Iowa spring flood outlook
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Iowa Spring Flood Outlook. March 5, 2010 Jeff Zogg, Senior Hydrologist. Topics. Outlook at-a-glance Definitions Current conditions Latest spring food outlook Questions. Outlook at-a-glance.

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Iowa Spring Flood Outlook

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Iowa Spring Flood Outlook

March 5, 2010

Jeff Zogg, Senior Hydrologist


Topics

  • Outlook at-a-glance

  • Definitions

  • Current conditions

  • Latest spring food outlook

  • Questions


Outlook at-a-glance

  • High risk of significant flooding continues in many areas. Has gone even higher in the Des Moines River basin above Saylorville Reservoir.

    • Highest risk areas include the Des Moines, Raccoon and Little Sioux River basins, as well as the Mississippi River from Davenport downstream and including its tributaries.

    • At least near to above normal risk of flooding statewide.

  • Time period when flooding is most likely extends from the middle third of March into late April for most streams.

    • Slightly later for the Mississippi and Missouri Rivers.


Outlook at-a-glance

  • Above normal risk of flash flooding especially from ice jams.

    • Ice jam flooding is not included in this outlook.

  • Risk of flooding and flash flooding will be higher than normal after the snow melts.

    • Will remain higher than normal at least into late spring.


Definitions


Definitions

  • Stage / Gage height

    • The level of the water surface of a stream above an established datum at a given location. The established datum may be a local elevation or sea level.

  • Reach

    • A section of stream between an upstream and downstream location, for which the stage or flow measured at a point somewhere along the section (e.g., gaging station or forecast point) is representative of conditions in that section of river or stream.


Definitions

  • Bankfull stage

    • An established gage height at a given location, above which a rise in water surface will cause the river or stream to overflow the river bank somewhere in the corresponding reach.

  • Flood stage

    • An established gage height for a given location at which a rise in water surface level begins to create a hazard to lives, property or commerce. The issuance of flood (or in some cases flash flood) warnings is linked to flood stage. Not necessarily the same as bankfull stage.


Definitions

  • Minor flooding

    • Minimal or no property damage, but possibly some public threat.

  • Moderate flooding

    • Some inundation of structures and roads near stream. Some evacuations of people and/or transfer of property to higher elevations.

  • Major flooding

    • Extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.


Definitions

  • Record flooding

    • Flooding which equals or exceeds the highest stage or discharge observed at a given site during the period of record keeping. The highest stage on record is not necessarily above the other three flood categories – it may be within any of them or even less than the lowest.

Stage – flood category relationship


Current conditions


Winter average temperatures


Winter precipitation


Snow depth—now


Snow depth vs. normal—now


Snow water equivalent—now


Snow pack—historical (2009)


Snow pack—historical (2008)


Stream levels—now

14-day average flow


Ground frost

Observed frost depth


Soil moisture—now

Soil moisture percentiles

Percentile key

~1: near record low

<10: much below normal

10 -24: below normal

25-75: normal

76-90: above normal

>90: much above normal

~99: near record high


Soil moisture—recent

Soil moisture percentiles—December 2009

Percentile key

~1: near record low

<10: much below normal

10 -24: below normal

25-75: normal

76-90: above normal

>90: much above normal

~99: near record high


Latest spring flood outlook


Assumptions / notes

  • This outlook is biased toward normal temperatures and precipitation (i.e., climatology) during the outlook period.

    • If conditions are not near normal then the probabilities in this outlook may not be valid.

  • Ice jams

    • This outlook does not take into account flooding from ice jams. Flooding from ice jams would be above and beyond the flooding potential indicated in this outlook.


50% flood category—now


50% flood category—second outlook


Risk of minor flooding—now

West 1/3 of Iowa (Missouri River basin)

East 2/3 of Iowa (Mississippi River basin)


Risk of minor flooding—second outlook

West 1/3 of Iowa (Missouri River basin)

East 2/3 of Iowa (Mississippi River basin)


Risk of moderate flooding—now

West 1/3 of Iowa (Missouri River basin)

East 2/3 of Iowa (Mississippi River basin)


Risk of moderate flooding—second outlook

West 1/3 of Iowa (Missouri River basin)

East 2/3 of Iowa (Mississippi River basin)


Risk of major flooding—now

West 1/3 of Iowa (Missouri River basin)

East 2/3 of Iowa (Mississippi River basin)


Risk of major flooding—second outlook

West 1/3 of Iowa (Missouri River basin)

East 2/3 of Iowa (Mississippi River basin)


Finding river forecast information


Finding river forecast information


Finding river forecast information


Final thoughts


Final thoughts

  • Most homeowners’ insurance policies do not cover flood-related losses.

    • To cover flood-related losses, flood insurance must typically be purchased. This is in addition to homeowners insurance.

    • Disaster assistance, if it’s available, is typically a loan that you must repay with interest.

    • In most cases, flood insurance requires a 30-day waiting period before it becomes effective.

    • Visit http://www.floodsmart.gov/ for more information.


Final thoughts

  • Will we see another “2008”?

    • A big factor in 2008 was the heavy rainfall from May-June.

    • High-end events such as the 2008 floods typically result from a sequence of additive factors. While one factor by itself may not lead to a high-end event, the combination of factors elevates the risk of a high-end event.

    • Factors which contribute to significant flooding this spring are lining up.

    • This does NOT automatically mean that we’ll have a repeat of 2008 though. We could luck out this spring—the flooding may be less severe than our outlook indicates.


Final thoughts

  • Spring flood outlook schedule

    • 1st Outlook: Friday, January 29, 2010.

    • 2nd Outlook: Friday, February 19, 2010.

    • 3rd Outlook: Friday, March 5, 2010.

    • 4th Outlook (Optional): Friday, March 26, 2010.


Questions

Contact information:

jeff.zogg@noaa.gov

515-270-4501

http://www.weather.gov/desmoines


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