Improving Forecasts

by Mark McLaughlin

The salubrious climate of Lake Tahoe is the perfect match for the region's spectacular alpine scenery and our year-round outdoor lifestyle. Few places on earth can boast a climate as inspiring for its contrast between soft summer serenity and dynamic winter weather. Lake Tahoe's celebrated climate is closely representative of the Sierra Nevada range as a whole. It is the interplay between the Pacifi c Ocean and the Sierra barrier that creates the sharply defi ned seasonal weather patterns that bring variety and change.

It's no secret that the Sierra Nevada gets blasted by some of the most intense winter storms in the world. The most powerful of these boast an atmospheric circulation exceeding 1,000 miles across, generate wind gusts approaching 200 miles per hour over mountain ridges and peaks, and produce snowfall accumulation measured by the yard.

With advanced technologies, largescale storms of this magnitude are easy to track as they roar across the Pacific Ocean or drop out of the Gulf of Alaska. Highaltitude images of advancing storms are now so common on television weather forecasts, it may be diffi cult to remember when there was nothing in space to give us this bird's-eye view. The launch of the world's fi rst weather satellite from Cape Canaveral on April 1, 1960, dramatically demonstrated the advantage of mapping the earth's cloud cover from satellite altitudes. Today, weather forecasters rely on geostationary satellites to provide much more than a picture of cloud structures. New technologies include infrared imaging to measure cloud top temperatures, which help determine precipitation intensity, as well as temperature and moisture profiles.

Forecasting snowfall across the Sierra Nevada is shared between three National Weather Service offi ces, San Francisco, Sacramento and Reno. Mike Alger, the chief meteorologist at KTVN-TV in Reno, Nevada, recently told me that computer processing capabilities and speed have grown exponentially in the last decade, which has helped forecasters synthesize and manipulate quickly the large volumes of data that are now available.

Jan Null, a certified consulting meteorologist and retired National Weather Service forecaster in San Francisco, has seen many forecasting improvements during the last 10 years or so. "The abundance of real-time weather observations in the Sierra is a great boon," he says. "I remember the 'old days' when Blue Canyon (along Interstate 80) was the only overnight observation station in the entire range." Null also credits higher resolution in numerical computer models, which "do a better job of resolving the Sierra's terrain and consequently precipitation patterns and amounts." He notes that computer models that not long ago could provide a forecast 3 to 5 days in advance, now go out 16 days (384 hours). While these extended forecasts generally focus on temperature and precipitation (precipitation forecasting skill lags behind temperature), the American Meteorological Society predicts that advances in observing systems, computer models and statistical techniques may allow skillful forecasts for the 8- to 14-day period in the near future.

Lake Tahoe and western Nevada weathercasters utilize a state-of-the-art NEXRAD (NEXT-generation RADar) Doppler radar perched on top of Virginia Peak at elevation 8,299 feet, located 15 miles northeast of downtown Reno. This elevated position gives the radar the ability to "read" the atmosphere 100 kilometers in any direction, which includes "upstream" activity beyond Donner Pass. The radar is helpful in analyzing atmospheric conditions over the high country of the Central Sierra, but its sweep is not low enough to see what's happening in the valleys of western Nevada.

Over the last few decades, there has been a revolution in the accuracy of weather forecasts, which have improved markedly as a direct outgrowth of research, technological developments and an increased understanding of how the atmosphere works. The accuracy of shortrange, 2- to 3-day forecasts continues to improve. Precipitation forecasts for this 36- to 60-hour time frame are now more accurate than 12- to 36-hour predictions were in the 1970s. The skill of a 5th day forecast has more than doubled in the last decade, although that skill confi dence drops substantially by the 7th day.

There are still many challenges to accurately predicting weather in mountainous regions. Short-term forecasting often relies on meteorological radar coverage to provide information on the intensity, extent and motion of a winter storm. There are signifi cant portions in the Sierra Nevada where coverage is lacking due to topographic blocking, and these "dark" areas in the radar scan can produce uncertain or even misleading information. Sometimes, realtime observations from weather watchers on the ground can fi ll in these blanks.

Weather and climate are different parts of the same animal. Climate is the accumulation of daily and seasonal weather events over a long period of time, while weather is the condition of the atmosphere at any particular time and place. The weather at Lake Tahoe has the ability to change moods in a fl ash, so visitors and locals should always pay attention to the forecast and keep their eyes to the sky. Author Robert Heinlein got it right when he wrote, "Climate is what you expect. Weather is what you get."