Above average precipitation occured in February from northern Arizona extending to just south of the Mogollon Rim. Below average precipitation was observed over the southern third of Arizona. Snow continued to accumulate at the highest elevations through mid February with melt occurring during the last week of the month. Snowmelt combined with rainfall earlier in the month resulting in February streamflow volumes well above median for the third consecutive month. Runoff volumes for the February through May period are expected to range from near 120 to 300 percent of median.

March through May runoff volumes in the Salt, Verde, and Tonto River Basins are expected to range from near 110 to 120 percent of the median. Rainfall and snowmelt runoff during February contributed to monthly volumes between 300 to 450 percent of median. Snowpack that remains in the highest elevations of the Salt and Verde Basins ranges from 120 to 260 percent of average. The snowpack will contribute to the seasonal runoff volumes but areal coverage extends across a small fraction of the basin. Additional rainfall during the March through May period may contribute significantly to runoff volumes due to saturated conditions in this area.

March through May runoff volumes in the Gila River Basin are expected to range from near 120 to 250 percent of median. Monthly runoff volumes in February due to rainfall and snowmelt ranged from 200 to 350 percent of median. Snowpack in the San Francisco Basin high elevations ranges from 90 to 260 percent of average. Additional rainfall during the March through May period may contribute significantly to runoff volumes due to saturated conditions.

March through May runoff volumes in the Little Colorado Basin are expected to range from near 120 to 270 percent of median. Monthly runoff volumes in February due to rainfall and snowmelt ranged from near 370 to near 900 percent of median. Snowpack in the Little Colorado southern headwaters ranges between 140 and 160 percent of average. February precipitation amounts ranged from 130 to 180 percent of average. Additional rainfall during the March through May period may contribute significantly to runoff volumes due to saturated conditions.

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