Tye W. Parzybok, Shauna Bokn, R. Jason Caldwell, Katie...

Tye W. Parzybok, Shauna Bokn, R. Jason Caldwell,

Katie Ward, Alyssa H. Dietrich, Brian Crow

MetStat, Inc.

Fort Collins, Colorado

2017 Dam Safety, San Antonio, TX Sept. 2017

What made the rainfall unique?

1. The large areal extent of extremely rare

rainfall

2. The storm efficiency caused 1000+ year

rainfalls over a large range

of durations – from 6-hours

to 5-days

Close to all-time US record

Detailed storm precipitation analysis with MetStorm®

Rain gauge data from numerous networks

✓ Total originally considered: 2,270

✓ Final gauge counts – 1,799

• Daily-reporting: 804

• Hourly-reporting: 995

5-minute Dual-Polarization radar-estimated precipitation mosaic

5-minute NEXRAD radar reflectivity

Hourly radar confidence grid

1-hour satellite-estimated precipitation

MetStorm® Total

Storm (5-day)

Precipitation

51.65”

September 25-30, 2017

• Possible under-catch

due to high winds –

maybe as much as

20%

• Considered storm

center for the 120h

total but 1h, 6h, 12, etc

might be different

places.

Average Recurrence Interval (ARI) of 100-years

=

Annual Exceedance Probability (AEP) of 0.01 (1/100) or 1.0 x 10-2

=

1% chance of occurring in any given year

ARI of rain does not equal ARI of flood

ARI of 250,000-years

=

AEP of 0.000004 (1/250,000) or 4.0 x 10-6

=

0.0004% chance of occurring in any given year

Available Precipitation Frequency Analyses for putting the rainfall

into a probabilistic perspective

Document Durations Frequencies

NOAA Atlas 14 (available in 2018) NA NA

Technical Memo NWS Hydro-35 (1977) 5 – 60 minute 2 – 100 years

Technical Paper 40 (1961) 1 – 24 hours 1 – 100 years

Technical Paper 49 (1964) 2 – 10 days 2 – 100 years

Depth-Duration Frequency of Precipitation for Texas

USGS Report 98-4044 (Asquith 1998)

15-minute – 7-days >= 1 year

MetStat Precipitation Frequency Analysis (2016) 6- and 24-hour 1 – 1,000 years

Depth-Duration Frequency of

Precipitation for Texas USGS Report 98-

4044 (Asquith 1998)

• Based on 38,120 cumulative years of

record

• Precip. Freq. = GEV(mean, L-skew, L-cv)

Annual exceedance probabilities (AEPs) provided are estimated

using GEV parameters from USGS Report 98-4044 (Asquith 1998).

We recognize that these estimates contain considerable uncertainty,

but represent the only 3- and 5-day precipitation-frequency data

publicly available at this time. Precipitation-frequency estimates by

storm type/mechanism are required to more accurately place this

event into proper historical/probabilistic perspective.

Based on MetStat 2016

17.43”

Based on MetStat 2016

30.56”

Based on Asquith 1998

46.88”364,000-year

(AEP = 2.8 x 10-6)

Annual exceedance probabilities (AEPs) provided in this graphic are estimated using GEV parameters from USGS

Report 98-4044 (Asquith 1998). We recognize that these estimates contain considerable uncertainty, but represent

the only 5-day precipitation-frequency data publicly available at present. Precipitation-frequency estimates by storm

type/mechanism are required to more accurately place this event into proper historical/probabilistic perspective.

51.65”542,000-year

(AEP = 1.9 x 10-6)

Based on Asquith 1998

Estimating Precipitation AEPs out to 10-9

Example Areal Precipitation-Frequency Relationship and 90%

Uncertainty Bounds for 48-hour Mid-Latitude Cyclone Storm

Precipitation Tennessee Watershed.

PMP ~ AEPs 10-4 to 10-7

Disconnect between PMP

“Limit” and shape of precip.

freq. curves

Attaining these AEPs

✓ Trading space for time

✓ Spatial mapping of

statistical parameters

✓ Include uncertainty

bounds

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

22.0

48

-HO

UR

P

RE

CIP

ITA

TIO

N (

in)

ANNUAL EXCEEDANCE PROBABILITY

10-910-3

Extreme Value Type 1 Plotting Paper

10-2 10-4 10-5 10-80.50.9 10-1 10-6 10-7

Holston River System - Cherokee Dam 3,425-mi2

Mean Frequency Curve Best-Estimate

90% Uncertainty Bounds 48-Hr PMP

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

22.0

48

-HO

UR

P

RE

CIP

ITA

TIO

N (

in)

ANNUAL EXCEEDANCE PROBABILITY

10-910-3

Extreme Value Type 1 Plotting Paper

10-2 10-4 10-5 10-80.50.9 10-1 10-6 10-7

Holston River System - Watauga Dam 467-mi2

Mean Frequency CurveBest Estimate

90% Uncertainty Bounds

Preliminary 2/16/2016

48-Hr PMP

PMP ~ AEPs sometimes to 10-9

Skew and distribution are

largest contributors

Distribution used: GEV

▪ Customary for extreme

precipitation (and flood)

frequency studies

USACE Hydrologic Hazards Development Project for the Trinity

River Basin in Texas

Late next year (2018) we will have the information to refine our

AEP analysis

PF estimates

for 4 storm types

TVA Hydrologic Hazards Project

500+ Year 24-hour Rainfall in Areas of the Southeast

Dry <1 yr 1yr 2yr 4yr 6yr 8yr 10yr 20yr 40yr 60yr 80yr 100yr 200yr 300yr 500yr

24-hour ARIs

ending @ noon

CDT Sept. 11, 2017

Tye W. Parzybok, CCM, GISP

tyep@metstat.com

CNBC

REAL – Houston, TX September 30, 2017

FAKE

Make sure to get your

information from reliable

sources!