected to remain unfavorable in 3-5 days time. I expect little from this wave but it will be watched. There is upper anticyclonic circulation over this feature aiding in the development of thunderstorms tonight, despite this, the westerlies will disrupt this circulation and make it difficult for any development to occur once it reaches say about 13N.
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1094. catastropheadjuster
12:44 AM GMT on June 22, 2008
I didn't get to go to the parade had to help hubby paint a boat. I miss all the fun.
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1093. Tazmanian
5:40 PM PDT on June 21, 2008
456 that tropical wave is are next player

where is it going and dos it have a ch at 92L??
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1092. jabjb2
12:37 AM GMT on June 22, 2008
yeah me too cat. We got some good rain yesterday with a good lightning show just after the parade downtown.
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1091. Tazmanian
5:39 PM PDT on June 21, 2008
way too soon to tell what will see this year
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1090. Weather456
8:27 PM AST on June 21, 2008
A rather vigourous tropical wave with define mid-low level turning on satellite imagery near 8.5N/36.5W.

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1089. catastropheadjuster
12:34 AM GMT on June 22, 2008
jabjb2- Hey I live in Satsuma. I hope we don't get a Ivan or a Katrina ever ever agian. I just want rain makers that's all to cool the evening down.
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1088. jabjb2
12:28 AM GMT on June 22, 2008
thats what I was thinking taz. Do you have any predictions for what storms I might get here in mobile AL? I hope we don't get another Ivan or katrina.
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1087. Tazmanian
5:23 PM PDT on June 21, 2008
jabjb2 the CATL wave may have a better ch then the gulf wave
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1086. txalwaysprepared
12:19 AM GMT on June 22, 2008
Just had a good storm move through. Took some pics, but took them a bit late. Didn't even know the storm was coming until it was almost on top of us. Tornado spotted... waiting to see if pearlandaggie made it through.

I asked for rain and boy did I get it lol
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1085. HadesGodWyvern (Mod)
12:21 AM GMT on June 22, 2008
National Hurricane Center - Miami, Florida

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%)
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1084. jabjb2
12:17 AM GMT on June 22, 2008
taz what do you think about the GOM and CATL blobs?
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1083. sporteguy03
12:12 AM GMT on June 22, 2008
A day without a hurricane threatening anyone is a good day!
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1082. Tazmanian
5:12 PM PDT on June 21, 2008
when sould we see 92L???
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1081. Tazmanian
4:36 PM PDT on June 21, 2008
nevere mine
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1080. InTheCone
7:22 PM EDT on June 21, 2008
Blog Sleepy Time!!

I just hope it will STAY that way - unfortunately I think I will be wrong....

Thanks to all for your insights - back to lurking...
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1079. Weather456
7:05 PM AST on June 21, 2008
This pretty much sums up the tropics:

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1078. catastropheadjuster
11:02 PM GMT on June 21, 2008
Gosh where did everyone go. Is there anything interesting going on in the tropics?
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1077. catastropheadjuster
11:01 PM GMT on June 21, 2008
Everyone gone? Hello (echo)
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1076. catastropheadjuster
10:57 PM GMT on June 21, 2008
Hey SJ how ya doing? just fine here. A little rain earlier. Fixing to grill some steaks.

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1075. WeatherfanPR
10:39 PM GMT on June 21, 2008
GFS and NAM models are forecasting a strong wave moving over Puerto Rico and also NAM model is forecasting a low developing just north of Puerto Rico and then a wind surge of 30 knots affecting the islands and Puerto Rico in about 50 to 60 hours.
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1074. StormJunkie
10:37 PM GMT on June 21, 2008
Afternoon all :~)

Get through that weather ok press?
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1072. StormHype
10:19 PM GMT on June 21, 2008
Did you enjoy the video nonetheless StormHype?

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.
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1071. txalwaysprepared
10:18 PM GMT on June 21, 2008
Thanks! It'd be nice to get some decent rain around here... but I don't want anything more than that.
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1070. 0741
10:10 PM GMT on June 21, 2008
what i was told by people who know model told me that NAM is not good with tropical system so we wait for other model pick what nam picking
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1069. txalwaysprepared
10:04 PM GMT on June 21, 2008
I like that NAM model thingy.. I would love for a little depression to come to the Houston area. Need the rain so bad!! But no more than tropical wave/depression.

So does this NAM forecast something small developing or has it changed its mind?

TampaSpin might want to see that ;)
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1068. presslord
10:01 PM GMT on June 21, 2008
Tigger we just got hammered again on Johns Island. Last nite was awful too
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1067. melwerle
10:05 PM GMT on June 21, 2008
nice weather here - great tstorm and REALLY HEAVY RAIN (yeah, I know I should turn off the computer...)

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...
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1066. moonlightcowboy
5:06 PM CDT on June 21, 2008
Hey, Tigger! Good to see you bouncing 'round! ;P
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1065. PSLHokie
9:51 PM GMT on June 21, 2008
Wow, low level convergence with upper level divergence right over florida. That will help the Thunderstorms!!
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1063. kmanislander
9:55 PM GMT on June 21, 2008
The Caribbean wave is only about 120 miles east of this buoy. Winds are sustained near 25 knots and gusting to 29. Would not want to be out sport fishing there today !

Pressure is approx 1012 mb and falling but that is relatively high.Link
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1062. tiggeriffic
9:51 PM GMT on June 21, 2008
blog seems to be dragging today
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1061. Stormchaser2007
9:43 PM GMT on June 21, 2008
93E looks decent..

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1060. Weather456
5:40 PM AST on June 21, 2008


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1057. tiggeriffic
9:37 PM GMT on June 21, 2008
raining in Charleston, SC, BAD storm last night, oldest son watched funnel cloud dropping down while at work last night, sooooo much rain, drainage could not handle it, destroyed part of a staples (connected to where he works) and part of their dining room.
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1056. msphar
9:33 PM GMT on June 21, 2008
The wave at 35- 40W is large but notice the shearing winds on the visible blowing up from the South. I think this will depower the wave as it trys to move Westward down the gauntlet.
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1055. Stormchaser2007
9:36 PM GMT on June 21, 2008
The discussion might be fake...but I just checked the NAM and it does have a front pass through and leave some kind of mess behind...Link
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1054. MississippiWx
4:35 PM CDT on June 21, 2008
JP...Look at the marine discussion on the current Houston forecast discussion.
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Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather

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