San Juan Water Supply Outlook, May 1, 2008

Note: This publication is scheduled for revision. If you have feedback on content, format, or publication frequency, please contact us.
Prepared by Cox
NOAA, National Weather Service
Colorado Basin River Forecast Center
Salt Lake City, Utah
www.cbrfc.noaa.gov

Contents

San Juan Summary

-Observed Streamflow...Observed streamflow for the month of April was above to much above average for the San Juan.

-Snowpack/Precipitation...Snowpack above Navajo Reservoir dropped to 120 percent of average. The Animas was near average for snowpack while the entire San Juan Basin dropped to 110 percent. Precipitation over the San Juan Basin for April was much below average with 45 percent. Seasonal average precipitation for the San Juan Basin dropped to 110 percent.

-Short Term Precipitation Forecast...The forecast models through five days keep the San Juan Basin relatively dry with near average temperatures. The CPC forecast days 6 through 10 have an increased chance for below normal temperatures and below normal precipitation.

-General Discussion...The current snowpack along with both statistical forecasts and ESP numbers suggest lowering the forecast across all of the headwaters of the San Juan Basin. Very dry conditions across the San Juan Basin over the past two months also reflect current thinking of lowering volume forecasts for the San Juan. CPC guidance for May 15th through June 1st continues to show an increased chance of above normal temperatures and below normal precipitation.



*Median of forecasts within each basin.


San Juan Basin Conditions




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


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-July245290129335
May-July195240125285
Carracas, Nr April-July460540133630
May-July350430132520
Navajo Res, Archuleta, Nr April-July84510301311240
May-July6057851281000
Farmington April-July138016001321820
May-July105012701291490
Bluff, Nr April-July125015801281910
May-July94012701301600
Rio Blanco
Pagosa Springs, Nr, Blanco Dam, Blo April-July607313889
May-July506314079
Navajo
Chromo, Nr, Oso Div Dam, Blo April-July7490130109
May-July597512994
Piedra
Arboles, Nr April-July255305133360
May-July163210122265
Los Pinos
Vallecito Res, Bayfield, Nr April-July205240117280
May-July170205111245
Animas
Durango April-July420515117625
May-July355450115560
Florida
Lemon Res, Durango, Nr April-July536411077
May-July455610669
La Plata
Hesperus April-July222610432
May-July16.22110027
Mancos
Mancos, Nr April-July264012154
May-July213512149
South Ck
Lloyd's Rsvr Nr Monticello, Abv March-July11.91453.2

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).

San Juan 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 42.4 34 103.0 82
untitled San Juan
Navajo Res, Archuleta, Nr 1701.3 1360.2 80 1622.0 95
untitled Florida
Lemon Res, Durango, Nr 39.8 21.3 54 36.0 90
untitled
TOTAL 1866.5 1423.9 76 1760.9 94

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: Cox