The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was near 65 percent of average in the Upper Colorado mainstem basin. April was another much below average precipitation month with 50 percent of average.

Snow:
May 1st snow water equivalent was near just 20 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, but this year snow melt began by the last half of March. Most SNOTEL sites below 10,500 feet in the basin do not have any snow left, and many of those melted out at the earliest time in their 30 year history.
--- Upper Colorado basin snow water equivalent plot

Streamflow:
April streamflow was near to above average in the basin as a whole. However, some sites in the upper part of the basin had volumes greater than 150% of average.

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

Climate Forecasts:
La Nina climate conditions existed throughout the winter 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 December. Near average precipitation occurred in January and February. March 2012 was one of the warmest and driest March's in the last 30 years in the Upper Colorado basin and April was another warm, dry month, although not to the same extent as March. As of May 1st the snow water equivalent was just 20 percent of average with seasonal precipitation near 65 percent of average. There were again large drops in the forecast runoff volumes from what was issued last month. Most of the May through July forecasts are for volumes that would be the 2nd or 3rd lowest on record if they occurred. Current May through July streamflow volume forecasts range between 25 and 50 percent of average with a median value of 40 percent. Current April through July streamflow volume forecasts range between 35 and 55 percent of average with a median value of 45 percent.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation ranged from 60 to 85 percent of average in most of the Gunnison River Basin.

April was another warm and dry month in the Gunnison Basin with precipitation generally between 30 to 65 percent of average.

Snow:
The May 1st snow water equivalent for the Gunnison Basin was near 30 percent of average. However snow has melted out at several measuring sites. Where snow remains it ranged between 10 and 55 percent of average on May 1st.

All areas have experienced a very early snow melt as above average temperatures occurred in March and April. Some snow measuring sites that were void of snow in April or early May melted out a full 4 to 6 weeks earlier than average.

Gunnison Basin snow water equivalent plot

Streamflow:
Streamflow volumes for April were near or above average, ranging from 100 to 145 percent of average at higher elevation and headwater locations. At locations further downstream April streamflow ranged from 60 to 85 percent of average, most likely due to lack of snow that was depleted prior to April, and increased irrigation demands due to the warm and dry conditions.

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:
Although October was wet and soil moisture was above average entering the season, the very dry and warm spring, lack of snow, and early snow melt have had a detrimental impact on the anticpated April-July streamflow runoff volumes. Forecast runoff volumes have been further reduced from those issued in April. Several forecasts are now the 2nd or 3rd lowest on record. Seasonal April-July streamflow runoff volumes are expected to range from near 15 to 50 percent of average. May-July streamflow runoff volumes are expected to range from near 10 to 45 percent of average.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation ranged between 70 and 95 percent of average in the Dolores Basin.

April experienced above average temperatures and much below average precipitation. For individual sites April precipitation ranged from 20 to 80 percent of average, with a Basin average of 60 percent.

Snow:
The May 1st snow water equivalent for the Dolores Basin was near 20 percent of average. However most snow measring sites had already melted out with remaining snow confined to the highest elevations and north facing or protected locations. Some snow measuring sites have melted out a full 4 to 8 weeks earlier than average.

Dolores Basin snow water equivalent plot

Streamflow:
Streamflow volumes for April were near or above average at high elevation headwater locations, ranging from 105 to 150 percent of average. At locations further downstream April streamflow ranged from 55 to 90 percent of average, most likely due to lack of snow that was depleted prior to April, and increased irrigation demands due to the warm and dry conditions.

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:
Although October was wet and soil moisture was near or slightly above average entering the season, the lack of snow during the winter combined with warmer and drier conditions during the spring has had a detrimental impact on anticipated April-July streamflow runoff volumes. Forecast runoff volumes have been further reduced from those issued in April. Some forecasts are now among the bottom 5 on record. Forecast April-July streamflow runoff volumes are expected to range from 45 to 50 percent of the 1981-2000 average. May-July streamflow runoff volumesrunoff volumes are expected to range between 30 and 40 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