Iowa Spring Flood Outlook

National Weather ServiceNational Weather ServiceDes Moines, IADes Moines, IA


Iowa Spring Flood OutlookIowa Spring Flood OutlookIowa Spring Flood OutlookIowa Spring Flood Outlook

March 5, 2010March 5, 2010

Jeff Zogg, Senior HydrologistJeff Zogg, Senior Hydrologist


TopicsTopicsTopicsTopics

• Outlook at-a-glanceOutlook at-a-glance• DefinitionsDefinitions• Current conditionsCurrent conditions• Latest spring food outlookLatest spring food outlook• QuestionsQuestions


Outlook at-a-glanceOutlook at-a-glanceOutlook at-a-glanceOutlook at-a-glance

• High risk of significant flooding continues in many High risk of significant flooding continues in many areas. areas. Has gone even higher in the Des Moines River Has gone even higher in the Des Moines River basin above Saylorville Reservoir.basin above Saylorville Reservoir.– Highest risk areas include the Des Moines, Raccoon and Little Highest risk areas include the Des Moines, Raccoon and Little

Sioux River basins, as well as the Mississippi River from Sioux River basins, as well as the Mississippi River from Davenport downstream and including its tributaries.Davenport downstream and including its tributaries.

– At least near to above normal risk of flooding statewide.At least near to above normal risk of flooding statewide.• Time period when flooding is most likely extends from Time period when flooding is most likely extends from

the middle third of March into late Aprilthe middle third of March into late April for most for most streams.streams.– Slightly later for the Mississippi and Missouri Rivers.Slightly later for the Mississippi and Missouri Rivers.


Outlook at-a-glanceOutlook at-a-glanceOutlook at-a-glanceOutlook at-a-glance

• Above normal risk of flash floodingAbove normal risk of flash flooding especially from ice especially from ice jams.jams.– Ice jam flooding is not included in this outlook.Ice jam flooding is not included in this outlook.

• Risk of flooding and flash flooding will be higher than Risk of flooding and flash flooding will be higher than normal after the snow melts.normal after the snow melts.– Will remain higher than normal at least into late spring.Will remain higher than normal at least into late spring.



DefinitionsDefinitionsDefinitionsDefinitions



• Stage / Gage heightStage / Gage height– The level of the water surface of a stream above an The level of the water surface of a stream above an

established datum at a given location. The established datum established datum at a given location. The established datum may be a local elevation or sea level.may be a local elevation or sea level.

• ReachReach– A section of stream between an upstream and downstream A section of stream between an upstream and downstream

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



• Bankfull stageBankfull stage– An established gage height at a given location, above which a An established gage height at a given location, above which a

rise in water surface will cause the river or stream to rise in water surface will cause the river or stream to overflow the river bank somewhere in the corresponding overflow the river bank somewhere in the corresponding reach.reach.

• Flood stageFlood stage– An established gage height for a given location at which a rise An established gage height for a given location at which a rise

in water surface level begins to create a hazard to lives, in water surface level begins to create a hazard to lives, property or commerce. The issuance of flood (or in some property or commerce. The issuance of flood (or in some cases flash flood) warnings is linked to flood stage. Not cases flash flood) warnings is linked to flood stage. Not necessarily the same as bankfull stage.necessarily the same as bankfull stage.



• Minor floodingMinor flooding– Minimal or no property damage, but possibly some public Minimal or no property damage, but possibly some public

threat.threat.• Moderate floodingModerate flooding

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

• Major floodingMajor flooding– Extensive inundation of structures and roads. Significant Extensive inundation of structures and roads. Significant

evacuations of people and/or transfer of property to higher evacuations of people and/or transfer of property to higher elevations.elevations.



• Record floodingRecord flooding– Flooding which equals or exceeds the Flooding which equals or exceeds the

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

Stage – flood category Stage – flood category relationshiprelationship



Current conditionsCurrent conditionsCurrent conditionsCurrent conditions


Winter average temperaturesWinter average temperaturesWinter average temperaturesWinter average temperatures


Winter precipitationWinter precipitationWinter precipitationWinter precipitation


Snow depth—nowSnow depth—nowSnow depth—nowSnow depth—now


Snow depth vs. normal—nowSnow depth vs. normal—nowSnow depth vs. normal—nowSnow depth vs. normal—now


Snow water equivalent—nowSnow water equivalent—nowSnow water equivalent—nowSnow water equivalent—now


Snow pack—historical (2009)Snow pack—historical (2009)Snow pack—historical (2009)Snow pack—historical (2009)


Snow pack—historical (2008)Snow pack—historical (2008)Snow pack—historical (2008)Snow pack—historical (2008)


