National Weather Service Flash Flood Modeling and Warning Services

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National Weather Service

Flash Flood Modeling and Warning

Services

Seann Reed, Middle Atlantic River Forecast Center Peter Ahnert, Middle Atlantic River Forecast Center

August 23, 2012

USACE Flood Risk Management and Silver Jackets Workshop Harrisburg, PA

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The Flash Flood Problem

Intense rainfall, ice jam, or dam failure

Rapid rise

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Issue timely watches and warnings

Collect data Analyze data and forecast

What we do

Ongoing Challenges:

TIMELINESS, LOCATION SPECIFICITY, UNGAUGED STREAMS, VERIFICATION

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Topics

• Observed and forecast rainfall

calculations

• Hydrologic modeling

• Verification

• Communicating warnings,

outreach

• The path forward, emerging

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RFC – WFO Roles in Flash Flood Services

• RFCs prepare Flash Flood Guidance and Site Specific Models

• WFOs use flash flood monitoring tools along with rainfall observations, forecasts and

monitoring tools to issue flash flood watches and warnings when appropriate

WFOs RFCs

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Observed and Forecast

Rainfall

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Observed and Forecast Rainfall

• Rain gauge data

• Radar data

• Satellite data

• Multi-sensor rainfall products

• QPF guidance

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Rain Gauge Data

• NWS cooperative observers (daily)

• CoCoRAHS (daily)

• Telemetered equipment (hourly, sub-hourly)

–

E.g. AFWS/IFLOWS (regional)

–

Automated Data Collection Platforms (DCPs) from

multiple state and federal agencies

• HADS – national processing, acquisition and

distribution of DCP data

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Stations with hourly or sub-hourly

reporting are most useful for flash flood modeling

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Automated Flood Warning Systems (AFWS) Using Integrated Flood Observing and Warning System (IFLOWS) Technology

http://afws.erh.noaa.gov/afws/national.php

• Joint effort between NWS and states

• 1663 rain gauges • 12 states

• 5 minute

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14,700 data collection locations

Real time data acquisition, processing and distribution. Data Collection Platforms (DCPs) operated by more than 100 collaborators

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Satellite and Radar Rainfall Estimates

Satellite 24-hour precipitation Ending 1200 UTC, 5 Jan 2007

More complete spatial coverage than gauge or radar estimates

Radar 24-hour precipitation Ending 1200 UTC, 5 Jan 2007

Generally higher accuracy than satellite estimates

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Quality-Controlled Precipitation Gage Measurement GOES Satellite Estimate Radar Estimate Climate patterns MPE Software Forecaster Analysis

Hourly Quantitative Precipitation Estimate (QPE) from Multi-Sensor Precipitation Estimator (MPE) – for RFCs

• Highest quality NWS precipitation

• Manual quality control

• Latency too long for very flashy events

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MPE Gauge-only Rainfall (~2.3 in.) _{MPE Radar-only Rainfall (~1.0 in.)}

Bias-adjusted Radar Rainfall (~1.7 in.) _{MPE Multisensor Rainfall (~2.2 in.)} Characteristics of Rain Gauge and Radar Estimates

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High Resolution Precipitation Estimator (HPE); High

Resolution Precipitation Nowcaster (HPN) – for WFOs

• HPE

– Rainfall accumulations at <5 minute intervals, 1km spatial resolution, < 1

hour latency (better than MPE but no manual QC)

– Uses radar-rainfall estimates from multiple radars

– Bias correction with recent MPE gauge/radar information

• HPN

– Extrapolation forecasts using “lag correlation” pattern matching

– 15 minute temporal resolution, 4km spatial resolution, 1-2 hour forecast

• Production

– Produced in AWIPS environment at each WFO

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Hour 1 HPN Forecast (mm) 23Z April 21st to 00Z April 22nd₂₀₀₉

HPE Precipitation (mm) 23Z April 21st to 00Z April 22nd₂₀₀₉

MPE Precipitation (mm) 23Z April 21st to 00Z April 22nd₂₀₀₉

Hour 2 HPN Forecast (mm) 23Z April 21st to 00Z April 22nd₂₀₀₉

O bs er va tio ns Fo rec as ts

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Probabilistic Precipitation Guidance from

NCEP’s

• Available every 6 hours out 3 days

• Good for situational awareness

• Can be used with Flash Flood Guidance (discussed next)

• No explicit spatial temporal patterns – cannot be used for high resolution hydrologic modeling

Probability of rainfall exceeding 0.25 in a given 6 hour period

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Hydrologic Modeling for Flash

Floods

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How much rain is needed for

Flash Flooding?

