The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 80 percent of average in the Upper Green basin as a whole. May precipitation was 45 percent of average. May was the third month in a row with much below average precipitation.

Snow:
On June 1st, only two SNOTEL locations in the Upper Green had any remaining snow. The snow water equivalent for these two sites was 25 percent of average. As of June 5th, these locations have melted out. It is important to note that the highest measuring location is at 9800 feet in the Upper Green; however, the basin extends to elevations up to 13,000 feet. Nearly half of the basin is not represented by the SNOTEL network. It is possible there is still small amounts of remaining snow at the highest elevations.
--- Upper Green basin snow water equivalent plot.

Streamflow:
May streamflow was near 70 percent of average.

Soil Moisture:
Modeled soil moisture states indicated near average to above average soil moisture conditions last fall prior to snow accumulation.

Climate Forecasts:
Climate forecasts were not a factor because there is not a strong correlation between La Nina conditions and winter precipitation in the Upper Green basin.

Forecast Summary:
The Upper Green had above average and much above average precipitation in October, November, January, and February. However, warm and dry conditions persisted from March through May resulting in decreased snow accumulation and early snowmelt. As a result of the warm and dry spring, the seasonal runoff forecasts have been continually reduced since March. Current April through July streamflow volume forecasts in the Upper Green have decreased by 5 to 15 percent since May. Forecasts for locations on the North Slope of the Uintas are in the bottom three of the historical record. April through July forecasts now range between 47 and 83 percent of average with a median value of 65 percent.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 60 percent of average in the Yampa/White basin. May precipitation was near 25 percent of average. May was the third month in a row with much below average precipitation.

Snow:
The snow had melted out of nearly all the SNOTEL measuring locations by June 1st. Many of these locations had record or near record early melt out dates.
--- Yampa basin snow water equivalent plot.

Streamflow:
May streamflow was 40 percent of average over the basin as a whole.

Soil Moisture:
Modeled soil moisture states indicated above average to much above average soil moisture conditions last fall prior to snow accumulation.

Climate Forecasts:
Climate forecasts were not a factor in the forecasts because there is not a strong correlation between La Nina conditions and winter precipitation in the Yampa/White basin.

Forecast Summary:
Near average precipitation in October and November was followed by multiple months, with the exception of February, of much below average precipitation and above normal temperatures. These conditions lead to decreased snow accumulation and record early snowmelt in the Yampa River Basin. As a result of the unseasonably warm and dry spring, seasonal runoff forecasts have continually decreased since March. Current April through July streamflow volume forecasts in the Yampa have decreased by 10 to 20 percent since May. Many of the April through July forecasts are in the bottom five of the historical record. April through July forecasts now range between 25 and 53 percent of average with a median value of 38 percent.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 70 percent of average in the Duchesne basin. May precipitation was 15 percent of average. May was the third month in a row with much below average precipitation.

Snow:
The snow had melted out of nearly all the SNOTEL measuring locations by June 1st. Many of these locations had record or near record early melt out dates.
--- Duchesne basin snow water equivalent plot.

Streamflow:
May streamflow was near 70 percent of average over the basin as a whole. However, some low elevation locations had monthly volumes less than 50 percent of average.

Soil Moisture:
Modeled soil moisture states indicated much above average soil moisture conditions last fall prior to snow accumulation.

Climate Forecasts:
Climate forecasts were not a factor in the forecasts because there is not a strong correlation between La Nina conditions and winter precipitation in the Duchesne basin.

Forecast Summary:
A wet October was followed by multiple months of below to much below average precipitation in the Duchesne basin. Warm and dry conditions persisted from March through May resulting in decreased snow accumulation and early snowmelt. As a result of the unseasonably warm and dry spring, seasonal runoff forecasts have continually decreased since March. Current April through July streamflow volume forecasts in the Duchesne have again decreased by 10 to 20 percent April through July forecasts now range between 11 and 56 percent of average with a median value of 44 percent.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 70 percent of average in the Lower Green basin. May precipitation was 10 percent of average. May was the third month in a row with much below average precipitation.

Snow:
The SNOTEL locations have been melted out since late April in the Lower Green. All of the snow in the Lower Green has melted. Many of these locations had record or near record early melt out dates.
--- Lower Green basin snow water equivalent plot.

Streamflow:
May streamflow was near 45 percent of average over the basin as a whole.

Soil Moisture:
Modeled soil moisture states indicated near average to above average soil moisture conditions last fall prior to snow accumulation.

Climate Forecasts:
Climate forecasts were not a factor in the forecasts because there is not a strong correlation between La Nina conditions and winter precipitation in the Lower Green basin.

Forecast Summary:
A wet October was followed by multiple months of below to much below average precipitation,with the exception of February, in the Lower Green. Warm and dry conditions persisted from March through May resulting in decreased snow accumulation and early snowmelt. As a result of the unseasonably warm and dry spring, seasonal runoff forecasts have continually decreased since March. Current April through July streamflow volume forecasts in the Lower Green have again decreased by 10 to 15 percent. Many of the forecasts in the Lower Green are in the bottom five of the historical record. April through July forecasts now range between 15 and 43 percent of average with a median value of 36 percent.

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.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was between 65 to 85 percent of average at most locations in the San Juan Basin.

May was the third consecutive month with very dry conditions. Precipitation in May ranged from 0 to near 55 percent of average over most of the San Juan Basin. The Basin average from the month of May was 25% of average.

Snow:
Snow melted off at measuring sites by early June. The warm and dry spring caused snow to melt out a full 4 to 6 weeks early at many locations.

Animas River Basin Snow Plot.

San Juan Basin (above Navajo Reservoir) Snow Plot.

Streamflow:
Streamflow volumes were much below average in May due to the lack of snow. Runoff volumes in May ranged from 40 to 60 percent of average.

Soil Moisture:
Modeled soil moisture was near to above average at highest elevations entering the winter season. Below average soil moisture conditions existed over the southern half of the basin and in tributaries further downstream.

Climate Forecasts:
La Nina climate conditions existed through the winter months. The La Nina climate condition suggests drier than average conditions possible over the San Juan Basin. The La Nina climate condition was considered when developing the forecasts.

Forecast Summary:
The 2012 season was defined by very dry and warm conditions that persisted throughout the spring that resulted in a very early snow melt. At times during the spring the snow pack was at record or near record low levels due to the dry and warm conditions. Runoff volume forecasts were reduced as the season progressed and the dry conditions persisted. Forecasts have been reduced slightly from those issued in early May and some are in the bottom 5 of the historical record. April-July runoff volumes are expected to range from 35 to 55 percent of the 1981-2010 average. June-July runoff volumes volumes are expected to range from near 10 to 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