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.Lake Powell Water Supply Outlook, June 1, 2011

Lake Powell Water Supply Outlook, June 1, 2011


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

Contents

Lake Powell Sub-Basin Summaries




*Median of forecasts within each basin.





*Median of forecasts within each basin.







*Median of forecasts within each basin.




Upper Green Basin Conditions

Forecasts for Hams Fork, Blacks Fork, and Flaming Gorge inflow are in the top five historical observed volumes.
The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 140 percent of average in the Upper Green basin. May precipitation was 200 percent of average in the basin as a whole.

Snow:
June 1st snow water equivalent was 360 percent of average in the basin; please note that this time of year the percent of average may not provide a valid measure of current conditions. The seasonal peak occurred near mid April and was near 135 percent of average. However, it should be noted that there are no snotel locations above 10,000 ft making it difficult to determine the current snowpack conditions.
--- Upper Green basin snow water equivalent plot.

Streamflow:
May streamflow was near 95 percent of average.

Soil Moisture:
Modeled soil moisture states were below average heading into the winter.

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:
Most forecast volumes increased from last month due to the much above average precipitation in May and cold temperatures which have delayed the snow melt. Snowpack and seasonal precipitation were still both much above average on June 1st. As a result of the current conditions, the April through July streamflow volume forecasts increased by approximately 5 to 10 percent from the May 1 forecasts. Current forecasts range between 115 and 181 percent of average with a median value of 145 percent. Forecasts for Hams Fork, Blacks Fork, and Flaming Gorge inflow are in the top five historical observed volumes.



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

Yampa/White Basin Conditions

All of these forecast volumes are at or above the record volume with the exception of the White River.
The following conditions influenced this month's forecasts:
Precipitation:
Seasonal October through May precipitation was 140 percent of average in the Yampa/White basin. May precipitation was 155 percent of average in the basin as a whole.

Snow:
June 1st snow water equivalent was 274 percent of average in the basin; please note that this time of year the percent of average may not provide a valid measure of current conditions. A wet and cold May has delayed snow melt and on June 1st there was still 135% of the seasonal peak, which typically occurs in mid April, at snotels above above 9,500ft. Many of the snotels in the basin have record snow water equivalent.
--- Yampa basin snow water equivalent plot.

Streamflow:
May streamflow was near 115 percent of average.

Soil Moisture:
Modeled soil moisture states were near average heading into the winter for the Yampa, Little Snake,and White River basins.

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:
Most forecast volumes increased from last month due to the much above average precipitation in May and cold temperatures which have delayed the snow melt. Snowpack and seasonal precipitation were still both much above average on June 1st. It is also important to note that many of the snotel locations have record snow water equivalent for June 1. As a result of the current conditions, the April through July streamflow volume forecasts have increased by approximately 5 to 10 percent. Current forecasts range between 161 and 242 percent of average with a median value of 200 percent. All of these forecast volumes are at or above the record volume with the exception of the White River.



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

Duchesne Basin Conditions

Many of these forecast volumes are in the top five historical observed volumes.
The following conditions influenced this month's forecasts:
Precipitation:
Seasonal October through May precipitation was 165 percent of average in the Duchesne basin. May precipitation was 200 percent of average the basin as a whole.

Snow:
June 1st snow water equivalent was 385 percent of average in the basin; please note that this time of year the percent of average may not provide a valid measure of current conditions. A wet and cold May has delayed snow melt and on June 1st there was still 100% of the seasonal peak, which typically occurs in mid April. However, the majority of the snow exists in areas above 10,000ft.
--- Duchesne basin snow water equivalent plot.

Streamflow:
May streamflow was 80 percent of average.

Soil Moisture:
Modeled soil moisture states were much below average to below average heading into the winter.

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:
Most forecast volumes increased from last month due to the much above average precipitation in May and cold temperatures which have delayed the snow melt. Snowpack and seasonal precipitation were still both much above average on June 1st. As a result of the current conditions, the majority of the April through July streamflow volume forecasts increased by approximately 5 to 10 percent. Current forecasts range between 108 and 275 percent of average with a median value of 188 percent. Many of these forecast volumes are in the top five historical observed volumes.



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

Lower Green Basin Conditions

Many of these forecast volumes are in the top five historical observed volumes.
The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 165 percent of average in the Lower Green basin. May precipitation was 190 percent of average.

Snow:
June 1st snow water equivalent was 445 percent of average in the basin; please note that this time of year the percent of average may not provide a valid measure of current conditions. Typically, the snow has melted out in this basin by June 1. However, currenlty there is still a small amount of snow located only at the highest elevations.
--- Lower Green basin snow water equivalent plot.

