After a warm and dry start to Water Year 2009, December precipitation in the Great Basin totaled 131% of average. These much above average totals greatly improved snow conditions throughout all the northern basin with indexed snow totals of 90% in the Weber and Ogden basins, 87% in the Provo basin, 87% in the Bear River basin and 95% in the Six Creeks basin. Antecedent soil moisture grids from our ESP model however indicate below to much below average soil moisture conditions which tend to lower the April through July volume forecasts. Water year precipitation in the overall Great Basin was 99% of average. However in the headwater regions where most volume is produced during the runoff season, values are below average.

Weather guidance from National Weather Service models are predicting generally dry conditions through mid month. Longer range predictions from NOAA's Climate Prediction Center indicate equal chances of wet or dry conditions through the spring of 2009. A forecast La Nina, released from CPC January 8th, based on current observations, recent trends and model forecast, La Nina conditions are likely to develop and continue into the Northern Hemishphere Spring of 2009.

Volumetric forecasts were weighted slightly lower due to dry soil condition, ranging from 78% of average on the Weber River at Echo to 95% at both Big and Little Cottonwood Canyons near Salt lake City.

As of January 1st, conditions across the Bear River basin are slightly below normal. Snowpack conditions are currently 87% of average. Monthly precipitation was 122% of average while seasonal numbers were 96% of average. Stream flow estimates for the Bear at the Utah/Wyoming border were 66% of average for December showing much below average baseflows, a good indication of low soil moisture conditions. Forecasts in the Bear River Basin ranged from 83% at Blacksmith Fork near Hyrum to 90% of average at Bear near Montpelier below Stewart Dam.

As of January 1st, conditions across the Weber and Ogden basins are slightly below normal. Monthly precipitation was at 120% of average and seasonal precipitation summed to 90% of average. Snow water equivalency is 90% of average. Estimated stream flow for the Weber at Oakley during December were 89% of average. The April through July volume forecasts range from 82% of average at Weber River at Gateway to 90% of average at East Canyon Creek Reservoir near Morgan.

As of January 1st, conditions across the Six Creeks basin near Salt Lake City are slightly below normal. Monthly precipitation for December was 139% of average and seasonal precipitation summed to 105% of average. Current snowpack conditions are 95% of average. Observed stream flow for Mill Creek near Salt lake City were below average for December at 82% of average. April through July volume forecasts range from 82% percent of average at Emigration Creek near Salt Lake City to 95% percent of average for Big and Little Cottonwood Canyon Creeks near Salt Lake City.

As of January 1st, conditions across the Utah Lake basin are slightly below normal. Monthly precipitation for Utah Lake Basin was 135% of average and seasonal precipitation totalled at 96% of average. Observed stream flows for Spanish Fork during December was 125% of average due to heavy rainfall. Currently the indexed snow for the Utah Lake Basin is 90% of average. December stream flow for the Provo at Woodland was 90% of average. Forecasts for Utah Lake ranged from 81% at Spanish Fork near Castilla to 90% of average for Provo at Hailstone.

Differences between the full period forecasts and the residual forecasts may not exactly equal the actual observed volumes due to rounding conventions (see Definitions section).

10% exceedance forecast: Given the current hydrometeorological conditions, i.e current snowpack, soil moisture and streamflow, the volume that has a 10% chance of being exceeded. Previously referred to as "Reasonable Maximum Forecast".

50% exceedance forecast: Given the current hydrometeorological conditions, i.e current snowpack, soil moisture and streamflow, the volume that has a 50% chance of being exceeded. Previously referred to as "Most Probable Forecast".

90% exceedance forecast: Given the current hydrometeorological conditions, i.e current snowpack, soil moisture and streamflow, the volume that has a 90% chance of being exceeded. Previously referred to as "Reasonable Minimum Forecast".

Acre-Foot (af): The volume equal to one acre covered one foot deep (43,560 cubic feet). See kaf below.

Average: The arithmetic mean. The sum of the values divided by the number of values. Values from 1971-2000 are used for this publication.

Categories: Much Above Average=Greater than 130%, Above Average=111-130%, Near Average=90-110%, Below Average=70-89%, Much Below Average=Less than 70%.

CBRFC: Colorado Basin River Forecast Center.

Forecast Period: The period from April 1 through July 31, unless otherwise noted.

kaf: Thousand Acre-Feet. See Acre-Foot above.

Inflow: The volume of water that flows into a reservoir or lake.

Median: The middle value of an ordered set of values. Half of the values are higher and half of the values are lower. When the set contains an even number of values the median is the average of the two middle numbers.

NOAA: National Oceanic and Atomospheric Administration.

NWS: National Weather Service.

Water supply forecasts take into consideration present hydrometeorological conditions and use average basin temperatures and precipitation for the forecast period. As the forecast season progresses, a greater portion of the future hydrologic and climatic uncertainty becomes known and monthly forecasts become more accurate. For more information on the tools we use, consult Water Supply Forecasting Tools.

Volume forecasts represent adjusted flows; that is, observed flows with upstream water use taken into account. Adjusted flows will closely approximate natural or unimpaired flows. However, not all upstream diversions or impoundments are measured or quantifiable. For specific adjustments used with each forecast point, consult the Guide to Water Supply Forecasting.

The Water Supply Outlook is issued monthly January through May by the Colorado Basin River Forecast Center. It represents a coordinated effort between the National Weather Service, Natural Resources Conservation Service, Bureau of Reclamation, U.S. Geological Survey and local water district managers.

Note: Data used in this report are provisional and are subject to revision.

For more information, or to be included on the mailing list, please contact:

Colorado Basin River Forecast Center
2242 W North Temple
Salt Lake City, UT 84116
(801) 524-5130
www.cbrfc.noaa.gov