MEDIUM-RANGE PRECIPITATION-and-TEMPERATURE FORECASTS FOR USE IN CITY OF SAN FRANCISCO/HETCH HETCHY TUOLUMNE RIVER STREAMFLOW OPERATIONS

from the CALIFORNIA APPLICATIONS PROJECT, SCRIPPS INSTITUTION OF OCEANOGRAPHY

Twelve 15-day precipitation and temperature forecasts from the current NCEP Medium-Range Forecast 00Z ensemble have been averaged for use by the City of San Francisco/Hetch Hetchy water/power system, in a prediction experiment planned for winter/spring 2002.(Precipitation is listed in inches/day scaled to the Hetch Hetchy station's 1991-2001 climatology; temperatures are mean daily temperatures, in degrees F scaled to the Tuolumne Meadows station's 1991-2001 climatology. First value is for the day of the time stamp, next value is for one day AFTER the time stamp, and so on. Beware of the longer-lead forecasts, with headers in red...less forecast skill is expected from them.)
DATE STAMP OF FORECAST: 12 8 2004

FORECASTDAY OF MONTH
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
TUOLUMNE MEADOWS TEMPERATURES (F) 34.4 35.2 36.6 37.5 37.4 36.0 36.6 34.5 34.8 34.6 34.3 35.4 36.4 36.5 36.3
HETCH HETCHY PRECIPITATION (in) 0.1 0.1 0.0 0.2 0.0 0.0 0.0 0.2 0.8 0.4 0.0 0.0 0.0 0.0 0.0
# of WET FORECASTS (among 12) 2 1 1 1 0 0 0 4 6 6 0 0 0 0 0

This downscaled forecast is a product of the California Applications Project at Scripps Institution of Oceanography.

EXPLANATION: These forecasts are derived from the NOAA National Centers for Environmental Prediction's daily (00Z) Medium-Range Forecast (MRF) ensemble, as archived at Scripps Institution of Oceanography. The forecasts are rescaled from the global grid of the MRF weather model (roughly 2 degrees of latitude by 2 degrees of longitude), and from the 850 millibar pressure level in the atmosphere, to be equivalent to the temperatures routinely observed by the Tuolumne Meadows SNOWTEL site. Historically, the downscaling has correctly captured 77% of the daily temperature variance there. Precipitation is scaled to have the same monthly means as the Hetch Hetchy precipitation station. In counting the numbers of "wet" ensemble members, daily precipitation forecasts were scaled and then ensemble members with more than 0.25 inches of precipitation on a given day were counted.

These forecasts will provide raw materials for an attempt during winter/spring 2002 to develop a Kalman filter-based medium-range forecast of inflows from the Tuolumne River into Hetch Hetchy reservoir.

Contact dettinge@meteora.ucsd.edu with comments and questions.

URL: http://meteora.ucsd.edu/~dettinge/hetchy/