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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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254. BoynSea
1:45 AM GMT on June 20, 2008
North Abaco, Bahamas

Well, be careful what you wish for! We have been under drought conditions for some time, wishing for a good rain. We got that today, plus some! The landscape and grass are very grateful, and the water cisterns are near full, but the soil is near saturation. This weather pattern began Wednesday, with a waterspout marching down our "backyard", causing no damage.

Supposed to clear out by tomorrow, that';s a good deal for us, 'cause we need to make a boat run.
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253. HadesGodWyvern (Mod)
1:46 AM GMT on June 20, 2008
ok later surfmom =)
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252. surfmom
1:40 AM GMT on June 20, 2008
i did not realize that either -- guess I was lucky. my b-day is coming so I want to update my pic. good thing I know this.

Well. I am signing off --early AM tomorrow
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251. aspectre
1:41 AM GMT on June 20, 2008
151. atmoaggie "Can you believe that most insurance companies will not write a policy for a home 15 miles inland from Lake Pontchartrain at 100 feet above sea level?"

Must be north near Bogalusa cuz darn near everything south of Interstates10&12 all the way past Beaumont,Texas is less than 30feet/9metres above mean*sea-level

or ~20feet/6metres above the shoreline**.

* Link to the mean sea-level map. On Firefox3, you can zoom in using Control Plus
and zoom out using Control Minus (This site's editor won't display the key symbols)
Larger interactive version.

** Not quite the same type of maps because the shorelines are defined by normal wave action riding on the highest high-tide superimposed upon projected rises in sea-level.
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250. HadesGodWyvern (Mod)
1:36 AM GMT on June 20, 2008
ya I know. Didn't think avator had to be weather related.
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249. weathermanwatson
1:36 AM GMT on June 20, 2008
Convection over the Yucatan looks interesting.
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248. surfmom
1:33 AM GMT on June 20, 2008
Oh well, guess you will have to be creative and come up w/something new...Ill wait patiently
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247. aspectre
1:26 AM GMT on June 20, 2008
Deleted cuz I double-posted below
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246. HadesGodWyvern (Mod)
1:27 AM GMT on June 20, 2008
hello surfmom

the avator got taken away by the administrators for not being weather related. (or so it said)
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244. surfmom
1:18 AM GMT on June 20, 2008
Hades... where your avatar photo?? it was one of my favorites ) reminded me of my fav movie Edward scissorhands...actually wonder if you were Johnny Dep in disguise
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243. surfmom
1:14 AM GMT on June 20, 2008
Thanks Hades for the update, btw you have a great "blogspace", Interesting enough I got my surf buddy's attention b/4 the PI weather news id. You caught this early and I was able to give him a jump start on the situation. He's got a bunch of wussy americans in PI to attend his wedding --he's a real surfer *few amenities in his lifestyle" The thought of his sister in the middle of this is almost funny.
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242. atmoaggie
1:07 AM GMT on June 20, 2008
209. AWeatherLover 11:43 PM GMT on June 19, 2008 Hide this comment.
atmo... It does seem weird like you are the only one posting... Dinner lull?

I think you got the wrong aggie there. Not me. No atmo in his handle.
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241. HadesGodWyvern (Mod)
1:10 AM GMT on June 20, 2008
At 0:00 AM UTC Typhoon Fengshen T0806 (970 hPa) located near 11.1N 126.5E has 10 minutes sustained winds of 70 knots with gusts up to 100 knots. The typhoon was reported moving west-northwest at 11 knots.

Storm-Force Winds
50 NM from the center

Gale-Force Winds
150 NM west from the center
120 NM east from the center

Forecast Position and Intensity
24 HRS: 13.2N 123.9E - 75 knots (CAT 3)
48 HRS: 15.1N 122.6E - 75 knots (CAT 3)
72 HRS: 17.4N 122.3E - 75 knots (CAT 3)
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240. surfmom
1:07 AM GMT on June 20, 2008
severe storm - Guess you can grin cause you don't have any visitors coming your way...LOL
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239. surfmom
1:05 AM GMT on June 20, 2008
severe, is that photo really from Zephryhills? You seem quite "high" --but I guess it's that hilly there
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238. surfmom
1:04 AM GMT on June 20, 2008
Never shoot the msgr.
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237. surfmom
1:03 AM GMT on June 20, 2008
Ohio --been there, well I'd rather face hurricanes, then winter --have a good night --enjoy your free time
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236. severstorm
8:59 PM EDT on June 19, 2008
well i have to admitt surfmom i was grinning at those comments
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235. nrtiwlnvragn
9:03 PM EDT on June 19, 2008
NOAA released a report today Weather and Climate Extremes in a Changing Climate

Chapter 3 covers hurricanes

PS: Please don't shoot the messenger
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234. PanhandleChuck
8:01 PM CDT on June 19, 2008
Yes... I really do not like Ohio very much. Glad I moved down here
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233. PanhandleChuck
7:59 PM CDT on June 19, 2008
Alright, I can see this is not working tonight. I can't put my thoughts into my fingers on the keyboard. Need to go to sleep so that I can function ar work tomorrow. Bye All
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232. surfmom
1:00 AM GMT on June 20, 2008
Sounds like a good plan! Don't you find it hard to leave FL and go north?
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231. PanhandleChuck
7:57 PM CDT on June 19, 2008
Well I do have to fly up next Friday for a couple of days, but I be at my parents most of the time while I'm there
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230. PanhandleChuck
7:55 PM CDT on June 19, 2008
Wow... that comment didn't make much sense... maybe had one too many beers.
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229. surfmom
12:56 AM GMT on June 20, 2008
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228. surfmom
12:53 AM GMT on June 20, 2008
usually I look to a hurricane for waves, as weird as it sounds, now I want a hurricane for "damage control"

It is my fault, but I love my home,my job w/the Polo horses, the beach and MY dog so much I never ever want to leave FL --now I will have dues to pay unless.... a "cane comes my way
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227. PanhandleChuck
7:53 PM CDT on June 19, 2008
Yes!! her in the kids left this morning and they just arrived in Ohio about 20 minutes ago
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226. DVG
12:45 AM GMT on June 20, 2008

Yesterday? one of the models showed something small heading n then a 90 rt turn across nfla. No longer there now.

