Upper North Platte River Basin (8)
May 1999

Snow
The snow courses above Seminoe Reservoir have about 103 percent of average snow water equivalent (SWE) recorded for this time of the year (117 percent of last year). SWE in the drainage area above Northgate is about 96 percent of average and 110 percent of last year at this time. SWE in the Encampment River drainage is about 97 percent of normal and 105 percent of last year. Brush Creek SWE for the year is about 111 percent of normal and 126 percent of last year's SWE. Medicine Bow and Rock Creek drainage SWE is about 96 percent of average and 114 percent of last year at this time. For more information see Basin Summary of Snow Courses at the beginning of this report.

Precipitation
Eight reporting stations indicate April precipitation was 153 percent of average and about 210 percent of last year's amount. April precipitation varied from 95 at South Brush creek to 432 percent of average at Wamsutter. Total water-year-to-date precipitation is about 118 percent of average for the basin, which is about 128 percent of last year's amount. Year to date percentage ranges from 94 to 149 for the 8 reporting stations.

Reservoirs
Seminoe Reservoir is currently storing about 183 percent of normal for this time of the year. Currently the reservoir is storing 104 percent of last year and 71 percent of capacity. Currently, Seminoe Reservoir storage is estimated to be 719,800 acre-feet (184 percent of average). Last year, at this time, the reservoir had 690,400 acre-feet in storage.

Streamflow
All the following yields are based on the fifty percent chance May through September yield. Yield for the North Platte River near Northgate is expected to be about 218,000 acre-feet (96 percent of average). Encampment River near Encampment is estimated to yield 160,000 acre-feet (108 percent of normal). North Platte River near Sinclair will yield about 636,000 acre- feet (101 percent of normal). Rock Creek near Arlington is estimated to yield 50,000 acre-feet (91 percent of average). Medicine Bow River above Seminoe Reservoir is expected to yield about 127,000 acre-feet (121 percent of normal). Seminoe Reservoir inflow should be about (761,000 acre-feet (104 percent of normal). See the following table for more detailed information on projected runoff.

==================================================================================================================================
                                                  UPPER NORTH PLATTE 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)
====================================================================|=======================|=====================================
North Platte River nr Northgate      MAY-SEP       150       191    |      218        96    |       245       286            228
                                                                    |                       |
Encampment River nr Encampment       MAY-SEP       125       146    |      160       108    |       174       195            148
                                                                    |                       |
North Platte River nr Sinclair       MAY-SEP       462       566    |      636       101    |       706       810            631
                                                                    |                       |
Rock Creek nr Arlington              MAY-SEP        41        46    |       50        91    |        54        60             55
                                                                    |                       |
Medicine Bow River ab Seminoe Reserv MAY-SEP        77       105    |      127       121    |       151       190            105
                                                                    |                       |
Seminoe Reservoir Inflow             MAY-JUL       507       617    |      692       103    |       767       877            671
                                     MAY-SEP       559       679    |      761       104    |       843       963            733
                                                                    |                       |
==================================================================================================================================
                     UPPER NORTH PLATTE RIVER BASIN                     |             UPPER NORTH PLATTE 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
========================================================================|=========================================================
SEMINOE                               1016.7    719.8    690.4    392.0 |   N PLATTE above Northgate    8       110        96
                                                                        |
                                                                        |   ENCAMPMENT RIVER            4       111       100
                                                                        |
                                                                        |   BRUSH CREEK                 5       126       111
                                                                        |
                                                                        |   MEDICINE BOW & ROCK CREEK   3       128       107
                                                                        |
                                                                        |   N PLATTE above Seminoe     20       117       103
                                                                        |
==================================================================================================================================
 * 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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