Upper North Platte River Basin (8)
May 1998

Snow
The 20 snow courses above Seminoe Reservoir average about 88 percent of average snow water equivalent (SWE) recorded for this time of the year (measured snow depths range from 13 to 67 inches). SWE in the drainage area above Northgate is about 88 percent of average and 66 percent of last year at this time. SWE in the Encampment River drainage is about 90 percent of normal and 65 percent of last year. Brush Creek SWE for the year is about 88 percent of normal and 74 percent of last year's SWE. Medicine Bow and Rock Creek drainage SWE is about 84 percent of average and 67 percent of last year at this time. For more information see Basin Summary of Snow Courses at the beginning of this report.

Precipitation
Twelve reporting stations indicate April precipitation was 70 percent of average and about 52 percent of last year's amount. April precipitation varied from 38 percent of average at Seminoe Dam to 88 percent of average at Old Battle SNOTEL. Total water-year-to-date precipitation is about 92 percent of average for the basin, which is about 75 percent of last year's amount.

Reservoir
Seminoe Reservoir is currently storing about 176 percent of normal for this time of the year. Currently the reservoir is storing about 13 percent more than last year. Seminoe Reservoir storage is estimated to be 690,400 acre feet. Last year the reservoir had 611,000 acre feet in storage. The reservoir is currently being drawn down in anticipation of spring runoff.

Streamflow
The entire drainage basin is expected to have below average runoff. The May through September 50 percent chance yield for the North Platte River near the Colorado state line is expected to be 178,000 acre feet (78 percent of average). Encampment River near Encampment is estimated to yield 138,000 acre feet (93 percent of normal). North Platte River near Sinclair will yield about 528,000 acre feet (about 84 percent of normal). Rock Creek near Arlington is estimated to yield 46,000 acre feet (about 84 percent of average). Medicine Bow River above Seminoe Reservoir is expected to yield about 67,000 acre feet (about 64 percent of normal). Seminoe Reservoir inflow should be about (591,000 acre feet (81 percent of normal). See the following table for more detailed information on projected runoff. 


==================================================================================================================================
                                                  UPPER NORTH PLATTE RIVER BASIN
                                               Streamflow Forecasts - May 1, 1998
==================================================================================================================================
                                              |  <<====== 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       110       151    |      178        78    |       205       246            228
                                                                    |                       |
Encampment River nr Encampment       MAY-SEP       103       124    |      138        93    |       152       173            148
                                                                    |                       |
North Platte River nr Sinclair       MAY-SEP       354       458    |      528        84    |       598       702            631
                                                                    |                       |
Rock Creek nr Arlington              MAY-SEP        37        42    |       46        84    |        50        55             55
                                                                    |                       |
Medicine Bow River ab Seminoe Reserv MAY-SEP        32        51    |       67        64    |        85       114            105
                                                                    |                       |
Seminoe Reservoir Inflow             MAY-JUL       357       467    |      542        81    |       617       727            671
                                     MAY-SEP       389       509    |      591        81    |       673       793            733
                                                                    |                       |
==================================================================================================================================
                     UPPER NORTH PLATTE RIVER BASIN                     |             UPPER NORTH PLATTE RIVER BASIN
             Reservoir Storage (1000 AF) - End of April                 |        Watershed Snowpack Analysis - May 1, 1998
==================================================================================================================================
                                      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    690.4    611.0    392.0 |   N PLATTE above Northgate    8        66        88
                                                                        |
                                                                        |   ENCAMPMENT RIVER            4        65        90
                                                                        |
                                                                        |   BRUSH CREEK                 5        74        88
                                                                        |
                                                                        |   MEDICINE BOW & ROCK CREEK   3        67        84
                                                                        |
                                                                        |   N PLATTE above Seminoe     20        66        88
                                                                        |
==================================================================================================================================
 * 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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