Learning how to make better "nowcasts" of weather

Researchers, forecasters discuss closer ties

July 2, 1999: Three days after heavy summer rains caused a 100-year flood in the southeast part of Huntsville, Ala., a group of meteorologists met on the other side of town to discuss what they are doing to improve "nowcasts." The timing was entirely coincidental - the meeting had been scheduled for several months - but it helped point up the need for nowcasts, or forecasts of what the weather will do in the next few hours.

Right: Water, water, everywhere - but where does it go? This GOES 8 infrared image of the Earth depicts water vapor in the atmosphere. Predicting where it will fall a few hours ahead of time is a new challenge being taken up by weather researchers and forecasters. Credit: NASA/Goddard Space Flight Center

On the afternoon and night of June 27, the skies over Huntsville opened up and dropped up to 6 inches of rain in two hours on sections of southeast Huntsville. Dozens of homes suffered extensive flooding, and a woman drowned when her car stalled on a flooded road and she tried to walk out. A warning of even a few hours could have let the city brace for a storm that statistically happens about once a century.

Meteorologists can measure in detail what is happening now, and predict in general terms what will happen over the next week. They now want to see if they can combine the wide range of operational tools - including Doppler radars that even TV stations have - and powerful numerical models of the atmosphere that can now be run on high-speed desktop computers to predict in detail what will happen over the next 30 minutes to six hours.

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The Collaborative Workshop on Mesoscale Modeling and Short-Term Convective Forecasting was held June 30-July 1 by the southern region headquarters of the National Weather Service at the Global Hydrology and Climate Center (GHCC). Also attending were scientists from the National Center for Atmospheric Research and other research institutions.

Left: The Potomac River, upstream from Washington, D.C., normally presents a picturesque view (left). But in January 1996, it overflowed its banks after a heavy winter rainstorm. Credit: NASA/Goddard Space Flight Center.

"The purpose of this workshop is to get the research and operational communities together to discuss how what we - the researchers - do can be translated to the operational community and thus to the public in some way," explained Dr. Bill Lapenta, a researcher at the GHCC.

"Technology transfer is important to NASA so we developed a collaboration with the National Weather Service Forecast Office in Birmingham to expedite the application of our research," he noted. While the GHCC is not involved in weather forecasting, "Our research tries to help the forecasters do their job" by providing improved tools.

Among the projects they discussed is the Summer Convective Rainfall in Alabama Prediction Experiment (SCRAPE). Tom Bradshaw of the National Weather Service Office in Birmingham described it as an experiment to improve short-term forecasts across North Alabama.

Test runs started in mid-June and began in earnest on July 1. They run through Sept. 15. Five NWS meteorologists in Birmingham have been detailed to work on SCRAPE nowcasts without being distracted by routine duties. They will integrate data from radar, weather satellites, and other tools in an MM5 model to produce a nowcast covering the next 6 to 12 hours.

Bradshaw hopes to get at least 40 nowcasts that his team will compare with what actually happens. The lessons will be fed back into operations, and then into SCRAPE 2 in 2000 when Bradshaw hopes to produce accurate 3-hour nowcasts.

Nowcasts today are limited, tending to be too broad and not taking in enough detail, Bradshaw said, or perishable in that they require a lot of work to update as the weather evolves. Because of that, "We tend to be a little dry in our forecasts," he continued, and in the summer, atmospheric convection tends to be highly irregular.

Developing accurate, durable nowcasts won't be easy, Bradshaw acknowledges: "We've got a challenge facing us."

Global Hydrology and Climate Center in Huntsville.

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National Weather Service, an element of the National Oceanic and Atmospheric Administration.

"We say Weather GO or NO GO." The NOAA/NWS Spaceflight Meteorology Group (represented at the workshop).

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Author: Dave Dooling Curator: Bryan Walls NASA Official: John M. Horack