The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through March precipitation was near 70 percent of average in the Upper Colorado mainstem basin. March precipitation was much below average at just 30 percent of average. This is the driest March on record at many of the SNOTEL sites in the basin.

Snow:
April 1st snow water equivalent was near 50 percent of average in the basin as a whole. In a normal year snow continues to accumulate through March, and into the middle of April at higher elevations. However this year melt occurred during March, especially toward the end of the month and even at SNOTEL sites above 11,000 feet. The April 1 snow water equivalent values are the lowest on record at many of the SNOTEL sites in the basin.
--- Upper Colorado basin snow water equivalent plot

Streamflow:
March streamflow was near 120 percent of average in the basin as a whole. It ranged from 110 percent to 220 percent of average at individual sites. The March volume at about one third of the forecast points in the Upper Colorado basin ranked in the top three of their historical records.

Soil Moisture:
Modeled soil moisture states were above average heading into the winter.

Climate Forecasts:
La Nina climate conditions exist but these do not show a strong correlation with winter precipitation in the Upper Colorado mainstem basin, and therefore were not influential in the development of streamflow volume forecasts.

Forecast Summary:
A wet October was followed by much below average precipitation in November and Decemeber. Near average precipitation occurred in January and February. As of March 1st the Upper Colorado had its best conditions of the season with both the seasonal precipitation and basin snow water equivalent near 80 percent of average. However, March 2012 was one of the warmest and driest March's in the last 30 years in the Upper Colorado basin and as of April 1st the snow water equivalent was just 50 percent of average with seasonal precipitation near 70 percent of average. Because of the unusual conditions during March there were very large drops in the April through July forecast runoff volumes from what was issued March 1st. Current April through July streamflow volume forecasts range between 46 and 68 percent of average with a median value of 55 percent.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through March precipitation ranged from 70 to 95 percent of average in the Gunnison Basin.

March precipitation was much below average over the entire Gunnison Basin. For individiual sites March precipitation ranged from 15 to 55 percent of average, with a Basin average of 35 percent.

Snow:
April 1st snow water equivalent was 60 percent of average in the Gunnison Basin, but varied with several locations below 50 percent of average and some below 40 percent of average. An early onset of the snow melt occurred at all elevation levels due to much above average temperatures in March.

Gunnison Basin snow water equivalent plot

Streamflow:
March streamflow volumes were above average due to the early snow melt. Runoff volumes in March ranged from near average to near 145 percent of average.

Soil Moisture:
Modeled soil moisture states were above average entering the winter season.

Climate Forecasts:
La Nina climate conditions existed throughout the winter but these do not show a strong correlation with winter precipitation or Apri-July runoff volumes in the Gunnison Basin. Therefore they were not influential in the development of streamflow volume forecasts.

Forecast Summary:
A wet October was followed by drier condtions in November and December and very dry conditions in March. Precipitation was near or above average at most locations in January and February. Seasonal precipitation was near 75 percent of average as of March 1st. Snow melt occurred at all elevations due to dry and warm conditions in March. By April 1st the Gunnison Basin snow water equivalent was reduced to 60 percent of average with several sites reporting less than 50 percent of average. Streamflow forecasts were reduced significantly from those issued March 1st. April-July streamflow runoff volumes are expected to range between 45 and 70 percent of the 1981-2010 average.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through March precipitation ranged between 75 and 100 percent of average in the Dolores Basin.

March precipitation was much below average. For individual sites March precipitation ranged from 25 to 50 percent of average, with a Basin average of 35 percent.

Snow:
April 1st snow water equivalent was near 50 percent of average in the Dolores Basin, with a few locations near or below 40 percent of average. An early onset of the snow melt occurred at all elevation levels due to much above average temperatures in March.

Dolores Basin snow water equivalent plot

Streamflow:
March streamflow volumes in the Dolores River Basin were near 105 percent of average.

Soil Moisture:
Modeled soil moisture states were near average entering the winter season.

Climate Forecasts:
La Nina climate conditions existed throughout the winter but these do not show a strong correlation with winter precipitation or April-July runoff volumes in the Dolores Basin. Therefore, it was not influential in the development of streamflow volume forecasts.

Forecast Summary:
Near average soil moisture existed in the Dolores River Basin entering the winter season. Seasonal precipitation was below average for most areas as of April 1st. Snow melt occurred at all elevations due to dry and warm conditions in March. By April 1st the Dolores Basin snow water equivalent was reduced to 50 percent of average with several sites reporting near 40 percent of average. Streamflow forecasts were reduced significantly from those issued March 1st. April-July streamflow runoff volumes are expected to range from 45 to 65 percent of the 1981-2010 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