The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October-March precipitation was near 190 percent of average in the Virgin Basin headwaters and ranged rom 165 to 200 percent of average elsewhere.

March precipitation was near 80 percent of average in the Virgin Basin headwaters, and 55 to 80 percent of average throughout the remainder of the Basin.

Snow:
April 1st snowpack in the Virgin River Basin ranged from 160 to 200 percent of average at elevations above 8000 feet.

Virgin River Basin Snow Plot.

Streamflow:
March streamflow volumes in the Virgin headwaters were near 165 percent the 1971-2000 median.

Soil Moisture:
Above average soil moisture conditions exist throughout the Virgin River Basin.

Climate Forecasts:
La Nina climate conditions have existed throughout the winter months and suggest slightly decreased chances of precipitation in this area. The La Nina influence was considered in generation of these forecasts.

Forecast Summary:
Below average precipitation occurred in March and snow melt was observed at elevations up to near 8000 feet. Despite the loss of snow at lower elevations, high elevation snowpack remains over 160 percent of average as of April 1st. Streamflow volumes in the Virgin and Santa Clara River headwaters exceeded 160 percent of median (120-150% of average) for March due to the lower elevation melt. With a large snowpack in place at higher elevations, and above average soil moisture conditions due in part to heavy rainfall in October and December, conditions are favorable for significant runoff volumes this spring.

April-July runoff volumes are forecast to range from near 270 to 300 percent of median, or 160 to 175 percent of average.

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 1981-2010 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