Snake River Basin (1)
June 1999

Snow
Snowfall has been a little above average in the basin so far this year. Cool weather has slowed the normal melting of snow -- consequently the comparison to average is exaggerated. Percentage of average snow water equivalent (SWE) for each drainage in the basin is: Snake above Jackson Lake -- 166 percent (182% of last year at this time), Pacific Creek -- 154 percent (145% of last year at this time), Gros Ventre River -- 130 percent (135% of last year at this time), Hoback River -- 178 percent (247% of last year at this time), Greys River -- 129 percent (197% of last year at this time), Salt River -- 139 percent (391% of last year at this time). Snake River Basin above Palisades is 145 percent of average (184% of last year at this time). See the Basin Summary of Snow Courses at the beginning of this report for a detailed listing of snow course information.

Precipitation
Precipitation across the basin was above average for last month. Monthly precipitation, for the basin, was 122 percent of average (106 percent of last May). May percentages range from 62 to 186 percent of average. Water-year-to-date precipitation is 108 percent of normal for the Snake River basin. This is 119 percent of last years total as of June 1. Year- to-date percentages range from 67 to 129 percent of average.

Reservoir
Current storage compared to average for the three storage reservoirs in the basin is as follows: Grassy Lake -100 percent of average (13,600 acre feet compared to 14,900 last year), Jackson lake - 120 percent of average (646,000 acre feet compared to 757,700 acre feet last year), and Palisades Reservoir -69 percent of average (723,300 acre feet compared to 1,121,100 acre feet last year).

Streamflow
The most probable June through September runoff yield forecast is above average for the basin. The Snake near Moran is expected to yield 671,000 acre-feet (116 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 2,370,000 acre-feet (130 percent of normal). The 50 percent chance yield near Heise is expected to be 3,300,000 acre-feet (126 percent of normal). Pacific Creek at Moran is expected to yield about 123,000 acre-feet (118 percent of average). Greys River above Palisades Reservoir is estimated to yield 258,000 acre-feet (107 percent of normal). Salt River near Etna is estimated to have a yield of 250,000 acre- feet (110 percent of normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               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)
====================================================================|=======================|=====================================
SNAKE near Moran (1,2)               JUN-SEP       515       622    |      671       116    |       720       827            579
                                                                    |                       |
SNAKE above Palisades (2)            JUN-SEP      2131      2273    |     2370       130    |      2467      2609           1823
                                                                    |                       |
SNAKE near Heise (2)                 JUN-SEP      2923      3148    |     3300       126    |      3452      3677           2622
                                                                    |                       |
PACIFIC CREEK at Moran               JUN-SEP        99       113    |      123       118    |       133       147            104
                                                                    |                       |
GREYS above Palisades                JUN-SEP       220       243    |      258       107    |       273       296            241
                                                                    |                       |
SALT near Etna                       JUN-SEP       204       231    |      250       110    |       269       296            228
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             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
========================================================================|=========================================================
GRASSY LAKE                             15.2     13.6     14.9     13.6 |   SNAKE above Jackson Lake    5       182       166
                                                                        |
JACKSON LAKE                           847.0    646.0    757.7    540.5 |   PACIFIC CREEK               2       145       154
                                                                        |
PALISADES                             1400.0    723.3   1121.1   1055.0 |   GROS VENTRE RIVER           2       135       130
                                                                        |
                                                                        |   HOBACK RIVER                5       247       178
                                                                        |
                                                                        |   GREYS RIVER                 3       197       129
                                                                        |
                                                                        |   SALT RIVER                  3       391       139
                                                                        |
                                                                        |   SNAKE above Palisades      17       184       145
                                                                        |
==================================================================================================================================
 * 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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