Streamflow:
May streamflow was near 115 percent of average.

Soil Moisture:
Modeled soil moisture states were much below to near average heading into the winter.

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:
Most forecast volumes increased slightly from last month due to the much above average precipitation in May and cold temperatures which have delayed the snow melt. As a result of the current conditions, the majority of the April through July streamflow volume forecasts increased, by approximately 5 to 10 percent. Current forecasts range between 163 and 212 percent of average with a median value of 190 percent.



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

Upper Colorado Mainstem Basin Conditions

Most of these forecast volumes are in the top two historical volumes of the last 40 years for both the April-July and the June-July period. The following points are forecast at or above the record volume: Lake Granby, Willow Creek Reservoir, Wolford Reservoir, Colorado River near Kremmling, and Colorado River near Dotsero

The following conditions influenced this month's forecasts:

Precipitation:
May precipitation was near 130 percent of average overall in the Upper Colorado mainstem basin making it above average for seven out of the eight months of this water year so far. Seasonal precipitation (the total since October) remained at 140 percent of average.

Snow:
June 1st snow water equivalent was near 290 percent of average in the basin as a whole; please note that this time of year the percent of average may not provide a valid measure of conditions. Another month of cooler than average temperatures has delayed the bulk of the snow melt and as of June 1st there was still 100% of the average seasonal peak, which usually occurs in mid April. Conditions are even more extreme in the headwaters of the basin where the June 1st snow water equivalent was near 130 percent of the average seasonal peak. Many snotel sites throughout the basin have snow water equivalent values that are at record levels when compared to their historical period (20-30 years).
--- Upper Colorado basin snow water equivalent plot

Streamflow:
April streamflow was above average at 110 percent.

Soil Moisture:
Modeled soil moisture states were near to above average heading into the winter.

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 Colorado mainstem basin.

Forecast Summary:
Most forecast volumes were raised from last month due to the much above average precipitation during May as well as cool temperatures which inhibited the snow melt. This has resulted in near to above record snow pack conditions throughout much of the basin. Current April through July streamflow volume forecasts range between 140 and 235 percent of average with a median value of 165 percent. Current June through July streamflow volume forecasts range between 165 and 315 percent of average with a median value of 190 percent. Most of these forecast volumes are in the top two historical volumes of the last 40 years for both the April-July and the June-July period. The following points are forecast at or above the record volume: Lake Granby, Willow Creek Reservoir, Wolford Reservoir, Colorado River near Kremmling, and Colorado River near Dotsero



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

Gunnison Basin Conditions

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 120 percent of average in the Gunnison basin. May precipitation was 115 percent of average.

Snow:
June 1st snow water equivalent was near 230 percent of average in the Gunnison basin; please note that this time of year the percent of average may not provide a valid measure of conditions.
--- Gunnison basin snow water equivalent plot

Streamflow:
May streamflow was 90 percent of average.

Soil Moisture:
Modeled soil moisture states were below to near average heading into the winter.

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

Forecast Summary:
May precipitation throughout the Gunnison basin was above average at 115%. Seasonal precipitation for the Gunnison did not change from last month's 120% of average. The current April through July streamflow volume forecasts dropped 5% from last month's forecast. Current April through July streamflow volume forecasts range between 100 and 133 percent of average with a median value of 125 percent.



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

Dolores Basin Conditions

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 105 percent of average in the entire Dolores basin. May precipitation was 150 percent of average.

Snow:
June 1st snow water equivalent was 140 percent of average in the Dolores basin; please note that this time of year the percent of average may not provide a valid measure of conditions.
--- Dolores basin snow water equivalent plot

Streamflow:
May streamflow was 70 percent of average.

Soil Moisture:
Modeled soil moisture states were average to slightly below average heading into the winter.

Climate Forecasts:
The correlation of La Nina and April through July water volumes in the Dolores basin is small that it did not influence the forecast process.

Forecast Summary:
May precipitation throughout the Dolores basin was much above average at 150%. Seasonal precipitation for the Dolores basin remained the same from last month at 105% of average. Due to the cool temperatures in May and above average monthly precipitation, this month's April-July forecast volumes rose slightly from last month. Current April through July streamflow volume forecasts range between 76 and 93 percent of average, with a median value of 80 percent.



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

San Juan Basin Conditions

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 100 percent of average, up 5 percent from last month. May precipitation was 145 percent of average, which was much above average.

