Theme for 2011:
Analyzing, Forecasting, and Managing Floods

The Problem: Estimating Extreme Floods — Too Much Statistics … No Common Sense

• Presentation format (PDF*, 738 KB)
• Notes format (PDF*, 624 KB)
• Hear the presentation (MP4†, 43:15 minutes)

New Regional Skew Values for California — Implications for Flood Frequency Analysis

• Presentation format (PDF*, 844 KB)
• Notes format (PDF*, 1.5 MB)
• Hear the presentation (MP4†, 38:25 minutes)

The new Bayesian GLS methodology produced a nonlinear function relating regional skew to mean basin elevation. The regional skew values ranged from -0.62 for a mean basin elevation of zero feet to 0.61 for a mean basin elevation of 11,000 feet. This relation between skew and elevation indicates a change in flood hydrology with an increase in elevation, primarily as a result of the increase in the effects of snow and snowmelt runoff. The equivalent record length for the new regional skew ranges from 52 to 65 years of record depending upon mean basin elevation, whereas the old regional skew map in Bulletin 17B reported skew values from -0.2 to 0.2 and an equivalent record length of only 17 years. Differences between flood-frequency estimates based on the old regional skew and the new regional skew are greatest for sites approaching the extremes in mean basin elevation and also having short record lengths.

Approximating the Probability of the Probable Maximum Flood

• Presentation format (PDF*, 1.0 MB)
• Hear the presentation (MP4†, 33:29 minutes)

This paper will summarize the extension methodology and provide the details on computing the estimated AEP of the PMF.

Communicating Risk and Uncertainty of Extreme Weather and Flood Events

• Presentation format (PDF*, 485 KB)
• Hear the presentation (MP4†, 36:49 minutes)

First, results will be presented from a nationwide survey examining people's perceptions, interpretations, and uses of uncertainty information in everyday weather forecasts. Next, results will be synthesized from a variety of studies of how people interpret hydrometeorological warnings and how they use those warnings in decisions. Finally, ongoing work will be described that analyzes people's warning decisions in extreme weather events, including mental modeling to understand experts' and laypeople's flash flood risk perceptions and a survey that includes questions on how people interpret and use flash flood warnings.

Forecast-Coordinated Operations: A Multi-Agency Flood Management Program Update

• Presentation format (PDF*, 1.0 MB)
• Notes format (PDF*, 1.0 MB)
• Hear the presentation (MP4†, 15:10 minutes)

Orographic Precipitation Processes in West Coast Mountains

• Presentation format (PDF*, 2.0 MB)
• Notes format (PDF*, 2.6 MB)
• Hear the presentation (MP4†, 32:30 minutes)

The first orographic process presented is the moisture flux. The moisture flux (upslope flow × integrated water vapor) is directly correlated to the coastal rain rate and amount when atmospheric rivers impact the terrain of California. Atmospheric river observatories and the Coastal Atmospheric River Monitoring and Early Warning System (aka "the flux tool") were developed to monitor the moisture flux and to provide enhanced situational awareness and forecast guidance for extreme precipitation events along the U.S. West Coast.

The second process described is non-brightband rain. Orographic rainfall can be partitioned into three rainfall types: non-brightband (NBB), bright band (BB), and hybrid rain. Of the three, NBB rain is found to contribute significantly (28%) to the total rainfall amount, and yet this key orographic precipitation process had not been documented until 2003. The characteristics of NBB rain are presented.

The final orographic process discussed is the Sierra barrier jet which significantly impacts the precipitation distribution in the Sierra Nevada. The Sierra barrier jet occurs when an airstream approaching the Sierra Nevada slows down and is deflected leftward as a result of a weakened Coriolis force when the Froude number is less than unity. A corridor of low-level blocked flow forms upstream and below the top of that barrier and usually contains a barrier jet paralleling the long axis of the high terrain. The presence of this orographically-induced jet tends to redistribute precipitation upwind of the barrier.

Improvements and Evolving Technologies at the California-Nevada River Forecast Center

• Presentation format (PDF*, 2.4 MB) — expanded slideset providing more information than speaking time allowed (slides shown at Symposium are marked with a star in the upper right corner)
• Presentation format (PDF*, 1.4 MB) — slideset shown at Symposium
• Hear the presentation (MP4†, 30:37 minutes)

Forecast precipitation and temperatures represent the greatest sources of uncertainty in hydrologic forecasts. CNRFC hydrologic models are fairly well calibrated and if we knew the future weather, we could issue some scary hydrologic forecasts. Recognizing this, the CNRFC has invested heavily in developing meteorological skill in its' HAS (Hydrometeorological Analysis and Support) Unit. The question is, "Has that investment paid off?" An analysis performed recently by Alan Haynes, CNRFC SCH, suggests it truly has. Verification information showing the level and pace of improvements will be provided.

Over the last 4 years, the NWS has been developing and deploying a new hydrologic forecasting architecture for River Forecast Centers. This new system called the Community Hydrologic Prediction System or CHPS is now operational at 5 of 13 RFCs. The process of implementation as well as the advantages of the new system will be described.

Few things in life are certain and hydrologic forecasts most definitely contain uncertainty. The question is, just how much. As we learn to estimate the uncertainty in our hydrologic forecasts, decision makers can begin to benefit from integrating "risk" into their mitigation and management efforts. The NWS has been focused on developing the ability to generate reliable ensemble-based hydrologic forecasts for the past five years. The new system, called the Hydrologic Ensemble Forecasting System or HEFS, has been designed and several key components are in prototype use. The presentation will describe the value of these new forecasts, some of the requirements, our status, and some early results.

Comparison of California and Pacific Northwest Atmospheric Rivers and Flooding

• Notes format (PDF*, 2.8 MB)
• Hear the presentation (MP4†, 1:08:38 minutes)

Both the Pacific Northwest and California experience multiple land-falling ARs each winter season. They can contribute substantially to the winter snowpack water content as well as to seasonal rainfall. On rare occasions however, an AR does produce extreme rainfall and runoff leading to flooding. This dual presentation, by experts from both regions, will focus on flood producing AR storms. You will begin to understand the challenges of flood forecasting shared by water management professionals along the entire west coast of North America. You will learn there are common critical issues and some important differences between these two locations with respect to flood and non-flood producing ARs. By recognizing the differences, you may become more confident in evaluating your own flood risk management strategies.