Iowa Spring Flood Outlook

National Weather ServiceNational Weather ServiceDes Moines, IADes Moines, IA


Iowa Spring Flood OutlookIowa Spring Flood Outlook

February 22, 2010February 22, 2010

Jeff ZoggJeff Zogg


TopicsTopics

• 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-glance

• High risk of significant flooding continuesHigh risk of significant flooding continues in many in many areasareas– 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 mid-March into late April.mid-March into late April.

• Above normal risk of Above normal risk of flooding from ice jams.flooding from ice jams.• Higher than normal risk of flooding also in areas that Higher than normal risk of flooding also in areas that

have have little to no snow cover.little to no snow cover.• The flooding potential is more sensitive than normalThe flooding potential is more sensitive than normal to to

the occurrence and timing of weather events such as the occurrence and timing of weather events such as rapid warm ups and heavy precipitation.rapid warm ups and heavy precipitation.



DefinitionsDefinitions



• 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 conditions


Winter average temperaturesWinter average temperatures


Winter precipitationWinter precipitation


Snow depth—nowSnow depth—now


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


Snow water equivalent—nowSnow water equivalent—now


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


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


Stream levels—nowStream levels—now

14-day average flow14-day average flow


Ground frostGround frost

Observed frost depthObserved frost depth


Soil 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—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 outlook


Assumptions / 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

– Any ice jams which do occur may result in locally higher river Any ice jams which do occur may result in locally higher river stages and rapid river rises. This outlook does not take into stages and rapid river rises. This outlook does not take into account potential flooding from ice jams. Any flooding from account potential flooding from ice jams. Any flooding from ice jams would be above and beyond the flooding potential ice jams would be above and beyond the flooding potential indicated in this outlook.indicated in this outlook.


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


50% chance flood category—last outlook50% chance flood category—last outlook


Risk 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—first outlook flooding—first outlook



Risk of Risk of moderatemoderate flooding—now flooding—now



Risk of Risk of moderatemoderate flooding—first outlook flooding—first outlook



Risk of Risk of majormajor flooding—now flooding—now



Risk of Risk of majormajor flooding—first outlook flooding—first outlook



90-day detail—Waterloo90-day detail—Waterloo


Weekly detail—WaterlooWeekly detail—Waterloo


90-day detail—Cedar Rapids90-day detail—Cedar Rapids


Weekly detail—Cedar RapidsWeekly detail—Cedar Rapids


90-day detail—Humboldt90-day detail—Humboldt


Weekly detail—HumboldtWeekly detail—Humboldt


90-day detail—DSM 2nd Ave90-day detail—DSM 2nd Ave


Weekly detail—DSM 2nd AveWeekly detail—DSM 2nd Ave


90-day detail—DSM Fleur90-day detail—DSM Fleur


Weekly detail—DSM FleurWeekly detail—DSM Fleur


90-day detail—Burlington90-day detail—Burlington


Weekly detail—BurlingtonWeekly detail—Burlington


90-day detail—Spencer90-day detail—Spencer


Weekly detail—SpencerWeekly detail—Spencer


Detail graphics—notesDetail graphics—notes

• The 90-day and weekly detail graphics are available The 90-day and weekly detail graphics are available online.online.– The following slides show how to find this information.The following slides show how to find this information.


Finding river forecast informationFinding river forecast information







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

http://www.floodsmart.gov/



• 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.



QuestionsQuestions

Contact information:Contact information:[email protected]

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

mailto:[email protected]

http://www.weather.gov/desmoines

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Iowa Spring Flood Outlook

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