Lake Powell Water Supply Outlook, June 1, 2008

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Prepared by CBRFC
NOAA, National Weather Service
Colorado Basin River Forecast Center
Salt Lake City, Utah
www.cbrfc.noaa.gov

Contents

Lake Powell Sub-Basin Summaries

Variable runoff conditions continue throughout the Green River Basin. Portions of the Yampa and White have reported average flows in May, while the Upper Green observed flows have been much below average. The outlooks for the rest of the runoff period ranges from above average in the Yampa to below average for portions of the Upper Green. For the majority of points, ESP forecast values were weighted higher than statistical methods.



*Median of forecasts within each basin.


The Upper Colorado mainstem and the Gunnison basin both received above average precipitation for the month, but the Dolores basin only received 80% of average. Temperatures averaged over the entire month were near to slightly below average in western Colorado during May, but daily temperatures within the month fluctuated between periods of much above and much below average temperatures. As a result, May runoff volumes were much above average in the Upper Colorado and Gunnison, but there is plenty of snow left to melt as June 1st snow water equivalent is much above average. May runoff volumes in the Dolores basin were near normal and June 1st snow water equivalent is below average. The median of the forecasts for the April through July runoff volumes increased in the Upper Colorado and Gunnison, but decreased in the Dolores.



*Median of forecasts within each basin.


-Observed Streamflow...Streamflow for the month of May was average to above average for the San Juan Basin.

-Snowpack/Precipitation...Snowpack above Navajo Reservoir dropped to 105 percent of average. The Animas was below average for snowpack while the entire San Juan Basin dropped to 90 percent. Precipitation over the San Juan Basin for May was average with 100 percent. Seasonal average precipitation for the San Juan Basin dropped slightly to 105 percent.

-Short Term Precipitation Forecast...The forecast models through the next five days forecast 2 to 6 tenths of an inch of water over the San Juan Basin into Thursday evening. Relatively dry conditions will return this weekend into early next week. Temperatures over the next five days will be near to slightly below normal into Friday but rebound to above normal by early next week. The CPC forecast for days 6 through 10 have an increased chance for above normal temperatures and below normal precipitation.

-General Discussion...The current snowpack along with both statistical forecasts and ESP numbers suggest keeping the same forecast across all of the headwaters of the San Juan Basin. Near average precipitation across the San Juan Basin during the past month also reflect current thinking of generally keeping the same volume forecasts for the San Juan. CPC guidance for the remainder of June continues to show an increased chance of above normal temperatures and near to slightly below normal precipitation.



*Median of forecasts within each basin.




Upper Green Basin Conditions

Precipitation in the Upper Green was above average in May. Basin wide, the snowpack is 115 percent of the June 1st average. Many Snotel sites along the Wind River Range are reporting snow free conditions, though upper elevation modeled snow states remain near average.



* Percent usable capacity, not percent average contents.
Click for multi-month Graph.

Yampa/White Basin Conditions

Substantial seasonal snowpack in the headwaters of the Yampa and Little Snake have translated into above average May observed flows for a number of forecasts points in the basin. Above average flows are forecasted for the rest of the runoff period.



* Percent usable capacity, not percent average contents.
Click for multi-month Graph.

Duchesne Basin Conditions

Forecasts for the remainder of the runoff period are in the near average to below average for points in the Duchesne Basin.



* Percent usable capacity, not percent average contents.
Click for multi-month Graph.

Lower Green Basin Conditions

Forecasts for the remainder of the runoff period range from near average in the Price drainage to below average in the in the headwaters of the San Rafael.



* Percent usable capacity, not percent average contents.
Click for multi-month Graph.

Upper Colorado Mainstem Basin Conditions

After two months of slightly below average precipitation, the Upper Colorado mainstem basin returned to wetter conditions during May with 110% of average precipitation for the month. This, combined with already high snowpack and near to slightly below average monthly temperatures, left the June 1st snow water equivalent values at much above average. The April through July streamflow forecasts were raised at most points in the basin, although some did go down because of low observed flows to date. These forecasts now range between 98% and 145% of average, with the exception of Mill Creek which is forecast at 66% of average.



* Percent usable capacity, not percent average contents.
Click for multi-mo