TWCA Annual Convention 2020

1. NWS West Gulf River Forecast
Center - And What You Need to
Know About Atlas 14
Gregory Waller
Service Coordination Hydrologist
NWS – West Gulf River Forecast
Center

2. NWS Mission
To provide weather, water, and climate data, forecasts and
warnings for the protection of life and property and
enhancement of the national economy.
NWS accomplishes this mission in close collaboration and
cooperation with State, Local, and Other Federal Agencies

3. Who We Are…
122 Weather
Forecast
Offices
13 River
Forecast
Centers
21 Center
Weather
Service Units

4. Perspective

5. The WGRFC Area
Diverse Water Issues
402,000 mi2 total area
87,000 mi2 in MX (Rio Grande headwaters)
320 forecast points, 15 major river systems
Over 65,336 miles of rivers (U.S. only)Snowpack
Water supply
• Hill Country Hydrology
• Flash flood threats
• Rapid river responses
• Cycles of Flood/Drought
Tropical cyclones
Storm surge
Coastal flooding
Prolonged river flooding
International Border – W
ater Allocation
Complex reservoir operations

6. It Is Important…
2019

7. Understanding River Forecast Categories
* NWS generally defines a river flooding as the point at which higher flows produce impacts. Common impacts are road closures, parks inundated, and water in structures.
River flooding does not have to be associated with other flood threats (flash, areal, coastal).
National Weather Service
www.weather.gov/wgrfc
Low Flow No Flooding ACTION STAGE
(AS)
MINOR
(MIN)
MODERATE
(MDT)
MAJOR
(MAJ)
River is within
banks, no
impacts
Buildings
threatened, roads
flooded and
impassable
River is higher,
possibly out of
banks
Impacts to
secondary roads
and lowlands near
floodplain
Significant impacts
to buildings,
structures and
roadways
Impacts to water
supply and
intake/outflow
structures
Only daily
operational points
will have forecasts
available
Flood Statements,
River Forecasts
Begin
Flood Warnings,
River Forecasts
Flood Warnings,
River Forecasts
• Evacuations may be
necessary
• Seek higher ground
• Potentially safer to
shelter in place
• Turn around don’t
drown!
• Homes and buildings
could be cut off as well
and should prepare
• Turn around don’t
drown!
• Stay away from
river’s edge as flow
speeds could be high
• Higher flows
• River could be out of
banks
• River can be running
higher than “normal”
so caution should still
be used
• Agriculture industry
will have to prepare
• Reservoir levels for
water storage
monitored
RECORD
(REC)
Impacts beyond
anything
previously
experienced
Flood Warnings,
River Forecasts,
Flood of Record
Statements
• Seek safety or
higher ground by
any means
• Impact areas
should be avoided
Flood Warnings,
River Forecasts
Only daily
operational points
will have forecasts
available SIGNIFICANT FLOODING

8. Understanding Flooding
West Gulf River Forecast Center NWSWGRFC @NWSWGRFC
Flooding Can Happen Anywhere. Are you “Flood
Prepared”?
Be Flood Aware. Turn Around. Don’t Drown!
Created March 8th 2015 @ 1:30pm CST
Urban / Small
Stream Advisory
issued when
flooding of small
streams, streets
and low-lying areas,
such as railroad
underpasses and
urban storm drains,
is occurring or is
imminent.
Flood Watch
issued when
flooding is
possible –
typically within a
6 to 48 hour time
frame before the
event.
Flash Flood
Watch
issued when flash
flooding is possible.
Flash Flood
Watches are
generally issued for
flooding that is
expected to occur
within 6 hours of the
event, which could
be heavy rainfall or
a dam /levee failure.
Flood Warning
issued when
flooding
conditions are
actually occurring
or are imminent.
Flash Flood
Warning
issued when flash
flooding is actually
occurring or
imminent. Flash
Flood Warnings are
issued for short-
term events, which
require immediate
action to protect
lives and property.
Flash Flood
Emergency
issued for a flash
flood situation that
presents a clear
threat to human life
due to extremely
dangerous flooding
conditions. This
signifies an area
that is witnessing
record flash
flooding.
How is a flash flood different from a standard flood?
Flash floods are characterized by rapid and heavy precipitation over a
short period of time (typically less than 6 hours) or levee/dam failures.
These can lead to raging water that can rip through river beds and
damage urban infrastructure.

9. The Vocabulary
• Public vs. “High End User”
Images courtesy of Jeff Lindner,
Harris County FCD

10. Now…The Hydrologic Cycle
Courtesy of the USGS Water Science School

11. River Forecasting 101

12. NOAA ATLAS 14 – VOLUME 11

13. NOAA Atlas 14 Meteorology
Research Incentive
§ What is it:
§ Precipitation frequency estimates
§ How much rain in a 100-year storm event
§ Non-regulatory
§ Benefits:
§ Better understanding of the risk from
extreme precipitation events
§ Infrastructure design – parking lots to
dams
§ Floodplain mapping (NFIP), where can
we safely construct new neighborhoods
§ Preparedness or mitigation planning
§ Schedule: Complete
§ Volume 11 (Texas) released September
2018
§ Documentation published January 2019
§ Further studies in the works with NOAA
NOAA Atlas 14

