Upper Yellowstone and Madison River Basins (2)
May 2000

Snow
Snowfall in the basin this year has been just below average, and very much below the SWE last year at this time. For this time of the year, snow water equivalent (SWE) is about 58 percent of average (46 percent of last year) in the Madison drainage. SWE in the Yellowstone drainage is about 69 percent of average (55 percent of last year at this time). See the "Snow Course Basin Summary" at the beginning of this document for more details on specific sites.

Precipitation
April precipitation in the Madison and Yellowstone drainage was about 91 percent of average (81 percent of previous year) for the 7 reporting stations -- percentage range was from 68 to 122 percent of average. Water- year-to-date precipitation is about 89 percent of average (73 percent of last year's amount). Year to date percentage ranges from 73 to 110 percent

Reservoir
Ennis Lake is storing 32,100 acre-feet (91 percent of average and 78 percent of capacity). Hebgen Lake is storing about 305,700 acre-feet of water (124 percent of average and 81 percent of capacity). Hebgen Lake is storing about 133 percent and Ennis Lake is storing about 97 percent of last year's volume.

Streamflow
All the following forecasts are the 50 percent chance runoff for the May through September runoff period. Yellowstone at Lake Outlet is expected to yield about 485,000 acre feet (64 percent of normal). Yellowstone at Corwin Springs will yield about 1,375,000 acre-feet (75 percent of normal). Yellowstone near Livingston will yield about 1,570,000 acre feet (74 percent of normal). Hebgen lake inflow is estimated to be 345,000 acre feet (81 percent of normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - May 1, 2000
==================================================================================================================================
                                              |  <<====== Drier ======  Future Conditions  =======  Wetter =====>>  |             
                                              |                                                                     |
  Forecast Point                     Forecast |  ==================== Chance Of Exceeding * ======================  |
                                      Period  |    90%       70%    |  50% (Most Probable)  |      30%       10%    |  30-Yr Avg.
                                              | (1000AF)  (1000AF)  |   (1000AF)  (% AVG.)  |    (1000AF)  (1000AF) |    (1000AF)
====================================================================|=======================|=====================================
YELLOWSTONE at Lake Outlet           MAY-SEP       393       448    |      485        64    |       522       577            756
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Spgs.    MAY-SEP      1230      1316    |     1375        75    |      1434      1520           1844
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    MAY-SEP      1341      1477    |     1570        74    |      1663      1799           2123
                                                                    |                       |
HEBGEN Reservoir Inflow              MAY-SEP       278       318    |      345        81    |       372       412            428
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of April                 |        Watershed Snowpack Analysis - May 1, 2000
==================================================================================================================================
                                      Usable |  *** Usable Storage ***  |                            Number     This Year as % of
Reservoir                            Capacity|  This      Last          |   Watershed                  of       =================
                                             |  Year      Year     Avg  |                          Data Sites   Last Yr   Average
========================================================================|=========================================================
ENNIS LAKE                              41.0     32.1     33.2     35.1 |   MADISON RIVER in WY         9        46        58
                                                                        |
HEBGEN LAKE                            377.5    305.7    229.2    246.1 |   YELLOWSTONE RIVER in WY    11        55        69
                                                                        |
==================================================================================================================================
 * 90%, 70%, 30%, and 10% chances of exceeding are the probabilities that the actual volume will exceed the volumes in the table.

The average is computed for the 1961-1990 base period.

(1) - The values listed under the 10% and 90% Chance of Exceeding are actually 5% and 95% exceedance levels.                  
(2) - The value is natural volume - actual volume may be affected by upstream water management.
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