New 1981-2010 Averages being used this year.
Note: This publication is currently undergoing major revisions. The current publication will be replaced with a new publication based on stakeholder requirements and scientific advances. We expect to begin sharing details on this soon. If you have input on content, format, or publication frequency at any time, please contact us at cbrfc.webmasters@noaa.gov.Virgin Water Supply Outlook, June 1, 2012

Virgin Water Supply Outlook, June 1, 2012


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

Contents

Virgin Summary






*Median of forecasts within each basin.


Virgin Basin Conditions

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October-May precipitation was below average at 70 percent of average in the Virgin Headwaters.

May precipitation was much below average at 5 percent of average in the Virgin Headwaters.

Snow:
The remaining portions of the snowpack melted out in May in the Virgin River basin, and the snowpack is gone as of June 1st in the Virgin River Basin.

Virgin River Basin Snow Plot

Streamflow:
May streamflow volume dropped from April in terms of percent of average and median. The May streamflow volume for the Virgin River basin was 30 percent of average and 50 percent of median.

Soil Moisture:
Modeled soil moisture content was above average in the Virgin River Headwaters going into the winter.

Climate Forecasts:
Conditions have transitioned from La Nina to neutral. La Nina/El Nino was not factored into this month's forecasts.

Forecast Summary:
The limited snowpack melted out in May, even at high elevations of the Virgin River Basin. May was very dry, with the monthly precipitation being much below average, at 5 percent of average. May streamflows were also much below average, at 30 percent of average.

Runoff during the June-July period is expected to range from 19 to 46 percent of average and 47 to 66 percent of median.

Full period April-July volumes are forecast to range between 32 and 43 percent of average and 49 to 63 percent of median.




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


Virgin Specific Site Forecasts (kaf)

Click site name for graph.
Forecast
Period
90%
Exceedance
Volume
50%
Exceedance
Volume
Percent
Average
10%
Exceedance
Volume
Santa Clara
Pine Valley, Nr April-July1.82402.4
June-July0.150.4190.76
Virgin
Virgin April-July23254328
June-July4.87469.6
Hurricane, Nr April-July17.2203224
June-July3.974311
Littlefield April-July18.9223426
June-July2.15299.2

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).

Precipitation Maps




Definitions

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.

Rounding Conventions:
RangeRound to
0-1.990.01
2.0-19.90.1
20-1991.0
200-9995.0
1000+3 significant digits


Streamflow: The volume of water that flows past a specific stream site.

Water Year: The 12-month period, October 1 through September 30. The water year is designated by the calendar year in which it ends. Thus, the year ending September 30, 2008, is called the "2008 water year."


Additional Information

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


Hydrologist: S. Bender