14. These Chips are Stale!
NWS Technical Paper No. 40 (1961)
NWS Technical Paper No. 49 (1964)
NWS Hydro-35 (1977)
Atlas of Depth-Duration
Frequency of Precipitation Annual
Maxima for Texas (2004)
NOAA/NWS USGS
TP40: 100-yr 24hr USGS: 100-yr 24hr

15. Importance of New Data
• Hydro-35/TP 40 - ~20-years of record
• USGS - ~35-years of record
• NA14 - ~60-years of record
***Insert Some Hearsay!!***

16. Nuts and Bolts of Atlas 14
• Approach: Regional frequency analysis approach based on L–moment statistics calculated from
annual maximum series (AMS)
Data Collection
Gathered data from 11,930 stations;
retained 3,900 stations
Extracted AMS from each station
Regionalization Approach
Trading space for time
Results on more accurate estimates of
extreme quantiles
Frequency Analysis
Fit multiple distributions to identify
best fit for each station and duration
Ultimately used GEV probability
distribution
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999
0
1
2
3
4
5
6
Dailyprecipitation(in)
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999
0
1
2
3
4
5
6
Year
Dailyprecipitation(in)
Source: NOAA Atlas 14 Volume 11 Version 2

17. More Nuts and Bolts
Uncertainty Bounds
• Utilized a Monte-Carlo approach to
estimate 90% confidence bounds
• Simulated 1,000 data sets for each
station and duration
Precipitation Grids
• Accounts for variations in terrain,
coastal proximity, mean annual
precipitation, and distance from station
Source: NOAA Atlas 14 Volume 11 Version 2

18. Precipitation Data
• Digitized pre-1948 data from NCEI’s Climate
Database Modernization Program
– Data screened for quality
– Merged nearby stations (within 3–5
miles, ~same elevation)
– Length of record (>30 years of data)
– Extracted AMS for durations between 15-
min and 60-day from precipitation
records across the state
• Average record length ~60 years
• Records extended through December 2017,
where available
– A few stations included data through June
2018

19. Regionalization
• Initial region for each station
consisted of closest 15 gages
• Included highest 1-hour or 1-day
peak within 60 miles
• Refinement of regional gages
based on:
– Distance from target station
– Topography
– Mean annual maxima
– Maximum recorded values
– Record lengths
• Analyzed L-moment statistics for
each gage within the region
• Typical density: 15-25 gages,
700-1,800 data years (daily
record)

20. Frequency Analysis
• Multiple probability distributions fit to each station and duration
• GEV distribution chosen for all durations based on multiple goodness of fit
tests
– Kolmogorov-Smirnov and Χ2 Test
• Precipitation frequency estimates based on regional L-moments from each
station
– Some smoothing required

21. Hurricane Harvey???
• Broke all multi-day rainfall records (not just Texas)
• Official highest amount of rainfall ever to fall on the
continental U.S.
• Harvey affected results of 2-day through 20-day estimates
for 1000-year event
• No significant skew for the 1000-year 24-hour or 1000-
year 30-day results

22. Uncertainty Bounds
• Monte Carlo simulation accounts for inter-station dependencies
• 1,000 simulations simulated for each station
• Accounts for natural variability (uncertainty in parameters) but not
knowledge uncertainty (selected distribution)

23. Geospatial Mapping
• Interpolated based on mean annual maxima and 2-
year spatial pattern using a hybrid statistical-
geographical approach developed by PRISM (Oregon
State University)
• Multiple iterations were made to insure satisfactory
spatial patterns and peer review comments
– Bulls eyes, geographic features, logical distribution of
precipitation

24. NOAA Atlas 14 Access
• All data and resources located on the
Precipitation Frequency Data Server (PFDS)
– http://hdsc.nws.noaa.gov/hdsc/pfds/

25. Results

26. Comparison To Previous Studies
NA 14
100-yr 24-hr
NA 14 – TP40
100-yr 24-hr

27. Comparison To Previous Studies
NA 14
100-yr 24-hr
NA 14 – USGS
100-yr 24-hr
Up to 30% increase for 100-year 24-hour precipitation from USGS!!!

28. Are We Done Yet???
• Areas of concern
• Short period of observations
• Relationship between extreme weather
variability and climate change/climate
variability
• Evaluation of non-stationarity with
respect to estimates
• NA 14 are point estimates
• Need to update Area Reduction Factors
(ARF)
• Need for additional studies
• Other methods to estimate precipitation
frequency Trend analysis
• Storm studies (design storms)
• Atlas 14 Upkeep
• Who will update in 10-20 years?
• How will it be funded?

29. NOAA Atlas 14 Impacts
• More accurate estimates spatially →
better preparedness and response
• Able to better quantify the degree or
risk of flooding at a location
• New delineation of floodplain maps
• Frequency of precipitation does not
equate to frequency of flooding
• Better planning/design of infrastructure
• More resilient towards future storms
• Non-Regulatory
• No current requirement for
communities or agencies to use
Atlas 14 values

30. Comparison To Previous Studies
NA 14
100-yr 6-hr
NA 14 – USGS
100-yr 6-hr

31. Comparison To Previous Studies
NA 14
100-yr 1-hr
NA 14 – USGS
100-yr 1-hr

32. Questions/Comments?
Gregory Waller
Service Coordination Hydrologist
NWS West Gulf River Forecast Center
greg.waller@noaa.gov