Precipitation Frequency Estimates For 

The Nation And Extremes – A Perspective

Geoff Bonnin

National Oceanic and Atmospheric Administration

National Weather Service

Office of Hydrologic Development

Workshop on

Probabilistic Flood Hazard Assessment (PFHA) January 29 - 31, 2013
Topics

• Status of NOAA Atlas 14, Precipitation Frequency Atlas 

of the United States

• Uncertainty of Estimates

• Potential Impact of Climate Change on Precip Frequency

• The Semantic Problem

• Exceedances

• Climate Change and PMP
NOAA Atlas 14

Precipitation Frequency Atlas

of the United States

• Begun in 2000

• Published as volumes by project area

– as funds become available

• Average Recurrence Interval: 1 – 1,000 years

• Durations: 5 minutes – 60 days

• Error Estimates: 90% confidence intervals

• Locally Relevant: 30 arc-sec resolution

• User Friendly: web based, interactive
NOAA Atlas 14 Status
+/- 30%: sparsely instrumented, shorter record; to 

+/- 10%: more densely instrumented, longer record
1.58 2 3 4 5 10 20 50 100 5001000

0

5

10

15

20

Average reccurence interval [yrs]

Precipitation (in)

Distribution:

GEV

GLO

GNO

GPA

KAP

PE3

WAK

ARI 

(years)

Potential PF 

range

100 11-13 in

1000 13 -22 in

Testing 7 distributions

- Estimates very sensitive to distribution selection 

for ARI > 100 years 

35

Distribution Selection
Potential Impact of Climate 

Change

“Management and mission-oriented agencies with 

public-sector responsibilities have been provided 

with marginally useful scientific information about 

the likely manifestations of future climate change.”

“There are insufficient interactions and knowledge 

exchange between climate scientists, water 

scientists, and engineers and practitioners to solve 

these challenges.”

Global Change and Extreme Hydrology: Testing Conventional Wisdom

National Research Council, Water Science and Technology Board, 2011
Climatology Semantics

•“It is likely that the frequency of heavy

precipitation events ... has increased over most 

areas.

”

– IPCC AR4, Climate Change 2007: Synthesis Report

•“Groisman et al. (2005) found significant 

increases in the frequency of heavy and very 

heavy (between the 95th and 99.7th percentile of 

daily precipitation events)”

– IPCC AR4 Working Group I 

• These and similar statements in the literature 

define terms such as

– “heavy”, “very heavy”, or “extreme” precipitation

– Sometimes differently!
For Example

• Groisman et al 2005

– “... we define a daily precipitation event as heavy

when it falls into the upper 10% and/or 5% of all 

precipitation events;

as very heavy when it falls into the upper 1% and/or 

0.3% of precipitation events;

and extreme when it falls into the upper 0.1% of all 

precipitation events.”

– “The return period for such events ... varies, for 

example, from 3 to 5 yr for ... very heavy

precipitation events.”
Civil Engineering Semantics

• Use precipitation frequency estimates

– average annual exceedance probabilities (AEP) 

or

– average recurrence intervals (ARI)

• Heavy, very heavy, and extreme rainfall:

– generally subjective terms

• Use many durations; not just daily

– NOAA Atlas 14 provides 5 min through 60 days
Let’s Count Exceedances

• Thresholds

– Use actual NOAA Atlas 14 thresholds

• Not a fixed value or a percentile of a time series

– For:

• 1 year – 1,000 year ARI

• Durations: 6 hours – 45 days

• Use Partial Duration Series

– Complies with ARI definition

• Count Number of Exceedances

– For each station

• Sum for each year over the all stations in the domain

– Normalize for varying number of stations each year

• Linear regression for all ARI/durations
NOAA Atlas 14 Status
Example Trends in Exceedances
Trends in Exceedances

Generally statistically significant except for 6 hour durations
Trends in Exceedances (continued)

Generally not statistically significant except for daily durations above 2 yr ARI
Spatial Coherence of AMS Means
Spatial Coherence of AMS Means
Precip Frequency Conclusions

• Climate community statements on trends in 

rainfall intensity

- Do not address frequencies and durations 

required for civil infrastructure

• Climate community statements are being 

misinterpreted

- by Civil Engineers and probably the public

• Historical trends in number of events

- Are small compared to uncertainty of IFD values

• Need better guidance on potential impact of 

climate change on IFD curves

- In range relevant to civil infrastructure
Can Climate Change Make the 

“Perfect Storm” More Perfect?
Geoff Bonnin

301-713-0640 x103 

Geoffrey.Bonnin@noaa.gov