Stream levels—nowStream levels—nowStream levels—nowStream levels—now

14-day average flow14-day average flow


Ground frostGround frostGround frostGround frost

Observed frost depthObserved frost depth


Soil moisture—nowSoil moisture—nowSoil moisture—nowSoil moisture—now

Percentile keyPercentile key

~1: near record low~1: near record low<10: much below normal<10: much below normal10 -24: below normal10 -24: below normal

25-75: normal25-75: normal

76-90: above normal76-90: above normal>90: much above normal>90: much above normal~99: near record high~99: near record high

Soil moisture percentilesSoil moisture percentiles


Soil moisture—recentSoil moisture—recentSoil moisture—recentSoil moisture—recent

Percentile keyPercentile key

~1: near record low~1: near record low<10: much below normal<10: much below normal10 -24: below normal10 -24: below normal

25-75: normal25-75: normal

76-90: above normal76-90: above normal>90: much above normal>90: much above normal~99: near record high~99: near record high

Soil moisture percentiles—Soil moisture percentiles—December 2009December 2009



Latest spring flood outlookLatest spring flood outlookLatest spring flood outlookLatest spring flood outlook


Assumptions / notesAssumptions / notesAssumptions / notesAssumptions / notes

• This outlook is biased toward normal temperatures This outlook is biased toward normal temperatures and precipitation (i.e., climatology) during the outlook and precipitation (i.e., climatology) during the outlook period.period.– If conditions are not near normal then the probabilities in this If conditions are not near normal then the probabilities in this

outlook may not be valid.outlook may not be valid.• Ice jamsIce jams

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


50% flood category—now50% flood category—now50% flood category—now50% flood category—now


50% flood category—second outlook50% flood category—second outlook50% flood category—second outlook50% flood category—second outlook


Risk of Risk of minorminor flooding—now flooding—nowRisk of Risk of minorminor flooding—now flooding—now

West 1/3 of Iowa (Missouri River basin)West 1/3 of Iowa (Missouri River basin) East 2/3 of Iowa (Mississippi River basin)East 2/3 of Iowa (Mississippi River basin)


Risk of Risk of minorminor flooding—second outlook flooding—second outlookRisk of Risk of minorminor flooding—second outlook flooding—second outlook



Risk of Risk of moderatemoderate flooding—now flooding—nowRisk of Risk of moderatemoderate flooding—now flooding—now



Risk of Risk of moderatemoderate flooding—second flooding—second outlookoutlook

Risk of Risk of moderatemoderate flooding—second flooding—second outlookoutlook



Risk of Risk of majormajor flooding—now flooding—nowRisk of Risk of majormajor flooding—now flooding—now



Risk of Risk of majormajor flooding—second outlook flooding—second outlookRisk of Risk of majormajor flooding—second outlook flooding—second outlook



Finding river forecast informationFinding river forecast informationFinding river forecast informationFinding river forecast information







Final thoughtsFinal thoughtsFinal thoughtsFinal thoughts



• Most homeowners’ insurance policies do not cover Most homeowners’ insurance policies do not cover flood-related losses.flood-related losses.– To cover flood-related losses, flood insurance must typically To cover flood-related losses, flood insurance must typically

be purchased. This is in addition to homeowners insurance.be purchased. This is in addition to homeowners insurance.– Disaster assistance, if it’s available, is typically a loan that you Disaster assistance, if it’s available, is typically a loan that you

must repay with interest.must repay with interest.– In most cases, flood insurance requires a 30-day waiting In most cases, flood insurance requires a 30-day waiting

period before it becomes effective.period before it becomes effective.– Visit http://www.floodsmart.gov/ for more information.Visit http://www.floodsmart.gov/ for more information.



• Will we see another “2008”?Will we see another “2008”?– A big factor in 2008 was the heavy rainfall from May-June.A big factor in 2008 was the heavy rainfall from May-June.– High-end events such as the 2008 floods typically result from High-end events such as the 2008 floods typically result from

a sequence of additive factors. While one factor by itself may a sequence of additive factors. While one factor by itself may not lead to a high-end event, the combination of factors not lead to a high-end event, the combination of factors elevates the risk of a high-end event.elevates the risk of a high-end event.

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

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



• Spring flood outlook scheduleSpring flood outlook schedule– 1st Outlook: Friday, January 29, 2010.1st Outlook: Friday, January 29, 2010.– 2nd Outlook: Friday, February 19, 2010.2nd Outlook: Friday, February 19, 2010.– 3rd Outlook: Friday, March 5, 2010.3rd Outlook: Friday, March 5, 2010.– 4th Outlook (Optional): Friday, March 26, 2010.4th Outlook (Optional): Friday, March 26, 2010.



QuestionsQuestionsQuestionsQuestions

Contact information:Contact information:[email protected]@noaa.gov

515-270-4501515-270-4501http://www.weather.gov/desmoineshttp://www.weather.gov/desmoines

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Iowa Spring Flood Outlook