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

The Flash Flood Problem Intense rainfall, ice jam, or dam failure Rapid rise Danger!

Issue timely watches and warnings Collect dataAnalyze data and forecast What we do Ongoing Challenges: TIMELINESS, LOCATION SPECIFICITY, UNGAUGED STREAMS, VERIFICATION Ongoing Challenges: TIMELINESS, LOCATION SPECIFICITY, UNGAUGED STREAMS, VERIFICATION

Topics Observed and forecast rainfall calculations Hydrologic modeling Verification Communicating warnings, outreach The path forward, emerging technologies

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

Observed and Forecast Rainfall

Rain gauge data Radar data Satellite data Multi-sensor rainfall products QPF guidance

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 8 Stations with hourly or sub-hourly reporting are most useful for flash flood modeling

Automated Flood Warning Systems (AFWS) Using Integrated Flood Observing and Warning System (IFLOWS) Technology Joint effort between NWS and states 1663 rain gauges 12 states 5 minute radio- telemetry reporting

14,700 data collection locations Real time data acquisition, processing and distribution. Data Collection Platforms (DCPs) operated by more than 100 collaborators

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

Quality-Controlled Precipitation Gage Measurement GOES Satellite Estimate Radar Estimate Climate patterns MPESoftwareMPESoftware 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 1 hour, 4 km resolution grids for river forecasts Highest quality NWS precipitation Manual quality control Latency too long for very flashy events 1 hour, 4 km resolution grids for river forecasts

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

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 Lead time = rainfall forecast period + basin response time

Hour 1 HPN Forecast (mm) 23Z April 21st to 00Z April 22 nd 2009 HPE Precipitation (mm) 23Z April 21st to 00Z April 22 nd 2009 MPE Precipitation (mm) 23Z April 21st to 00Z April 22 nd 2009 Hour 2 HPN Forecast (mm) 23Z April 21st to 00Z April 22 nd 2009 Observations Forecasts 1 hour forecast 2 hour forecast

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

Hydrologic Modeling for Flash Floods (Current Practice)

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)

Flash Flood Guidance (FFG) (in) Threshold Runoff In this basin, FFG ranges from 2 to more than 4 inches depending on antecedent moisture Flash Flood Guidance (FFG) Curves

Flash Flood Guidance (FFG) Products Issued by RFCs for different rainfall durations Updated 1-4 times daily Gridded FFG Headwater FFG

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 21

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.

Distributed hydrologic modeling – combining physical, conceptual, statistical Leveraging Community Hydrologic Prediction System (CHPS) technology Interagency collaboration Hydrologic Modeling for Flash Floods, Moving Forward

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

25 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 4. Parameterization

26 14 UTC15 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. 6. Statistical-distributed

Community Hydrologic Prediction System (CHPS) Community Hydrologic Prediction System (CHPS) 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.

Recent Interagency Collaboration on Dam Break Forecasting NWS GeoSMPDBK GIS Pre-processor for SMPDBK Produces SMPDBK input file 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 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

NWS Use of Unsteady HEC-RAS Models Current HEC-RAS model applied to mainstem only 2183 mi 2 upstream 3200 mi 2 at outlet/coastal boundary Experimental HEC-RAS applications to smaller basins (~ 100 km2 ) will allow flood forecast mapping at flash flood scales

Communicating warnings, outreach

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 End users Communication Mechanisms

Flood Safety Education – Best Practices Check for the latest forecast Get Insurance index.shtml

Flood Safety Awareness - Outreach

Verification

Flash Flood Verification Data ( all options have limitations ) 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–

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 (pre-FFMP) to those from (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) %47 min %64 min0.56

Example Verification for Distributed Hydrologic Modeling – Threshold Frequency Technique Binghamton WFO, October 1 st, 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

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 – IWRSS, National Water Center

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 km 2 57,500 km 2 60,000 km 2 Pittsburgh Binghamton Balt/Wash

NEW! National Distributed Hydrologic Models Surface Soil Saturation: 4 km Grid Experimental National gridded flash flood guidance New inputs for decision support

# of historical peaks available at each point since 1986 USGS stations with Area < 100 mi 2 [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 National gauge-based verification: A future possibility?

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)

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 interagency collaboration

Thank You! Contact Information: Seann Reed Middle Atlantic River Forecast Center 328 Innovation Blvd. (STE 330) State College PA,