Upper Yellowstone and Madison River Basins (2)
June 1999

Snow
Snowfall in the basin this year has been well above average. Cool weather has delayed the snowmelt. For this time of the year, snow water equivalent (SWE) is about 224 percent of average (220 percent of last year) in the Madison drainage. SWE in the Yellowstone drainage is about 134 percent of average (213 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
May precipitation in the Madison and Yellowstone drainage was about 119 percent of average (149 percent of last May) for the 8 reporting stations -- percentage range was from 84 at Tower Falls to 244 percent of average at Old Faithful. Water-year-to-date precipitation is about 120 percent of average (133 percent of last year's amount). Year to date percentage ranges from 100 to 145 for the 8 reporting stations.

Reservoir
Ennis Lake is storing 34,600 acre-feet (97 percent of average and 84 percent of capacity). Hebgen Lake is storing about 298,800 acre-feet of water (96 percent of average and 79 percent of capacity). Hebgen Lake is storing about 85 percent and Ennis Lake is storing about 95 percent of last year's volume.

Streamflow
All the following forecasts are the 50 percent chance runoff for the June through September runoff period. Yellowstone at Lake Outlet has is expected to yield about 870,000 acre feet (126 percent of normal). Yellowstone at Corwin Springs will yield about 1,830,000 acre-feet (123 percent of normal). Yellowstone near Livingston will yield about 2,140,000 acre feet (124 percent of normal). Hebgen lake inflow is estimated to be 360,000 acre feet (117 percent of normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - June 1, 1999
==================================================================================================================================
                                              |  <<====== 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           JUN-SEP       839       858    |      870       126    |       882       901            691
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Spgs.    JUN-SEP      1665      1763    |     1830       123    |      1897      1995           1484
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    JUN-SEP      1941      2059    |     2140       124    |      2221      2339           1721
                                                                    |                       |
HEBGEN LAKE Inflow                   JUN-SEP       330       348    |      360       117    |       372       390            307
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of May                   |        Watershed Snowpack Analysis - June 1, 1999
==================================================================================================================================
                                      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     34.6     36.6     35.8 |   MADISON RIVER in WY         6       220       224
                                                                        |
HEBGEN LAKE                            377.5    298.8    351.6    309.8 |   YELLOWSTONE RIVER in WY     8       213       134
                                                                        |
==================================================================================================================================
 * 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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