•

Rainfall needed dependent on:

– Antecedent conditions – Basin characteristics

•

Threshold Runoff

– Depth of runoff required to exceed

flood flow in a channel (over a certain time and drainage area)

•

Flash Flood Guidance

– Depth of rain required to exceed

flood flow in a channel (over a certain time and drainage area)

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0 1 2 3 4 5 0 1 2 3 4 5 C umul at iv e R unof f (i n)

Cumulative Rainfall (in)

SMI = 0.5 2.5 2.0 1.5 3.0 3.5

Flash Flood Guidance (FFG) (in)

Thr eshol d R unof f

In this basin, FFG ranges from 2 to more than 4 inches depending on antecedent moisture

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Flash Flood Guidance (FFG) Products

•

Issued by RFCs for

different rainfall

durations

•

Updated 1-4 times

daily

Gridded FFG Headwater FFG

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FFG Pros and Cons

Pros

• Easy to use product –

used by WFOs and other outside organizations

• Can use with

probabilistic

precipitation forecasts

• Used internationally w/

satellite data – Central America / Caribbean (Hydrologic Research Center)

Cons

• Methodology inconsistencies exist

among neighboring RFCs

• West – rainfall rate more

important than soil moisture

• Gridded FFG does not include

routing

• Difficult to verify

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• Add image

from Pete J

and text….

Flash Flood Monitoring and Prediction (FFMP) software allows Forecasters to see current radar derived rainfall rates, rainfall totals, and compares data to Flash Flood Guidance. This software helps forecasters identify areas of possible flooding sooner.

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• Distributed hydrologic modeling –

combining physical, conceptual,

statistical

• Leveraging Community Hydrologic

Prediction System (CHPS) technology

• Interagency collaboration

Hydrologic Modeling for Flash Floods,

Moving Forward

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1. Rainfall and soil properties averaged over basin

2. Single rainfall/runoff model

computation for entire basin or sub-basin

3. Prediction/verification only at outlet point

Lumped Distributed

Lumped Versus Distributed Models

1. Rainfall, soil properties vary by grid cell 2. Rainfall/runoff model applied separately to

each grid cell

3. Prediction/verification at any grid cell Distributed models are well-suited for flash flood prediction and monitoring, offering high-resolution streamflow at outlet and interior points with ability to route flow

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Hydrologic Soil Group

Surface Texture

UZTWM for SAC Model

UZFWM / UZTWM (Koren et al., 2000; Koren et al., 2001; Anderson et al., 2005) From Soil Properties to SAC-SMA Model Parameters

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14 UTC 15 UTC

16 UTC _{17 UTC}

Distributed Hydrologic Model –Threshold Frequency Approach (DHM-TF)

4 Times on 1/4/1998

Frequencies are derived from routed flows on a 2 km network.

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Community Hydrologic Prediction System

(CHPS)

http://www.nws.noaa.gov/ohd/hrl/chps/index.html • New operational hydrologic modeling software architecture

• Easier to leverage models and data from other groups in

operations • Configurable/flexible GUIs • Client-server architecture FC FEWS FEWS Models NWS Models USACE Models Other Models CHPS

Client server architecture may allow WFO to run flash flood hydrology models

maintained on the RFC CHPS systems.

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Recent Interagency Collaboration on Dam Break

Forecasting

NWS GeoSMPDBK

• GIS Pre-processor for SMPDBK

• Produces SMPDBK input file

FEMA GeoDam- BREACH

Geospatial Dam Break,

Rapid EAP, Consequences, and Hazards

• Includes GeoSMPDBK

functionality and much more. . .

