32nd Conference on Broadcast Meteorology/31st Conference on Radar Meteorology/Fifth Conference on Coastal Atmospheric and Oceanic Prediction and Processes

Tuesday, 12 August 2003: 9:30 AM
Winter Weather Nowcasting
Roy M. Rasmussen, NCAR, Boulder, Colorado
Short term forecasting of winter weather is important to a variety of sectors of the U.S. economy, including transportation, energy, construction, recreation and maritime. It is also very important for public safety. Transportation impacted areas include: aviation, roads, and railways. Freezing rain accumulation on power lines is a significant concern to power companies. The construction sector is concerned with snow loading on structures. The recent blizzard in Colorado, for instance, caused millions of dollars of damage to structures in the Denver metropolitan area. Avalanches are an ever-present danger in mountainous terrain and affect both recreational skiers and can block roads and railroad tracks. Rapidly deepening winter storms can cause high winds and waves that present a significant safety hazard to ships on both the ocean and inland waterways such as the Great Lakes. Deicing of aircraft requires one minute updates of liquid equivalent snowfall rate at airports. Prevention of snow on airport runways also requires information on liquid equivalent snowfall rate as well as snow depth. The excellent sensitivity of the NexRad radars (down to 28 dBZ in clear air scan mode) allows the horizontal distribution of snow to be well mapped (most snow is between 10 to 40 dBZ). However, since radars are mainly sensitive to the sixth power of the snow particle diameter, it is difficult to determine snow density from the radar alone. Thus, additional information from other instruments such as snowgauges are required to accurately determine liquid equivalent snowfall rate over a domain. Current methods of nowcasting snow out to one hour rely on extrapolating snowband movement based on tracking the radar reflectivity of the storm and combining this with real-time snowgauge information to determine the actual liquid equivalent snowfall rate. Beyond one hour, results show rapidly decreasing skill based on echo tracking alone, thus new techniques are required. A recently developed technique for 1-12 hour snow forecasting using assimilation of radar data into mesoscale models is showing promise for improved longer time snow forecasts. This paper will review some of the above-described methods to nowcast winter weather, and also areas for future development.

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