A week long period of widespread convection near the end of May provided the majority of the precipitation for the month and caused large variations in the percent average precipitation across the Upper Colorado, Gunnison, and Dolores basins. The Gunnison River headwaters were a favored location for some of the heaviest showers and ended the month with 120% of average precipitation. Southern areas of the Upper Colorado Basin and the upper Dolores River saw near to above average precipitation. On the other hand, the San Miguel River and the north and extreme eastern portions of the Upper Colorado Basin headwaters had below average precipitation with 85% and 80% of average, respectively. The majority of the snow was already melted in all basins by June 1, with most remaining snow above 11,000 feet. Overall, the June 1st April through July forecast runoff volumes increased from the May 1st forecasts in the Upper Colorado and Gunnison basins and showed little change in the Dolores Basin.

-Model Soil Moisture/Observed Streamflow... Soil moisture was near to above average in the Upper Colorado heading into the winter. May streamflow was 165% of average. According to the USGS, many sites had observed flows that were greater than the 75th percentile, with some above the 90th percentile, of the historical monthly flow and many of the unregulated volumes calculated by the CBRFC ranked in the top five historical volumes. While the overall monthly average maximum temperatures during May in the Upper Colorado basin were near normal, the monthly average minimum temperatures were significantly above average. In addition, widespread thunderstorms during the last ten days of the month helped to keep the flows up when the temperatures dropped.

-Snowpack/Precipitation... The basin-wide monthly precipitation for May was 105% of average. However, some of the headwater areas only received 80% of average while the Blue River drainage had 110% of average and areas near the bottom of the basin had close to 120% of average. Snow water equivalent was 35% of average on Jun 1st and most of the remaining snow is above 11,000 feet.

-Short Term Precipitation Forecast... As of June 8th, meteorological models were predicting increased chances for below normal temperatures and above normal precipitation through the middle of June.

-General Discussion... Most of the April through July volume forecasts stayed the same or increased slightly from the May 1st issuance, and the forecasts now range between 56% of average on Mill Creek to 116% of average on Williams Fork. The June through July streamflow volume forecasts are much lower percentage-wise ranging between 43% and 92% of average since most of the snow had melted by June 1st.

-Model Soil Moisture/Observed Streamflow... Soil moisture for most of the upper basins in the Gunnison was near to slightly below average in the fall. Observed streamflow for the month of May was much above average for the Gunnison due to much above average temperatures during the first 7 to 14 days of May.

-Snowpack/Precipitation...The June 1st snow water equivalent percent of average was 5 percent. May precipitation for the upper Gunnison Basin was 120 percent of average. This was due to a week long period of convection with similarities to the monsoon. This helped raise the April-July volume forecast at Blue Mesa 10 percent from last month's forecast. The entire Gunnison Basin received 110 percent of average precipitation in May. Seasonal precipitation for the upper Gunnison was 110 percent of average while the entire Basin was 105 percent.

-Short Term Precipitation Forecast... As of June 8th, meteorological models were predicting increased chances for below normal temperatures and above normal precipitation through the middle of June.

-General Discussion... April-July forecasts for the most part were raised 5-10 percent from last month's forecast. The median April-July forecast for the Gunnison Basin increased to 105 percent of average.

-Model Soil Moisture/Observed Streamflow... Soil moisture for most of the upper basins in the Dolores was below average in the fall. This was most likely due to limited monsoonal precipitation during the summer months. Observed streamflow for the month of May was average for the Dolores.

-Snowpack/Precipitation... As of June 1st, snow water equivalent across the Dolores was 0 percent of average. The Dolores Basin recieved average precipitation for May, while seasonal precipitation for the Dolores Basin is at 90 percent of average.

-Short Term Precipitation Forecast... As of June 8th, meteorological models were predicting increased chances for below normal temperatures and above normal precipitation through the middle of June.

-General Discussion... With much of the snow pack below 10,000 feet melted and stream flow being below average in the Dolores Basin, April-July volume forecasts dropped around 3 to 5 percent from last month's forecast. The median April-July forecast is 80 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 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