Building a Weather-Ready Nation

Increasingly Common Heavy Rainfall Events

in IowaJeff Zogg – NWS Des Moines, IA

Doug Kluck – NOAA Central Region Climate Services

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Building a Weather-Ready Nation

Agenda

The ProblemResearch & MethodologySummary & Impacts

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Building a Weather-Ready Nation

Flooding is Iowa’s #1 weather-related hazard.43 of 55 (~80%) Presidential Disaster

declarations.Iowa ranks #2 in the U.S. for flood-related

losses.Flooding—happening more frequently?

A major driver of flooding—especially flash flooding—in Iowa is heavy rainfall.Heavy rainfall events seem to

be occurring more frequently.Perception or reality?

The Problem

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Ames/Iowa State University, Aug 2010 –Des Moines Register

Building a Weather-Ready Nation

Mallakpour & Villarini (2015)Flood frequency increasing, not severity

Support of Flooding Trends

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Changes in flood magnitude,1962-2011

Changes in flood frequency,1962-2011

Northern U.S.

Building a Weather-Ready Nation

Used two frequency publications for IowaRainfall Frequency Atlas of the Midwest

(1992)Huff & Angel – MRCC & Illinois SWS

NOAA Precipitation-Frequency Atlas 14, Volume 8 (2013)

Research & Methodology

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Overview

Building a Weather-Ready Nation

Base: 275 stations (IA: 41) – NWScoop sites w/ POR > 50 yrs; supplemented w/ other data

Max precip conversion factors≥ 1 day: NWS empirical factors used

(& verified)< 1 day: factors from 1948-1983

data for 55 recording rain gages in IL & surrounding states – compared & verified with other studies

Partial Duration Series (PDS) extracted

MRCC Rainfall Frequency Atlas

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Data & Analytical Approach

Building a Weather-Ready Nation

3 techniques evaluatedLog-log graphical (Huff-Angel)Maximum likelihoodL-moments

No significant differencesBy design, L-moments tends to give more

conservative (i.e., lower) precip values for same recurrence interval

Huff-Angel technique selectedAllows analyst to incorporate professional

knowledgeCutoff near 100-yr event

MRCC Rainfall Frequency Atlas

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Statistical Methods

Building a Weather-Ready Nation

Point-basedIsohyetal maps derivedMore susceptible to subjectivity

& inherent variabilityShows small-scale variability

Areal-basedTabular data derivedNWS climate divisions used (& verified

appropriate)Average frequency distributions usedMitigates impacts of sampling errors

MRCC Rainfall Frequency Atlas

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Output

Iowa NWS climate divisions

Building a Weather-Ready Nation

Base: IA – 276Most were NWS coop sites

Max precip conversion factors≥ 1 day: similar to Bull71< 1 day: hourly data used then

correction factors applied; resultsbelieved similar to Bull71

Annual Maximum Series (AMS) extracted, then PDS obtained from AMS

NOAA Atlas 14, Volume 8

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Data & Analytical Approach

Iowa daily stations used

Building a Weather-Ready Nation

L-moments techniqueLess impacts of outliers

Upper & lower 90%confidence intervalscalculatedMonte-Carlo simulation approach

Longer recurrence intervalsRegion of influence approach

NOAA Atlas 14, Volume 8

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Statistical Methods

Interactively removing stations from aregion—Minnesota

Building a Weather-Ready Nation

MRCC Bulletin 71

Both NOAA Atlas 14,

Vol 8

2-month

3-month

4-month

6-month

9-month

1-year 1-year 1-year

2-year 2-year 2-year

5-year 5-year 5-year

10-year 10-year 10-year

25-year 25-year 25-year

50-year 50-year 50-year

100-year 100-year 100-year

200-year

500-year

1000-year

Overlapping Return Pds & Durations

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MRCC Bulletin 71 Both NOAA Atlas 14, Vol 8

