Sierra weather forecasting: The return of La Nia
October 16, 2007
It happens every year like clockwork. October brings shorter days and colder nights. Its also that time of year when weather prophets come forth with their winter forecasts. Its not a job for the meek. From ski resort managers and hydrologists, to the average guy buying firewood, it seems that everyone wants to know, What will this winter be like? Besides putting faith in the folksy Old Farmers Almanac or observing how busy the squirrels are, can anyone really predict the weather months in advance?When it comes to the Tahoe-Truckee region, weather prediction is no simple task. It is the interplay between the Pacific Ocean and the Sierra Nevada that creates the sharply defined seasonal weather patterns which bring variety and change.The Sierra gets blasted by some of the most intense winter storms in the world. The most powerful of these Pacific weather systems boast an atmospheric circulation exceeding 1,000 miles across, can generate wind gusts approaching 200 mph over mountain ridges and peaks and produce snowfall measured by the yard. Large-scale storms of this magnitude are easy to track as they roar across the Pacific Ocean or drop out of the Gulf of Alaska. Satellite-derived images of advancing storms are now commonplace on television weather forecasts. The launch of the worlds first weather satellite from Cape Canaveral on April 1, 1960, dramatically demonstrated the advantage of mapping the earths cloud cover from satellite altitudes. Today, 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. 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, Null said. 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 Sierras terrain and consequently precipitation patterns and amounts. Computer models that several years ago could only provide a forecast three to five days in advance, now go out to 16 days (384 hours). Mike Alger, the chief meteorologist at KTVN-TV in Reno, Nev., recently told me that computer processing capabilities and speed have grown exponentially in the last decade, which has helped forecasters to synthesize and manipulate quickly the large volumes of data that are now available.For real-time observations, weathercasters at Lake Tahoe and in western Nevada utilize a NEXRAD (NEXTgenerationRADar) 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 whats happening in the valleys of western Nevada. Over the last few decades, there has been a revolution in the accuracy of weather forecasts, a direct outgrowth of research, technological developments and an increased understanding of how the atmosphere works. The accuracy of short-range, two- to three-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 five-day forecast has more than doubled in the last decade. Yes, but what about a forecast for this winter? Last year, a weak El Nio event sparked hopes for a big winter, but only 275 inches of snow fell on Donner Summit, well below the average of 410 inches in a normal year. This year, however, prognosticators are eyeing a moderate La Nia chilling in the Pacific. In contrast to the warmer waters associated with an El Nio event, a La Nia episode is characterized by cooler than normal sea surface temperatures in the eastern and central equatorial Pacific. El Nio and La Nia are opposite counterparts of a cyclical atmospheric pressure swing known as ENSO (El Nio – Southern Oscillation). La Nias occur after some, but not all El Nio years. This see-saw of pressure affects equatorial wind speed and direction and influences weather patterns in the California Sierra, as well as around the globe. An El Nio event tends to bring wetter-than-normal conditions to Southern California while La Nia increases the likelihood of above average precipitation in the Pacific Northwest. Testament to the challenges associated with predicting weather based on ENSO events last years El Nio left Southern California parched with Los Angeles receiving the least amount of rainfall on record.The La Nia phase is often characterized by warm winters in the Southeastern United States, colder than normal winters from the Great Lakes to the Pacific Northwest, and unsettled winters in the Northeast and Middle Atlantic states. Nationwide, La Nia events tend to bring wetter than average conditions to the northern states and drier than average in the south. Along the California coast, Eureka should be wet, Los Angeles most likely dry. Typical jet stream and cloud patterns in the Pacific Ocean during a La Nia event feature a well-developed subtropical jet stream bringing moisture to the Northwest from Hawaii, known locally as the Pineapple Express. This Maui Connection occurred during the moderate La Nia of 1995-1997, which generated several large flood events, including the catastrophic 1997 New Years Flood on the Truckee River. Occasionally, blocking high pressure in the Gulf of Alaska will pull the polar jet stream into the mix. Dense cold air wedging under moist sub-tropical clouds streaming up from Hawaii can produce heavy snowfall. A moderate La Nia during the winter of 1906-07 dumped more than 73 feet of snow on the Sierra, still the all-time record. Professional weather pundits are paid by utility companies, farmers, ski resorts and the hydroelectric industry, but despite their best efforts, accurately predicting weather patterns months in advance is virtually impossible. At this time, the National Weather Services Climate Prediction Center is predicting above average precipitation from about Tahoe north into the Pacific Northwest and Northern Rockies with a probability of warmer than normal temperatures from Tahoe south. The extent of the temperature anomaly and the timing of the ENSO pulse are critical to the Pacifics winter storm track. Strong La Nias produce more powerful polar jet streams that tend to be very cold, but rather dry. The last strong cold ENSO occurred during the parched winter of 1988-89. In the Tahoe-Truckee area, La Nia-influenced winters tend to be drier than normal, but not exceptionally so. Historically, moderately cold ENSO events bring a wetter than normal November and December to the Northern Sierra. January through March tend to be hit and miss and precipitation less reliable. At best, any long-range winter forecast for our region is an educated guess. When forecasting Sierra weather patterns, there are no easy answers. But dont give El Nio credit for all the big winters. Remember, when it comes to heavy snow in the Sierra Nevada, La Nia can hold her own, and then some. Mark McLaughlins column, Weather Window, appears every other week in the Sierra Sun. He is a nationally published writer and photographer whose award-winning books, The Donner Party: Weathering the Storm, Sierra Stories: True Tales of Tahoe, Vol. 1 & 2, and Western Train Adventures: The Good, the Bad & the Ugly are available at local stores. Mark, a Carnelian Bay resident, can be reached at firstname.lastname@example.org.