Precipitation in December ranged from near average to over 300 percent of average across most of the Lower Colorado River Basin. The exception was the extreme southeast corner of Arizona where much below average precipitation was received. December streamflows were near or slightly below median values in the upper Gila and parts of the Little Colorado Basin. Much above median streamflow was observed in the Salt River Basin.

Significant snowpack exists for this time of year at the higher elevations, particularly in the Upper Salt Basin, White Mountains, Verde, and Little Colorado drainages. In these areas average annual seasonal peak snow has already been exceeded. Snowmelt, in general, is a smaller component than rainfall to seasonal streamflow volumes. In the absence of rainfall events during the forecast period, snowmelt is the primary contributor. However, since snow is limited to the higest elevations, areal coverage is only a small percentage of most basins. Runoff volumes due solely to snowmelt are much smaller than runoff produced from basin-wide rain events over the course of a season.

January through May runoff volumes in the Salt River Basin are expected to range from near to slightly above median. December precipitation ranged from 200 to 300 percent of average with significant snow falling at higher elevations. Snowpack in the upper Salt, Verde, and Tonto River Basins has reached levels as high as 320 percent of average, exceeding average seasonal peaks at many locations. December streamflow in the Salt River Basin exceeded 400 percent of the median.

January through May runoff volumes in the Gila River Basin are expected to range from 80 to 105 percent of median. December precipitation ranged from 65 to 200 percent of median at most locations while December streamflow was generally between 80 and 110 percent of median. Snowpack, while not a large contributor to seasonal runoff in much of the Gila Basin, is between 60 to 125 percent of average.

January through May runoff volumes in the Little Colorado Basin are expected to range from near median to 150 percent of median. Smaller, high elevation streams, that draw more runoff from snow melt, are forecast to exceed median values by the greatest amounts. December precipitation ranged from 150 to over 300 percent of average in the Little Colorado with much of it falling as snow above 7500 feet. Snowpack in the Little Colorado headwaters exceeds average annual peaks and ranges from 200 to 280 percent of average. December streamflows in the Little Colorado were near 90 percent of median.

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