5-minute

6-minute

10-minute 10-minute 10-minute

15-minute 15-minute 15-minute

30-minute 30-minute 30-minute

1-hour 1-hour 1-hour

2-hour 2-hour 2-hour

3-hour 3-hour 3-hour

6-hour 6-hour 6-hour

12-hour 12-hour 12-hour

18-hour

24-hour 24-hour 24-hour

2-day 2-day 2-day

3-day 3-day 3-day

4-day

5-day

7-day

10-day 10-day 10-day

20-day

30-day

45-day

60-day

Building a Weather-Ready Nation

Clip geographic area to IowaCompute zonal statistics

84 individual files, each with 228,347 gridsIndex each of the 84 rasters to

between 0 & 1Create average index value

GIS Analysis Procedure

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NOAA Atlas 14, Volume 8

South Skunk River @ Colfax, Iowa(Aug 2010)

𝑥𝑖=𝑥−𝑟𝑚𝑖𝑛

𝑟 𝑚𝑎𝑥−𝑟𝑚𝑖𝑛

Building a Weather-Ready Nation

GIS Results

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Building a Weather-Ready Nation 14

Building a Weather-Ready Nation

Documented Bull71 & Atlas14 results for all 9 Iowa climate sections

Calculated statewide averagesCompared all 12 common durations

of Bull71 vs. Atlas14 for all 7 common Tr s

Data Analysis Procedure

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Spreadsheet

Cedar River @ Cedar Rapids, Iowa (2008) – Scott Olson/Getty Images

Building a Weather-Ready Nation 16

ResultsData Analysis Procedure

Great Flood of 1993 – Des Moines –Des Moines Water Works

𝑦=(𝑐 ln 𝑥 )+𝑏x = Tr (yr); y = precip value (in)

𝑥=𝑒( 𝑦−𝑏 )

𝑐

Building a Weather-Ready Nation

All 7 common Tr s (↕) of Bull71 vs. Atlas14 for all 12 common durations (↔)Bull71 equation correlation: 0.9968Atlas14 equation correlation: 0.9934# Atlas14 elements > Bull71: 78 (93%)# Atlas14 elements < Bull71: 6 (7%)

Data Analysis Procedure

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Results

Iowa Hwy 92—Muchakinock Creek near Oskaloosa (Aug 2010) –

NWS Des Moines

Building a Weather-Ready Nation 18

ResultsData Analysis Procedure

Building a Weather-Ready Nation

BUT…Not simple relationship for width vs.

duration & Tr but (Atlas14 – Bull71) remains within 90% intervals15-min, 2-yr event: |∆| = 0.004 in6-hr, 100-yr event: |∆| = 0.834 in (Atlas14 ∆ ≈

1.54 in)

HOWEVER...

90% Confidence Intervals

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Remember Them?

Building a Weather-Ready Nation

Bull71 – YesExamine ratio of 2nd 40-yr period

to 1st 40-yr periodRatio > 1.1

“The increases appear to be greaterthan expected from natural climaticvariability”

“Findings suggest that the assumption of a stationary time series for fitting statistical distributions to historical precipitation data may be invalid.”

“An update on the order of every 20 years would be appropriate to capture any substantial changes.”

Rainfall Frequency Distribution

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Changing Over Time?

Building a Weather-Ready Nation

Studies suggest flood frequency—not severity—increasing over timeAlso suggest increases in heavy rainfall

days but not rainfall severityAtlas14 values > Bull71 values, but

Bull71 values fall within the Atlas14 90% confidence intervals

Bull71 suggests heavy rainfall frequency distribution is changing over time, Atlas 14 says precip AMS is not changingDoes not address precip PDS trend

Summary & Impacts

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Building a Weather-Ready Nation

Mallakpour & Villarini (2015)Findings similar to 3rd Nat’l Climate

Assessment (2014)

Support of Precip PDS Trends

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Changes in heavy rainfall magnitude,1948-2012

Changes in heavy rainfall frequency,1948-2012

Northern U.S.

Building a Weather-Ready Nation

Most likely explanationPrecip PDS trends are changing – heavy

rainfall becoming more frequent but not more severe

What if rainfall trends are to blame?Under-designed municipal storm water

systems?More frequent flash flooding

especially urbanIncreased soil erosion

Timing of rainfall important

Summary & Impacts

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Central Iowa soil erosion –ISU Extension / Drake Larson

Continued

Building a Weather-Ready Nation

Thank You

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For questions & additional information:NWS Des Moines, IA

http://www.weather.gov/desmoinesEmail: jeff.zogg@noaa.gov

Phone: 515-270-4501