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Lower Colorado Water Supply Outlook, February 1, 2009


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

Contents

Lower Colorado Summary

Precipitation in January was much below average across all of the Lower Colorado River Basin. Snowmelt also occurred in many locations, particularly at elevations up to 7500 feet. Snowpack conditions remain above average across most higher elevations with the exception of the Gila Basin where it is 70 percent of average or less. Streamflow during January was much above median in the Salt River Basin due to a combination of snow melt and precipitation that occurred later in the month. Elsewhere streamflow volumes were near to below median.

An active weather pattern bringing preciptation to Arizona is anticipated the first half of February. As a result February streamflow volumes are forecast to meet or exceed median levels. Seasonal volume forecasts vary widely, with near to above median volumes expected in the Little Colorado Basin, near to slightly below median values expected in the Salt River Basin, and below median values expected in the Gila Basin. Weak La Nina conditions are forecast to extend into the spring months. La Nina usually correlates with drier weather conditions across the Lower Colorado River Basin region.



*Median of forecasts within each basin.


Salt Basin Conditions

Precipitation for the month of January was near 40 percent of average in the Salt River Basin. February 1st snowpack ranged from near 115 to 140 percent of average in the higher elevations. Precipitation combined with low and mid elevation snowmelt during the last week of January and resulted in monthly streamflow volumes that ranged from 140 to 220 percent of median.

The runoff response from the late January precipitation and melt was indicative of moist soil conditions. Efficient runoff is anticipated from future precipitation events that may occur for the next several weeks. With additional precipitation expected during the first half of February, monthly volumes are forecast to exceed median values. Seasonal forecast volumes for February through May range from 80 to 95 percent of median.



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

Gila Basin Conditions

Precipitation in the Upper Gila Basin during January was near 35 percent of average. Snowpack conditions decreased from 180 percent of average on January 1st to 60 percent of average on February 1st. Streamflow conditions in January were near median in the Gila Basin headwaters, and ranged from 65 to 85 percent of median in the San Francisco drainage and elsewhere.

Seasonal streamflow volumes for February through May are expected to range from 60 to 80 percent of median across most of the upper Gila River Basin.



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

Little Colorado Basin Conditions

January precipitation ranged from near 40 to 70 percent of average in the Little Colorado Basin. Seasonal precipitation varies from near 70 percent of average at lower elevations to over 110 percent of average at higher elevation headwater sites where snowpack exists. While snowpack expressed as a percent of average decreased over the last month, the actual snowpack increased slightly. Current snowpack in the Little Colorado headwaters is near 150 percent of average. February through May runoff volumes in the Little Colorado Basin are expected to range from near median to 130 percent of median. Smaller, high elevation streams, that draw more runoff directly from snow melt, are forecast to exceed median values by the greatest amounts.



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


Salt Specific Site Forecasts (kaf)

Click site name for graph.
Forecast
Period
90%
Exceedance
Volume
50%
Exceedance
Volume
Percent
Median
10%
Exceedance
Volume
Salt
Roosevelt, Nr February-May14933093620
Tonto Ck
Roosevelt, Nr, Gun Ck, Abv February-May7.94386126
Verde
Blo Tangle Ck, Abv Horsehoe Dam February-May7417085325


Gila Specific Site Forecasts (kaf)

Click site name for graph.
Forecast
Period
90%
Exceedance
Volume
50%
Exceedance
Volume
Percent
Median
10%
Exceedance
Volume
Gila
Gila, Nr February-May20377061
Virden, Nr, Blue Ck, Blo February-May16435797
Solomon, Nr, Head Of Safford Vly February-May259365230
San Carlos Res, Coolidge Dam, At February-May106274165
San Francisco
Glenwood, Nr February-May10.9208333
Clifton February-May184678102
San Pedro
Charleston February-May1.72695


Little Colorado Specific Site Forecasts (kaf)

Click site name for graph.
Forecast
Period
90%
Exceedance
Volume
50%
Exceedance
Volume
Percent
Median
10%
Exceedance
Volume
Little Colorado
Lyman Lk, Abv, St. Johns, Nr February-June2.5811318.3
Woodruff February-May1310721
Rio Nutria
Ramah, Nr February-May0.152.4809.8
Zuni
Black Rock Res, Abv February-May0.481.2852.3
Cebolla Ck
Ramah Res February-May0.011.3805.8
East Clear Ck
Blue Ridge Res, Pine, Nr February-May6.4169832
Clear Ck
Winslow, Nr February-May53510379
Chevelon Ck
Winslow, Nr, Wildcat Cyn, Blo February-May41611536
Walnut Ck
Lake Mary February-May2.351049.3

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).

Lower Colorado Reservoir Contents (kaf)

Usable
Capacity
EOM
Contents
Percent
Usable
Capacity
Last Year
EOM
Last Year
%Capacity
Salt
Roosevelt 1653.0 1642.5 99 1248.4 76
Horse Mesa 245.0 238.9 98 231.3 94
Mormon Flat 58.0 55.6 96 56.1 97
Stewart Mountain 70.0 64.5 92 68.5 98
Horseshoe 109.2 30.2 28 104.2 95
Bartlett 178.0 118.4 66 176.9 99
untitled
TOTAL 2313.2 2150.1 93 1885.4 82
Little Colorado
Lyman Lake 31.0 14.4 46 9.0 29
Bill Williams
Alamo 1045.0 154.8 15 148.7 14
Agua Fria
Lake Pleasant 1145.0 613.0 54 680.8 59
Gila
San Carlos 885.0 227.5 26 248.7 28
Painted Rock 2476.0 0.0 0 0.0 0
Colorado
Lake Powell 24322.0 13154.6 54 10880.4 45
Lake Mead 27380.0 12573.0 46 13005.0 47
Lake Mohave 1810.0 1647.4 91 1671.0 92
Lake Havasu 619.0 559.3 90 556.0 90
untitled
TOTAL 59713.0 28944.0 48 27199.5 46

Monthly Streamflows













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 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.

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: G. Smith