West Coast Winter Storms Provides NOAA With Shower Of Data

Researchers with the National Oceanic and Atmospheric Administration (NOAA) are taking advantage of the soggy start of 2006 on the U.S. West Coast to better understand, predict and prepare for a series of intense precipitation events, officials stated on Jan. 20.

A field study that began in December 2005 and will continue until March is designed to provide some long-term, as well as short-term, results to improve the predictability of floods to better protect the public from losses of life and property while minimizing economic risks associated with water management.

"Our need to improve the predictability of floods dictates that we have a responsive and adaptable research program," said Richard W. Spinrad, NOAA assistant administrator for oceanic and atmospheric research. "NOAA and university scientists demonstrated how to obtain critical data under some of the most extreme environmental conditions."

The study, Hydrometeorological Testbed-West (HMT-West), targets California's flood-vulnerable American River Basin, or ARB, near Sacramento, which is considered a high risk for a flood catastrophe. NOAA has been engaged in research aimed at better forecasts of land-falling winter storms on the West Coast for a number of years, and improving the nation's coastal observing system. Recently, these efforts have focused on water management issues on the American River and other coastal watersheds. A pilot program was conducted two years ago on the nearby Russian River.

"Severe flooding on the West Coast requires the right sequence of storms," said William Neff, director of the NOAA Earth System Research Laboratory's Physical Sciences Division in Boulder, Colo. "Typically, early storms saturate the soil. Later storms fill the reservoirs. If the storms continue, then the stage is set for potential flooding and decisions must be made about draining the reservoirs."

Neff noted that better forecasts of rainfall and runoff will help decision makers balance the need to protect life and property against the requirement to retain water for the dry summer months.

This winter, precipitation totals in the ARB from Nov. 28, 2005, through Jan. 8, 2006, have already exceeded 42 inches of liquid. Most of this precipitation fell in December 2005, which was more than three times wetter than the climatological average for December.

The early-season storms that produced this precipitation have caused flooding on nearby watersheds, such as the Russian River, Napa River and Sacramento River in California, and the Truckee River in Nevada. Also, a debris flow occurred on the edge of the ARB that closed Interstate 80 between Reno and Sacramento. Serious flooding caused by runoff from the ARB was avoided because reservoir operators were very proactive, based on forecasts issued by the NOAA National Weather Service. They began releasing water prior to the strong late December storms and continued to do so for more than a week. In addition, all of the Sacramento area flood control bypass weirs were opened to handle the immense runoff, thus creating temporary lakes over much of the agricultural land around Sacramento.

"There are two parts of this study," said Neff. "We want help NOAA's Office of Hydrologic Development test and improve their streamflow models as well as make better predictions of where, when and how much rain will fall."

The Hydrometeorological Testbest (HMT) is designed to accelerate the testing and infusion of new technologies, models and scientific results from the research community into daily forecasting operations of the NOAA National Weather Service, including the NOAA National Centers for Environmental Prediction, weather forecasting offices and river forecasting offices.

"A half an inch or an inch of rain really doesn't mean anything to the average person," explained Pedro Restrepo, senior scientist with the NOAA Office of Hydrologic Development at the NOAA National Weather Service. "But I can put that half inch or inch into a computer and it can tell me if an area is going to flood or not, depending on a number of factors, such as how that rain is distributed over space and time, the characteristics of the area and previous precipitation."

One NOAA meteorologist, who knew that some wet weather was on the way, was able to use data from the project for his forecasts.

"The NOAA National Centers for Environmental Prediction guidance was forecasting 10 to 20 inches of rain over Northern California in a five-day period," said David Reynolds, the meteorologist in charge of the NOAA National Weather Service Monterey Weather forecast office. "In addition, daily conference calls for the HMT-West experiment in the American River Basin allowed several of the impacted NWS forecast offices to discuss this situation daily."

An array of technology is employed, including wind profilers, transportable and mobile scanning precipitation radars, precipitation profiling radars, Global Positioning System sensors to measure precipitable water vapor, precipitation gauges, raindrop disdrometers to measure number and size of raindrops, surface meteorological stations, soil moisture/temperature probes, radiosonde balloon releases and stream level loggers that automatically measure the depth of streamflow at key points. Most of the HMT-West instruments operate unattended around the clock.

"The data we collect with these instruments are very important because much of the precipitation forms below the coverage of the NEXRAD radars, and the existing NEXRAD radar techniques are not able to accurately estimate rainfall," said Marty Ralph, program manager for the NOAA Weather and Water/Science, Technology and Infusion Program. Through HMT, NOAA is exploring how best to fill gaps in our ability to monitor and predict precipitation. New radar techniques, known as "polarimetric" hold great promise, and are being tested through deployment of a prototype polarimetric scanning radar by the ESRL-PSD. Another key technology being tested, known as wind profilers, focuses on measuring winds, precipitation and the snow level aloft -- variables that are otherwise difficult to measure continuously.

These profilers, along with the NOAA National Severe Storms Laboratory's scanning Doppler radar and other PSD radars, are useful because they can fill critical information gaps where coastal mountains block NEXRAD coverage. These instruments from the research community also monitor the atmosphere with greater temporal and/or spatial resolution than is generally available from existing NOAA operational platforms. Also, in mountainous terrain, the snow level is a critical variable to measure because a higher snow level can lead to increased runoff whereas a lower snow level results in greater storage of water for the following summer.

The data collection also is aimed at improving the understanding of the physical processes that cause the precipitation, and then to evaluate numerical weather prediction models in terms of their ability to represent these processes. A key phenomenon being explored is the landfall of narrow regions of very strong low-altitude winds and large water vapor contents, known as "atmospheric rivers." As pointed out by Ralph, "Although these regions are just a couple of hundred kilometers wide, they can have winds of hurricane force and are responsible for most of the water vapor transport that ultimately strikes the coast and Sierras. Better understanding of these features is critical to improving prediction of precipitation in storms like those that have struck California and Nevada last month. "

Several high-resolution weather prediction models are being run by ESRL's Global Systems Division to supplement the operational model guidance already available to the NOAA National Weather Service.

"When the storms occur plays a major role, as well," Neff said. "If they come one right after another and the soil is already saturated and the streams and reservoirs are filled to capacity, then additional precipitation makes conditions right for flooding."

NOAA Hydrometeorological Testbed: HMT-West-2006 Operations: http://www.etl.noaa.gov/programs/2006/hmt

This article originally appeared in the 01/01/2006 issue of Environmental Protection.

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