Forecast Note
Forecasts in this publication are now being referenced to the 1971-2000 median value. Median represents the mid point of all the April-July volumes observed during this 30 year period. Half of the observed volumes are above this value, and half below this value. Historically forecasts were represented as a percent of the 1971-2000 average (or mean) value. However, the average values were skewed by a few very wet years and are not considered the best metric on which to reference forecasts.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October-April precipitation was near 125 percent of average in the Virgin Headwaters, and ranged from 100 to 145 percent of average basin wide.

April precipitation was near 110 percent of average in the Virgin Headwaters and ranged from 50 to 115 precent of average basin wide.

Streamflow:
April streamflow volumes in the Virgin River basin ranged from 100 to 130 percent of average, or 125 to 160 percent of median.

Snow:
Snowpack conditions in the Virgin River Basin ranged from 150 to 270 percent of average at elevations above about 8500-9000 feet. Lower elevation snow was depleted during the month of April.

Virgin River Basin Snow Plot.

Soil Moisture:
Below average modeled soil moisture conditions existed entering the winter season. Soil moisture conditions beneath areas of persistent winter snowpack remained dry prior to the onset of snow melt.

Climate Forecasts:
El Nino conditions existed during the winter and spring season. These El Nino conditions influenced early season forecasts. El Nino conditions are expected to dissipate by this summer.

Forecast Summary:
As anticipated runoff so far has exceeded median levels. Dry soils that existed under the persistent snowpack may have acted to reduce overall runoff volumes to some extent. Above average snowpack exists, but it remains only at highest elevations and is limited in areal coverage. While streamflow runoff volumes have been reduced slightly from those issued last month, above median flow is still expected. May-July runoff volumes are forecast to range from 180 to 240 percent of median. For the entire April-July runoff period volumes near 175% of median (or 110% of average) are expected.

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