Snake River Basin (1)
May 1999

Snow
Snowfall has been a little above average in the basin so far this year. Percentage of average snow water equivalent (SWE) for each drainage in the basin is: Snake above Jackson Lake -- 121 percent (133% of last year at this time), Pacific Creek -- 142 percent (136% of last year at this time), Gros Ventre River -- 116 percent (111% of last year at this time), Hoback River - - 130 percent (125% of last year at this time), Greys River -- 113 percent (127% of last year at this time), Salt River -- 110 percent (133% of last year at this time). Snake River Basin above Palisades is 121 percent of average (127% 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 131 percent of average (144 percent of last April). April percentages range from 69 to 248 percent of average. Water-year-to-date precipitation is 106 percent of normal for the Snake River basin. This is 120 percent of last years total as of May 1. Year-to- date percentages range from 65 to 130 percent of average.

Reservoir
Current storage compared to average for the three storage reservoirs in the basin is as follows: Grassy Lake -115 percent (13,400 acre feet compared to 7,500 last year), Jackson lake - 117 percent (533,100 acre feet compared to 663,200 acre feet last year), and Palisades Reservoir -58 percent (555,400 acre feet compared to 910,600 acre feet last year).

Streamflow
The most probable May through September runoff yield forecast is above average for the basin. The Snake near Moran is expected to yield 945,000 acre-feet (116 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 3,095,000 acre-feet (125 percent of normal). The 50 percent chance yield near Heise is expected to be 4,430,000 acre-feet (121 percent of normal). Pacific Creek at Moran is expected to yield about 182,000 acre-feet (116 percent of average). Greys River above Palisades Reservoir is estimated to yield 378,000 acre-feet (108 percent of normal). Salt River near Etna is estimated to have a yield of 360,000 acre- feet (100 percent of normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               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)
====================================================================|=======================|=====================================
SNAKE near Moran (1,2)               MAY-SEP       818       905    |      945       116    |       985      1072            814
                                                                    |                       |
SNAKE above Palisades (2)            MAY-SEP      2859      3000    |     3095       125    |      3190      3331           2475
                                                                    |                       |
SNAKE near Heise (2)                 MAY-SEP      4041      4272    |     4430       121    |      4588      4819           3672
                                                                    |                       |
PACIFIC CREEK at Moran               MAY-SEP       154       171    |      182       116    |       193       210            157
                                                                    |                       |
GREYS above Palisades                MAY-SEP       338       362    |      378       108    |       394       418            350
                                                                    |                       |
SALT near Etna                       MAY-SEP       297       335    |      360       106    |       385       423            339
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             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
========================================================================|=========================================================
GRASSY LAKE                             15.2     13.4      7.5     11.7 |   SNAKE above Jackson Lake    6       133       121
                                                                        |
JACKSON LAKE                           847.0    533.1    663.2    456.5 |   PACIFIC CREEK               2       136       142
                                                                        |
PALISADES                             1400.0    555.4    910.6    950.0 |   GROS VENTRE RIVER           3       114       120
                                                                        |
                                                                        |   HOBACK RIVER                6       125       130
                                                                        |
                                                                        |   GREYS RIVER                 4       138       116
                                                                        |
                                                                        |   SALT RIVER                  5       133       110
                                                                        |
                                                                        |   SNAKE above Palisades      25       129       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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