• inundation maps +

velocity maps + time to peak maps

• EAPs • other

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NWS Use of Unsteady HEC-RAS Models

Current HEC-RAS model applied to mainstem only 2183 mi2 upstream 3200 mi2_{at outlet/coastal boundary} Experimental HEC-RAS applications to smaller basins (~ 100 km2 ) will allow flood forecast mapping at flash flood scales

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Communicating warnings,

outreach

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NO BENEFIT unless the end user gets the warning

in time and understands it so they can act!

• Homeowners • Business owners • Drivers • Police/emergency responders • EMT • Transportation Department • Local Emergency Official • recreational boaters

etc. . . .

Communicating the Warning Message

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Flood Safety Education – Best Practices

Check for the latest forecast

Get Insurance

http://www.weather.gov/floodsafety/ index.shtml

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Flash Flood Verification Data

(

all options have limitations

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• NWS Storm Database – reports from local

observers, officials, police, etc.

• Gauge-based verification

• Severe Hazards Analysis and Verification

Experiment (SHAVE)

– Gourley, Jonathan J., Jessica M. Erlingis, Yang Hong, Ernest B. Wells, 2012: Evaluation of Tools

Used for Monitoring and Forecasting Flash Floods in the United States. Weather and Forecasting, 27, 158–173.

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The use of FFMP has improved the quality

and timeliness of warnings

•

A study conducted in 2010 compared national

verification statistics

– Study looked at data from 1997-2000 (pre-FFMP) to those from

2006-2009 (post FFMP installation). Years of Study Probability of Detection (POD) % of warnings with > 0 min lead time Average lead time for

warnings False Alarm Ratio (FAR) 1997-2000

0.85 66%

47 min

0.42

2006-2009

0.91 79%

64 min

0.56

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Example Verification for Distributed Hydrologic Modeling – Threshold Frequency Technique

Binghamton WFO, October 1st_{, 2010 Flood Event}

• Excellent spatial agreement among areas of 2+ year return periods and local storm reports • Isolated areas with > 100 year return periods • = Local Storm Reports = NWS Storm Reports #’s = USGS Gauge Return Period

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The Path Forward

• Regional pilot projects - build a little, test

a little, field a little

• MARFC flash flood improvement pilot

planned

• Larger nationally coordinated projects –

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• DHM-TF pilot studies are underway in coordination with NWS Weather Forecast Offices (WFOs) and River Forecast Centers (RFCs)

– Binghamton and Pittsburgh WFO domains on WFO servers

– Baltimore/Washington WFO domain on OHD server (transitioning to WFO)

Getting Ready for Weather Ready Nation Flash

Flood Pilot

Pittsburgh, Binghamton, and Balt/Wash WFO Domains

89,000 km2 57,500 km2 60,000 km2 Pittsburgh Binghamton Balt/Wash

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NEW! National Distributed Hydrologic Models

Surface Soil Saturation: 4 km Grid

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xp

eri

m

en

tal

08/11/2012 @12z National gridded flash flood guidance

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# of historical

peaks available at each point since 1986

USGS stations with Area < 100 mi2 _{[2781 stations]}

Average Recurrence Interval Associated w/Flooding

Many gauges exist for small streams but minor, moderate, and major flood levels are

not defined

Could make better use of small gauges, national distributed hydrologic models, and frequency thresholds for systematic verification

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Summary

• Keys to life saving flash flood forecasts –Timeliness, lead time

–Model accuracy and resolution –Warning and response

–Education and outreach

• NWS Improvements since 1980s have led to improved

precipitation measurements, QPF, hydrologic modeling, forecaster analysis tools, warning dissemination

methods, and education and outreach ( BUT DIFFICULT TO QUANTIFY IMPROVEMENT)

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What’s Next?

• New technologies and modeling techniques to leverage in next 5 –

10 years

 High resolution distributed models  CHPS

 Dual pol radar

 Combine multisensor-extrapolative and higher-resolution meteorological outputs to develop more precise and accurate 0-6 hour precipitation forecasts  Innovative verification

 Hydraulic models models for smaller streams – public private partnerships  Uncertainty estimates via ensembles

• IWRSS establishes formal program at the national level to facilitate

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Thank You!

Contact Information: Seann Reed

Middle Atlantic River Forecast Center 328 Innovation Blvd. (STE 330)

State College PA, 16803