Snake River Basin (1)
May 2000

Snow
Percentage of average snow water equivalent (SWE) for each drainage in the basin is: Snake above Jackson Lake -- 54 percent (45% of last year at this time), Pacific Creek -- 64 percent (45% of last year at this time), Gros Ventre River -- 59 percent (51% of last year at this time), Hoback River -- 63 percent (48% of last year at this time), Greys River -- 67 percent (57% of last year at this time), Salt River -- 55 percent (50% of last year at this time). Snake River Basin above Palisades is 57 percent of average (48% 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 below average for last month. Monthly precipitation, for the basin, was 71 percent of average (52 percent of last year). April percentages range from 27 to 126 percent of average. Water-year-to-date precipitation is 76 percent of normal for the Snake River basin (72 percent of last year at this time) Year-to-date percentages range from 66 to 92 percent of average.

Reservoir
Current storage compared to average for the three storage reservoirs in the basin is as follows: Grassy Lake -111 percent of average (13,000 acre feet compared to 13,400 last year), Jackson lake - 157 percent of average (716,600 acre feet compared to 533,100 acre feet last year), and Palisades Reservoir -122 percent of average (1,161,100 acre feet compared to 555,400 acre feet last year).

Streamflow
The most probable, 50 percent chance, May through September runoff yield forecast is below average for the basin. The Snake near Moran is expected to yield 642,000 acre-feet (79 percent of normal). Yield from the Snake River above Palisades Reservoir is estimated to be 1,930,000 acre-feet (78 percent of normal). The 50 percent chance yield near Heise is expected to be 2,840,000 acre-feet (77 percent of normal). Pacific Creek at Moran is expected to yield about 117,000 acre-feet (75 percent of average). Greys River above Palisades Reservoir is estimated to yield 231,000 acre-feet (66 percent of normal). Salt River near Etna is estimated to have a yield of 217,000 acre-feet (64 percent of normal). 

==================================================================================================================================
                                                        SNAKE RIVER BASIN
                                               Streamflow Forecasts - May 1, 2000
==================================================================================================================================
                                              |  <<====== 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       515       602    |      642        79    |       682       769            814
                                                                    |                       |
SNAKE above Palisades (2)            MAY-SEP      1694      1835    |     1930        78    |      2025      2166           2475
                                                                    |                       |
SNAKE near Heise (2)                 MAY-SEP      2451      2682    |     2840        77    |      2998      3229           3672
                                                                    |                       |
PACIFIC CREEK at Moran               MAY-SEP        89       106    |      117        75    |       128       145            157
                                                                    |                       |
GREYS above Palisades                MAY-SEP       191       215    |      231        66    |       247       271            350
                                                                    |                       |
SALT near Etna                       MAY-SEP       154       192    |      217        64    |       242       280            339
                                                                    |                       |
==================================================================================================================================
                            SNAKE RIVER BASIN                           |                    SNAKE RIVER BASIN
             Reservoir Storage (1000 AF) - End of April                 |        Watershed Snowpack Analysis - May 1, 2000
==================================================================================================================================
                                      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     13.0     13.4     11.7 |   SNAKE above Jackson Lake    6        45        54
                                                                        |
JACKSON LAKE                           847.0    716.6    533.1    456.5 |   PACIFIC CREEK               2        45        64
                                                                        |
PALISADES                             1400.0   1161.1    555.4    950.0 |   GROS VENTRE RIVER           4        54        59
                                                                        |
                                                                        |   HOBACK RIVER                6        48        63
                                                                        |
                                                                        |   GREYS RIVER                 5        57        67
                                                                        |
                                                                        |   SALT RIVER                  5        50        55
                                                                        |
                                                                        |   SNAKE above Palisades      25        48        57
                                                                        |
==================================================================================================================================
 * 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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