Upper Yellowstone and Madison River Basins (2)
February 1999

Snow
Snowfall in the basin this year has been above average. Snow water equivalent (SWE) is about 128 percent of average (142 percent of last year) in the Madison drainage. SWE in the Yellowstone drainage averages 125 percent of average (132 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
January precipitation in the Madison and Yellowstone drainage was about 143 percent of average (102 percent of last January) for the 7 reporting stations. Percentages range from 85 to 203 percent. Water-year-to-date precipitation is about 122 percent of average (125 percent of last year's amount). Year to date percentages range from 92 to 158.

Reservoir
Ennis Lake is storing 28,500 acre-feet (84 percent of average). Hebgen Lake is storing about 307,800 acre-feet of water (125 percent of average and 82 percent of capacity). Hebgen Lake is storing about 110 percent of last year's volume.

Streamflow
All the following forecasts are the 50 percent chance runoff for the April through September runoff period. Yellowstone at Lake Outlet has is expected to yield about 960,000 acre feet (121 percent of normal). Yellowstone at Corwin Springs will yield about 2,325,00 acre-feet (120 percent of normal). Yellowstone near Livingston will yield about 2,705,000 acre-feet (121 percent of normal). Madison River near Grayling has a 50 percent chance of yielding about 525,000 acre-feet (108 percent of normal). See the following page for detailed runoff volumes. 

 
==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - February 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           APR-SEP       805       897    |      960       121    |      1023      1115            792
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Spgs.    APR-SEP      2016      2200    |     2325       120    |      2450      2634           1937
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    APR-SEP      2402      2583    |     2705       121    |      2827      3008           2241
                                                                    |                       |
HEBGEN LAKE Inflow (2)               APR-SEP       438       490    |      525       108    |       560       612            486
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of January               |        Watershed Snowpack Analysis - February 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     28.5     29.5     34.0 |   MADISON RIVER in WY         9       142       128
                                                                        |
HEBGEN LAKE                            377.5    307.8    280.2    246.8 |   YELLOWSTONE RIVER in WY    11       132       125
                                                                        |
==================================================================================================================================
 * 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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