Snow:
Current snow water equivalent for the entire basin is 115 percent, which is above average. The Animas basin is 135 percent and above Navajo is 100 percent. All three are up from the May 1st percentages.
--- Animas River Basin Snow Plot.
--- Above Navajo Basin Snow Plot.

Streamflow:
Streamflow for the entire San Juan Basin was much below average for May at 55%. Seasonal streamflow since October continued to be much below average at 60%, due to the much below average fall precipitation.

Soil Moisture:
Modeled soil moisture as of November 1st, 2010 was average to below average across the entire San Juan Basin.

Climate Forecasts:
The correlation of La Nina and April through July water volumes in the San Juan basin does suggest slightly drier than average December through May precipitation, but it did not influence the June forecast process.

Forecast Summary:
The April-July forecast volumes went up 5% from last month due to the much above average precipitation in May. Forecast volumes for June range between 69 and 102 percent of average with a median value of 80%.



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


Upper Green Specific Site Forecasts (kaf)

Click site name for graph.
Forecast
Period
90%
Exceedance
Volume
50%
Exceedance
Volume
Percent
Average
10%
Exceedance
Volume
Green
Daniel, Nr, Warren Bridge, At April-July315350132390
June-July265300160340
Pine Ck
Fremont Lk, Abv April-July113128123143
June-July105120143135
New Fork
Big Piney, Nr April-July455535135620
June-July390470159555
Green
Fontenelle Res, Fontenelle Nr April-July103012301431460
June-July7709751711200
Big Sandy
Farson, Nr April-July526611482
June-July415514171
Green
Green River, Wy, Nr April-July105012701451500
Hams Fork
Frontier, Nr, Pole Ck, Blo April-July95110169125
June-July658025095
Viva Naughton Res April-July140162182187
June-July6890243115
Blacks Fork
Robertson, Nr April-July131153161177
June-July108130194154
Ef Smiths Fork
Robertson, Nr April-July405017261
June-July354521456
Green
Flaming Gorge Res, Flaming Gorge Dam, At April-July162018901592180
June-July113014001921690


Yampa/White Specific Site Forecasts (kaf)

Click site name for graph.
Forecast
Period
90%
Exceedance
Volume
50%
Exceedance
Volume
Percent
Average
10%
Exceedance
Volume
Yampa
Stagecoach Rsvr, Abv April-July556317073
June-July263419744
Steamboat Springs April-July465520186580
June-July295350254410
Elk
Milner, Nr April-July625700215780
June-July375450271530
Elkhead Ck
Long Gulch, Abv, Hayden, Nr April-July125141199160
June-July244040859
Yampa
Maybell, Nr April-July179019902012210
June-July103012302801450
Little Snake
Slater, Nr April-July310345217385
June-July182220306260
Dixon, Nr April-July650740224845
June-July310400303505
Lily, Nr April-July7808852421000
June-July405510345630
White
Meeker, Nr April-July405465160535
June-July280340213410
Watson, Nr April-July435495162560
June-July290350205415


Duchesne Specific Site Forecasts (kaf)

Click site name for graph.
Forecast
Period
90%
Exceedance
Volume
50%
Exceedance
Volume
Percent
Average
10%
Exceedance
Volume
Big Brush Ck
Vernal, Nr, Red Fleet Res, Abv April-July192210527
June-July8.21212016.5
Ashley Ck
Vernal, Nr April-July445811275
June-July375118268
West Fork Duchesne
Vat Diversion, Blo April-July344222551
June-July263434343
Duchesne
Tabiona, Nr April-July168205195235
June-July121160262190
Rock Ck
Upper Stillwater Res April-July119142173167
June-July107130228155
Mountain Home, Nr April-July132157176185
June-July115140226168
Currant Ck
Currant Ck Res April-July475622466
June-July213025440
Strawberry
Soldier Springs, Nr April-July122139236159
June-July324930469
Duchesne, Nr April-July255295242335
June-July83120255163
Duchesne
Duchesne, Nr, Knight Div, Abv April-July310360191430
June-July250300256370
Lake Fork
Moon Lake Res, Mtn Home, Nr April-July106126185148
June-July90110234132
Yellowstone
Altonah, Nr April-July97115185135
June-July82100222120
Whiterocks
Whiterocks, Nr April-July6683148102
June-July437018497
Uinta
Neola, Nr April-July123143181163
June-July110130228150
Duchesne
Myton April-July580690260835
June-July365475319620
Randlett, Nr April-July7058902741110
June-July475660346880


Lower Green Specific Site Forecasts (kaf)