Lately the local forecats have been repeatedly altered to reflect rain then not or vice versa. ( NFla )

It seems conditions are fluid as reflected in the altered forecasts.

Not trained in this, but my thinking is that if and when something pops up it will be fairly unannounced and appear suddenly.
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225. surfmom
12:52 AM GMT on June 20, 2008
She was smart!!! I want to run away, but since I can't I am calling on mother nature to help me out. Hope you are enjoying the P & Q
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224. surfmom
12:49 AM GMT on June 20, 2008
Well, that 'cane in the phillipines is timed perfectly for my surfer buddy's wedding...... this weekend w/ a handful of American relatives in the midst of it all. Wonder if he'll skip the wedding to go surfing ...only kidding
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223. PanhandleChuck
7:49 PM CDT on June 19, 2008
Hey surfmom, my wife went back up north without me for a visit for the same reason. lol
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222. surfmom
12:46 AM GMT on June 20, 2008
Well, I know to be careful for what I wish for...but my mom is planning to visit in July --I'd like just enough of a hurricane prior to her visit to scare her into staying up north. She and my hubby are fighting dogs and I'd rather face a 'cane then the two of them together
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221. PanhandleChuck
7:46 PM CDT on June 19, 2008
Hey all

Pretty quiet in here tonight
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220. weatherg8r
8:45 PM EDT on June 19, 2008
Evening all...I just got home helping with Damage Assessment training at work for hurricane season...I guess better late than never!
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219. surfmom
12:41 AM GMT on June 20, 2008
Don't know the exact amount but it was a nice steady sprinkle, cooled things off and penetrated the ground. My mango gave thanks and is holding on to her fruit now. the last time she was this loaded with mango's was 2004 and we know what happened that year. I am waiting to see if the tree will be my hurricane predictor. It was enough rain not to have to work out east at the Polo barn -- and that was a gift from the heavens so to speak.
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218. surfmom
12:39 AM GMT on June 20, 2008
weatherlvr - might just might be a 2ft bump wind swell from the west wind on sunday --surf site noted a possibility ==I am so hungry for waves
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217. severstorm
8:39 PM EDT on June 19, 2008
hey surfmom how much rain did you get this morning? i'm in zephyrhills only got .19
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216. surfmom
12:36 AM GMT on June 20, 2008
Pottery --justr LOVED the photo - what a cute grom - is he yours??? Geeze I am sooo tired of paddling buoy to buoy just to keep ibn shape waiting for some waves to come through...looks like a nice board too!
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215. surfmom
12:34 AM GMT on June 20, 2008
Texas gulf YOU ARE TOOO Funny re 210 -- save those definitions, they're a hoot and great when there is a full head count on the blog playground
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214. surfmom
12:33 AM GMT on June 20, 2008
Hey Taz, I am here so it's not dead NO more
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213. Tazmanian
5:10 PM PDT on June 19, 2008
this blog is dead tonight
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212. wxman69
11:45 PM GMT on June 19, 2008
In reference to the question by lowerbamagirl, the conversion of "kts" to "mph", is simply the value in knots multiplied by 1.15
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211. SLU
11:43 PM GMT on June 19, 2008
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210. TexasGulf
11:18 PM GMT on June 19, 2008
Perhaps when the next 500-year flood happens in another 8-years from now... we can discuss CHANGING the definition of a 500-year flood!

Maybe just a change in terminology would do fine. Rather than calling it a "100-year" or "500-year" flood, one of the following might be better; Progressing from smaller (25-year events) to larger (10,000-year flood events)... I give you the LIST of New Titles.

UBER-FLOOD (sort of German... but catchy!)25-yr

MEGA-FLOOD (kind of 1970-ish)50-yr

PanUltimate FLOOD (for those awaiting our final extinction in a massive flood event)100-yr

Elephantine FLOOD (for animal lovers) 500-yr

Titanic FLOOD (ironic)1,000-yr

Prodigious FLOOD (for techno-geeks)5,000-yr

Cosmic FLOOD (for science fiction fans)10,000-yr

Any other suggestions? I feel that different labels will at least not give people the wrong impression about a likelihood of flooding. After all... you can have three simultaneous years of Prodigious Floods without anyone questioning.
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209. AWeatherLover
11:40 PM GMT on June 19, 2008
atmo... It does seem weird like you are the only one posting... Dinner lull?
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208. pearlandaggie
11:39 PM GMT on June 19, 2008
i fell into a blog black hole!
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207. pearlandaggie
11:28 PM GMT on June 19, 2008
is anyone else here? i feel like i'm posting on the wrong blog or something! LOL
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206. pearlandaggie
11:25 PM GMT on June 19, 2008
hello, typhoon!

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205. pearlandaggie
11:23 PM GMT on June 19, 2008
for a laugh... :)

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204. pearlandaggie
11:12 PM GMT on June 19, 2008
190. basscat71, here ya go...

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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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