Precipitation in January was much below average across all of the Lower Colorado River Basin. Snowmelt also occurred in many locations, particularly at elevations up to 7500 feet. Snowpack conditions remain above average across most higher elevations with the exception of the Gila Basin where it is 70 percent of average or less. Streamflow during January was much above median in the Salt River Basin due to a combination of snow melt and precipitation that occurred later in the month. Elsewhere streamflow volumes were near to below median.

An active weather pattern bringing preciptation to Arizona is anticipated the first half of February. As a result February streamflow volumes are forecast to meet or exceed median levels. Seasonal volume forecasts vary widely, with near to above median volumes expected in the Little Colorado Basin, near to slightly below median values expected in the Salt River Basin, and below median values expected in the Gila Basin. Weak La Nina conditions are forecast to extend into the spring months. La Nina usually correlates with drier weather conditions across the Lower Colorado River Basin region.

Precipitation for the month of January was near 40 percent of average in the Salt River Basin. February 1st snowpack ranged from near 115 to 140 percent of average in the higher elevations. Precipitation combined with low and mid elevation snowmelt during the last week of January and resulted in monthly streamflow volumes that ranged from 140 to 220 percent of median.

The runoff response from the late January precipitation and melt was indicative of moist soil conditions. Efficient runoff is anticipated from future precipitation events that may occur for the next several weeks. With additional precipitation expected during the first half of February, monthly volumes are forecast to exceed median values. Seasonal forecast volumes for February through May range from 80 to 95 percent of median.

Precipitation in the Upper Gila Basin during January was near 35 percent of average. Snowpack conditions decreased from 180 percent of average on January 1st to 60 percent of average on February 1st. Streamflow conditions in January were near median in the Gila Basin headwaters, and ranged from 65 to 85 percent of median in the San Francisco drainage and elsewhere.

Seasonal streamflow volumes for February through May are expected to range from 60 to 80 percent of median across most of the upper Gila River Basin.

January precipitation ranged from near 40 to 70 percent of average in the Little Colorado Basin. Seasonal precipitation varies from near 70 percent of average at lower elevations to over 110 percent of average at higher elevation headwater sites where snowpack exists. While snowpack expressed as a percent of average decreased over the last month, the actual snowpack increased slightly. Current snowpack in the Little Colorado headwaters is near 150 percent of average. February through May runoff volumes in the Little Colorado Basin are expected to range from near median to 130 percent of median. Smaller, high elevation streams, that draw more runoff directly from snow melt, are forecast to exceed median values by the greatest amounts.

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