Click site name for graph.
Forecast
Period
90%
Exceedance
Volume
50%
Exceedance
Volume
Percent
Average
10%
Exceedance
Volume
White
Blo Tabbyune Ck, Soldier Summit, Nr April-July333620739
June-July6.5921412
Fish Ck
Scofield, Nr, Reservoir, Abv April-July626821374
June-July344034846
Price
Scofield Res, Scofield, Nr April-July7388191107
June-July355033169
Green
Green River, Ut April-July555062901987100
June-July366044002575210
Huntington Ck
Electric Lake April-July252918533
June-July16.22034524
Huntington, Nr April-July778717898
June-July556524176
Seeley Ck
Joes Vly Res, Orangeville, Nr April-July7595164118
June-July557520898
Ferron Ck
Ferron, Nr April-July556416473
June-July415021759
Muddy Ck
Emery, Nr April-July283417141
June-July19.92621833
Seven Mile Ck
Fish Lake, Nr April-July0000
June-July0000
-0000


Upper Colorado Mainstem Specific Site Forecasts (kaf)

Click site name for graph.
Forecast
Period
90%
Exceedance
Volume
50%
Exceedance
Volume
Percent
Average
10%
Exceedance
Volume
Colorado
Lake Granby, Granby, Nr April-July385430191475
June-July310355228400
Willow Ck
Willow Ck Res, Granby, Nr April-July108120235133
June-July637531388
Fraser
Winter Park April-July263015034
June-July232717631
Williams Fork
Williams Fork Res, Parshall, Nr April-July143160168178
June-July109126185144
Muddy Ck
Wolford Mtn Res, Blo April-July95115192139
June-July456531089
Blue
Dillon Res April-July240265159295
June-July197225189255
Green Mtn Res April-July405455163510
June-July325375188430
Colorado
Kremmling, Nr April-July143016001841770
June-July100011702111340
Eagle
Gypsum, Blo April-July445510152580
June-July350415184485
Colorado
Dotsero, Nr April-July225025001742770
June-July160018502042120
Frying Pan
Ruedi Res, Basalt, Nr April-July173200142225
June-July139166173193
Roaring Fork
Glenwood Springs April-July88010001411100
June-July700820167925
Colorado
Glenwood Springs, Blo April-July308034901623820
June-July227026801913010
Cameo, Nr April-July361040001654410
June-July262030101973420
Plateau Ck
Cameo, Nr April-July117178155205
June-July50111236140
Colorado
Cisco, Nr April-July597066001427280
June-July390045301785210
Mill Ck
Moab, Nr, Sheley Tun, At April-July4.45.31066.4
June-July2.53.41364.5
Colorado
Lake Powell, Glen Cyn Dam, At April-July110001260015914400
June-July7800940020311200


Gunnison Specific Site Forecasts (kaf)

Click site name for graph.
Forecast
Period
90%
Exceedance
Volume
50%
Exceedance
Volume
Percent
Average
10%
Exceedance
Volume
Taylor
Taylor Park Res April-July115135131157
June-July86106156128
Almont April-July180200121230
June-July131151144180
East
Almont April-July230255133280
June-July165190161215
Gunnison
Gunnison, Nr April-July460510131565
June-July330380158435
Tomichi Ck
Gunnison April-July6481100102
June-July385512876
Lake Fork
Gateview April-July122140111160
June-July99117133137
Gunnison
Blue Mesa Res April-July820895124970
June-July575650153725
Morrow Point Res April-July9009701241070
June-July625695153815
Crystal Res April-July98010801181200
June-July685780149905
Muddy Ck
Paonia Res, Bardine, Nr March-June114127127142
April-July116134131154
June-June415420069
June-July486620086
Nf Gunnison
Somerset, Nr April-July350380125410
June-July175205158235
Surface Ck
Cedaredge April-July17.72112325
June-July9.81315716.9
Uncompahgre
Ridgway Res April-July105119117135
June-July7690130106
Colona April-July134163117195
June-July91120136152
Delta April-July112123105165
June-July6475119117
Gunnison
Grand Junction, Nr April-July174019401242160
June-July100012001531420


Dolores Specific Site Forecasts (kaf)

Click site name for graph.
Forecast
Period
90%
Exceedance
Volume
50%
Exceedance
Volume
Percent
Average
10%
Exceedance
Volume
Dolores
Dolores April-July18821581250
June-July7710593139
Mcphee Res April-July23026081290
June-July8711588147
San Miguel
Placerville, Nr April-July10612393143
June-July7390111110
Dolores
Cisco, Nr April-July44046576545
June-July17119587275


San Juan Specific Site Forecasts (kaf)

