West Coast Storms Affected By El Niño To Be Studied This Winter

December 16, 1997

Winter storms in California and Oregon -- with the possibility of increased rainfall due to El Niño -- are being studied by government and university scientists hoping to improve forecasts of heavy rain, snow and wind along the West Coast.

The study, called CALJET (California Land-Falling Jets Experiment), began on Dec. 1, the Department of Commerce announced today, and includes scientists and forecasters from the National Oceanic and Atmospheric Administration, the U.S. Navy, and various universities. It is particularly timely with the onset of the severe storms last week along the California coast. The study will run through March of 1998, which is the wet season in that area.

According to Martin Ralph, the principal investigator for the project from NOAA's Environmental Technology Laboratory in Boulder, Colo., "One of the main causes of the severe weather experienced along the West Coast during winter is the low-level jet which is a river of wind that occurs near cold fronts in winter storms. Many times this jet contains a great deal of moisture, and can cause extreme coastal rains when it hits the mountains."

These land-falling winter storms can cause extensive damage along the West Coast due to the high winds and heavy rain produced in a short period of time as they come ashore. Because of limited data concerning these events, accurate warnings are not always as timely as are needed. The hope is that by improving the observations of the low-level jet, forecasts will be improved. This may prove to be a great benefit for people living along the coast, if El Niño causes the increased rainfall that has been predicted.

CALJET will make use of a wide variety of special observing systems to augment existing operational systems. These include the NOAA P-3 aircraft with a variety of instruments such as GPS (Global Positioning System) dropsondes, which take meteorological measurements as they are dropped from the aircraft, dual-Doppler-capable radar, a gust probe, and other instruments. A special observing period will run from Jan.18 to Feb. 28 when the NOAA P-3 and the University of Oklahoma's Doppler on Wheels, a Doppler radar mounted on a truck, will participate, operating out of Monterey, Calif.

The P-3, flown by NOAA Corps pilots, will be sent to find the low level jets associated with storms that occur within this time period. It will detect the jets about 12 hours before the storm strikes the coast. The Doppler on Wheels will then be deployed to measure the heart of the heavy rains when the storm strikes.

In addition, 20 wind profilers are continuously monitoring upper-level winds and temperature. They have been positioned along the coast from San Diego to Seattle. A system of 30 drifting buoys will be installed by the NWS to measure conditions offshore 12-36 hours before the storms strike the coast. Data collected from the P-3, buoys, and wind profilers will be used in nowcasting and operational numerical weather prediction. Special observing areas will be created along the coast near Santa Rosa, Calif.; in the area between Santa Barbara and Crescent City; and in the California Bight area from Santa Barbara to San Diego. These locations were designated due to the possibility of heavy rainfall in those areas and the high population density.

Ralph says that "one unique aspect of CALJET is that we intend the P-3 data to be transmitted in nearly real-time for direct use by operational weather forecasters in winter storms approaching California. This approach is similar to what is done for hurricane forecasting during hurricane season. In addition, the data will be put into mesoscale research numerical models in a way that should provide improved short-term model guidance to forecasters." The result of this, says Ralph, is to improve operational warnings to the public 0-12 hours before the land-fall of winter storms during the experiment.

"One of the biggest challenges facing weather forecasters in California is accurate quantitative precipitation forecasts," says Dave Reynolds, a principal investigator from NOAA's National Weather Service in Monterey, Calif. "Being able to accurately predict the low level wind and moisture structure of a storm is critical to predicting the precipitation amounts. And precipitation amounts depend on the rate at which moisture condenses. On the West Coast, the strength of the winds blowing in off the Pacific Ocean combined with how the winds interact with the coastal terrain, determines how much precipitation is produced during a storm."

"It is critical that we be able to accurately predict the timing and intensity of precipitation to improve flood forecasts. To date, we do a good job at 24 hour precipitation amounts but lack the skill to forecast in the three to six hour time frame that produces most floods. We're hoping to use the wealth of real-time observations from CALJET to better predict rainfall this winter," Reynolds says.

According to Reynolds, these observations will provide input to a locally run weather model that will be set up for the San Francisco Bay Area. "This model will be run twice daily and should allow us to evaluate how these observations and forecasts are improving rainfall forecasts on a daily basis."

Scientists involved in the project:

Martin Ralph, Environmental Technology Laboratory, Boulder, Colo. - 303-497-7099

Dave Reynolds, National Weather Service, Monterey, CA - 408-656-1717

Ola Persson, Environmental Technology Laboratory, Boulder - 303-497-5078

Wendall Nuss, Naval Postgraduate School, Monterey, CA - 408-656-2308

Note to Editors: A press day will be held in mid-January in Monterey, with opportunities to film or photograph the NOAA P-3, Doppler on Wheels, etc. A media advisory outlining the details will be issued at that time.

Note: Satellite photos of a land-falling storm from Dec. 5, 1997 and from a damaging storm last year are available from NOAA Public Affairs in Washington, D.C. at 202-482-6090.
-end-


National Oceanic and Atmospheric Administration

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