Upper Yellowstone and Madison River Basins (2)
May 1999

Snow
Snowfall in the basin this year has been well above average. Snow water equivalent (SWE) is about 132 percent of average (146 percent of last year) in the Madison drainage. SWE in the Yellowstone drainage is about 122 percent of average (148 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 115 percent of average (141 percent of last April) for the 8 reporting stations -- percentage range was from 61 at Old Faithful to 156 percent of average. Water-year-to-date precipitation is about 121 percent of average at Parkers peak (130 percent of last year's amount). Year to date percentages range from 100 to 148 for the 8 reporting stations.

Reservoir
Ennis Lake is storing 33, 200 acre-feet (95 percent of average and 81 percent of capacity). Hebgen Lake is storing about 229,200 acre-feet of water (93 percent of average and 61 percent of capacity). Hebgen Lake is storing about 87 percent and Ennis Lake is storing about 101 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 has is expected to yield about 935,000 acre feet (124 percent of normal). Yellowstone at Corwin Springs will yield about 2,260,00 acre-feet (123 percent of normal). Yellowstone near Livingston will yield about 2,620,000 acre-feet (123 percent of normal). Hebgen lake inflow is estimated to be 535,000 acre feet (125 percent of normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - May 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           MAY-SEP       843       898    |      935       124    |       972      1027            756
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Spgs.    MAY-SEP      2115      2201    |     2260       123    |      2319      2405           1844
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    MAY-SEP      2391      2527    |     2620       123    |      2713      2849           2123
                                                                    |                       |
HEBGEN LAKE Inflow                   MAY-SEP       468       508    |      535       125    |       562       602            428
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of April                 |        Watershed Snowpack Analysis - May 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     33.2     32.8     35.1 |   MADISON RIVER in WY         9       146       132
                                                                        |
HEBGEN LAKE                            377.5    229.2    264.5    246.1 |   YELLOWSTONE RIVER in WY    10       148       121
                                                                        |
==================================================================================================================================
 * 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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