The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was near 65 percent of average in the Upper Colorado mainstem basin. May was the third month in a row with much below average precipitation coming in at just 35 percent of average.

Snow:
As of June 1st there was essentially no snow left at any of the SNOTEL sites within the Upper Colorado mainstem basin. Many of these sites melted out at the earliest time in their 30 year history this year.
--- Upper Colorado basin snow water equivalent plot

Streamflow:
May streamflow was much below average in the basin as a whole. Monthly volumes at individual sites ranged from 20 to 70 percent 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, April and May were warm and dry causing the early meltout of a near record low snowpack. As a result, the seasonal runoff forecasts have been continually reduced the last few months. Most of the June through July forecasts are for volumes that would be the 2nd lowest on record if they occurred, while most of the April through July forecasts would be in the bottom 3 of their historical records. Current June through July streamflow volume forecasts range between 12 and 35 percent of average with a median value of 30 percent. Current April through July streamflow volume forecasts range between 30 and 50 percent of average with a median value of 40 percent.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation ranged from 50 to 80 percent of average in most of the Gunnison River Basin.

For the third consecutive month very dry conditions were experienced in the Gunnison Basin. May precipitation ranged from 10 to 50 percent of average at most locations. The Basin average precipitation was 30 percent of average.

Snow:
The snow had melted out of nearly all measuring sites by June 1st. The snow melt was very early this season due to the warm dry spring. Many sites were void of snow a full 4 to 6 weeks earlier than average.

Gunnison Basin snow water equivalent plot

Streamflow:
Streamflow volumes for May were much below average due to the lack of snow. Runoff volumes for the month of May ranged from 25 to 70 percent of average at most locations. Volumes below 20 percent of average occurred at lower elevation streams where snow had melted out during April.

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:
The 2012 season was defined by extremely dry and warm conditions during the spring, with record or near record low snow amounts due the dry conditions and early snow melt. Seasonal runoff forecasts were progressively reduced throughout the season as the dry conditions persisted. Forecast runoff volumes were reduced slightly from those issued in early May. Several April-July forecasts are the 2nd or 3rd lowest on record. For June-July many some volume forecasts are the lowest on record.Record Seasonal April-July streamflow runoff volumes are expected to range from near 10 to 50 percent of the 1981-2010 average. June-July streamflow runoff volumes are expected to range from near 5 to 30 percent of average.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation ranged between 60 and 85 percent of average at most locations in the Dolores Basin.

May was the third consecutive month with very dry conditions in the Dolores Basin. May precipitation ranged from 0 to 15 percent of average at most locations. The Basin average precipitation was near 5 percent of average.

Snow:
The snow had melted out of nearly all measuring sites by June 1st. The snow melt was very early this season due to the warm dry spring. Many sites were void of snow a full 4 to 6 weeks earlier than average.

Dolores Basin snow water equivalent plot

Streamflow:
Streamflow volumes for May ranged from 35 to 65 percent of average in the Dolores Basin.

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:
The 2012 season was defined by extremely dry and warm conditions during the spring, with record or near record low snow amounts due the dry conditions and early snow melt. Seasonal runoff forecasts were progressively reduced throughout the season as the dry conditions persisted. Forecast runoff volumes were reduced slightly from those issued in early May. Some forecasts are among the bottom 5 on record. Forecast April-July streamflow runoff volumes are expected to range from 30 to 45 percent of the 1981-2010 average. June-July streamflow runoff volumes are expected to range between 15 and 30 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