Upper Yellowstone and Madison River Basins (2)
June 1998

Snow
Snow water equivalent (SWE) averages about 19.1 inches; that is about 102 percent of average (58 percent of last year) in the Madison drainage. SWE in the Yellowstone drainage averages about 12.3 inches; that is about 63 percent of average (47 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 84 percent of average (95 percent of last year) for the 7 reporting stations. Tower Falls had the lowest percentage of average (35 percent) and Thumb Divide SNOTEL had the highest percentage of average (108 percent). Water-year-to-date precipitation is about 92 percent of average (61 percent of last year's amount).

Reservoir
Reservoir storage is near normal. Ennis lake is currently storing about 36,600 acre feet of water (102 percent of average and 89 percent of capacity). Hebgen Lake is storing about 351,600 acre feet of water (82 percent of average and 93 percent of capacity). Ennis Lake is storing 112 percent of last year at this time and Hebgen Lake is storing about 106 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 565,000 acre feet (82 percent of normal). Yellowstone at Corwin Springs will yield about 1,300,00 acre feet (88 percent of normal). Yellowstone near Livingston will yield about 1,515,000 acre feet (88 percent of normal). Madison River near Grayling has a 50 percent chance of yielding about 230,000 acre feet (75 percent of normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - June 1, 1998
==================================================================================================================================
                                              |  <<====== 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       473       528    |      565        82    |       602       657            691
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Springs  JUN-SEP      1139      1235    |     1300        88    |      1365      1461           1484
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    JUN-SEP      1316      1434    |     1515        88    |      1596      1714           1721
                                                                    |                       |
MADISON RIVER near Grayling (2)      JUN-SEP       191       214    |      230        75    |       246       269            307
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of May                   |        Watershed Snowpack Analysis - June 1, 1998
==================================================================================================================================
                                      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     36.6     32.8     35.8 |   MADISON RIVER in WY         6        58       102
                                                                        |
HEBGEN LAKE                            377.5    351.6    330.5    309.8 |   YELLOWSTONE RIVER in WY     8        47        63
                                                                        |
==================================================================================================================================
 * 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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