Click site name for graph.
Forecast
Period
90%
Exceedance
Volume
50%
Exceedance
Volume
Percent
Average
10%
Exceedance
Volume
San Juan
Pagosa Springs April-July19420591230
June-July10011199135
Carracas, Nr April-July29033583390
June-July13818599240
Navajo Res, Archuleta, Nr April-July47554569620
June-July19026076335
Farmington April-July825945781050
June-July450570100675
Bluff, Nr April-July815950771110
June-July440575103735
Rio Blanco
Pagosa Springs, Nr, Blanco Dam, Blo April-July39458553
June-July17.5249632
Navajo
Chromo, Nr, Oso Div Dam, Blo April-July47547863
June-July24319140
Piedra
Arboles, Nr April-July15617476195
June-July628090101
Los Pinos
Vallecito Res, Bayfield, Nr April-July16017786195
June-July9311095128
Animas
Durango April-July410450102490
June-July280320128360
Florida
Lemon Res, Durango, Nr April-July37447652
June-July21288536
La Plata
Hesperus April-July16.118.37321
June-July5.887310.7
Mancos
Mancos, Nr April-July24278231
June-July8.8127315.9
South Ck
Lloyds Rsvr Nr Monticello, Abv March-July0.370.44320.56

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).

Reservoir Monthly Inflow Forecasts














Green End of Month Reservoir Contents (kaf)

Usable
Capacity
EOM
Contents
Percent
Usable
Capacity
Last Year
EOM
Last Year
%Capacity
untitled Green
Fontenelle Res, Fontenelle Nr 344.8 120.0 35 116.8 34
Flaming Gorge Res, Flaming Gorge Dam, At 3752.0 3150.2 84 3196.2 85
untitled Strawberry
Strawberry Res 1105.9 1046.0 95 995.8 90
Starvation Res, Duchesne, Nr 165.3 128.9 78 157.8 95
untitled Price
Scofield Res, Scofield, Nr 65.8 48.9 74 39.9 61
untitled Seeley Ck
Joes Vly Res, Orangeville, Nr 61.6 35.4 58 44.7 73
untitled Huntington North Reservoir Storage
4.2 4.0 94 3.5 83
untitled
TOTAL 5499.6 4533.4 82 4554.6 83


Upper Colorado End of Month Reservoir Contents (kaf)

Usable
Capacity
EOM
Contents
Percent
Usable
Capacity
Last Year
EOM
Last Year
%Capacity
untitled Colorado
Lake Granby, Granby, Nr 490.3 311.0 63 359.6 73
untitled Willow Ck
Willow Ck Res, Granby, Nr 9.1 4.0 44 10.1 111
untitled Williams Fork
Williams Fork Res, Parshall, Nr 96.9 79.6 82 94.3 97
untitled Muddy Ck
Wolford Mountain Reservoir 66.0 67.4 102 67.5 102
untitled Blue
Dillon Res 254.0 184.9 73 259.6 102
Green Mtn Res 146.9 67.9 46 116.1 79
untitled Homestake Ck
Homestake Res 43.0 14.7 34 29.0 67
untitled Frying Pan
Ruedi Res, Basalt, Nr 102.0 62.5 61 86.1 84
untitled Plateau Ck
Vega Res, Collbran, Nr 32.9 30.2 92 33.7 103
untitled Taylor
Taylor Park Res 106.2 61.1 58 80.5 76
untitled Gunnison
Blue Mesa Res 829.5 493.4 59 601.6 73
Morrow Point Res 117.0 114.0 97 112.6 96
Crystal Res 17.5 17.1 97 16.8 96
untitled Muddy Ck
Paonia Res, Bardine, Nr 18.5 2.7 15 16.0 86
untitled Uncompahgre
Ridgway Res 83.2 57.5 69 78.9 95
untitled Dolores
Mcphee Res 381.1 366.0 96 381.4 100
untitled
TOTAL 2794.1 1933.9 69 2343.7 84
Colorado
Lake Powell 24322.0 14097.8 58 14405.3 59
untitled
TOTAL 24322.0 14097.8 58 14405.3 59


San Juan End of Month Reservoir Contents (kaf)

Usable
Capacity
EOM
Contents
Percent
Usable
Capacity
Last Year
EOM
Last Year
%Capacity
untitled Los Pinos
Vallecito Res, Bayfield, Nr 125.4 111.3 89 112.9 90
untitled San Juan
Navajo Res, Archuleta, Nr 1701.3 1427.8 84 1505.7 89
untitled Florida
Lemon Res, Durango, Nr 39.8 23.5 59 31.5 79
untitled
TOTAL 1866.5 1562.6 84 1650.1 88

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 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