Hydrologic Conditions

SPRING SNOWMELT FLOOD OUTLOOK...CORRECTED
NATIONAL WEATHER SERVICE CHEYENNE WY
850 AM MST FRI FEB 12 1999

SUMMARY...SPRING SNOWMELT FLOOD OUTLOOK FOR WYOMING AND THE
          NEBRASKA PANHANDLE
 
THIS SPRING SNOWMELT OUTLOOK IS FOR THE NORTH PLATTE RIVER DRAINAGE WHICH
INCLUDES RIVERS IN WYOMING, AND THE NEBRASKA PANHANDLE.  ALSO INCLUDED IS
THE LITTLE SNAKE RIVER DRAINAGE IN THE GREEN RIVER BASIN.
 
   ASSUMPTIONS
 
THIS OUTLOOK CONTAINS A GENERALIZED SUMMARY OF SPRING SNOWMELT FLOOD
POTENTIAL.  ABOVE NORMAL FUTURE SNOW ACCUMULATIONS COMBINED WITH
HEAVY RAINS AND RAPID MELT WILL INCREASE THE CURRENT FLOOD POTENTIAL
WHILE BELOW NORMAL FUTURE PRECIPITATION AND GRADUAL OR INTERMITTENT
FREEZING AND THAWING WILL DECREASE THE MAGNITUDE OF THE CURRENT
ASSESSMENT.
 
THESE PROJECTIONS OF RIVER STAGES AND RESERVOIR LEVELS ARE BASED ON
CURRENT OBSERVED STATES OF STREAMFLOW, SOIL MOISTURE, AND SNOW PACK,
COUPLED WITH FUTURE PRECIPITATION AND TEMPERATURE PATTERNS AND
ANTICIPATED OPERATIONAL HYDROLOGIC CHANGES SUCH AS RESERVOIR RELEASES
AND CANAL DIVERSIONS.  "OUTLOOKS" ARE PROVIDED FOR LONG-RANGE (WEEKS
TO MONTHS) PROJECTIONS BASED ON CLIMATOLOGICAL PATTERNS OF
PRECIPITATION AND TEMPERATURE.  "FORECASTS" ARE PROVIDED FOR
SHORT-TERM (DAYS) PROJECTIONS BASED ON FUTURE FORECASTED PATTERNS OF
PRECIPITATION AND TEMPERATURE.  THE UNCERTAINTY OF THESE PRODUCTS
VARIES FROM SEASON TO SEASON AND SITE TO SITE.  IN RECENT YEARS,
OUTLOOK CRESTS HAVE BEEN ABOVE THE OBSERVED CREST ABOUT AS OFTEN AS
THEY HAVE BEEN BELOW THE OBSERVED CREST.  THE UNCERTAINTY OF FORECASTS
TEND TO BE LESS THAN THE UNCERTAINTY OF OUTLOOKS DUE TO THEIR SHORTER
LEAD TIME.  USERS OF THESE PRODUCTS ARE ENCOURAGED TO CONTACT THEIR
NEAREST NATIONAL WEATHER SERVICE FORECAST OFFICE FOR CONTINUED UPDATES
OF METEOROLOGICAL CONDITIONS WHICH CAN HAVE SIGNIFICANT IMPACTS ON
FLOOD PLANNING AND FLOOD FIGHTING ACTIVITIES.
 
   CURRENT SNOW CONDITIONS
 
OTHER THAN IN MOUNTAINOUS AREAS LITTLE OR NO SNOW IS REPORTED IN
EASTERN WYOMING OR THE NEBRASKA PANHANDLE.
 
SNOWPACK IN THE MOUNTAINS OF MONTANA, WYOMING AND COLORADO AVERAGE FROM
ABOUT 80% TO 140% OF AVERAGE WITH SOME REPORTS OF OVER 150% OF NORMAL
IN PARTS OF THE YELLOWSTONE, BIGHORN AND SHOSHONE RIVER BASINS.  THE
CURRENT SNOWPACK FOR THE GREEN RIVER BASIN ABOVE FLAMING GORGE AND
LITTLE SNAKE DRAINAGES IS AVERAGING 93 PERCENT OF NORMAL.  IN THE NORTH
PLATTE THE CURRENT SNOWPACK IS AROUND 95 PERCENT OF NORMAL.
 
   CURRENT SOIL MOISTURE CONDITIONS
 
MOST OF THE EASTERN PORTION OF THE MISSOURI RIVER BASIN HAS NORMAL TO
ABOVE NORMAL SOIL MOISTURE CONDITIONS FOR THIS TIME OF YEAR.  EASTERN
PORTIONS OF MONTANA, SOUTH DAKOTA, KANSAS, AND ALL OF MISSOURI AND
NORTH DAKOTA HAVE ABOVE NORMAL SOIL MOISTURE.  THE REST OF THE MISSOURI
RIVER BASIN GENERALLY HAS NEAR NORMAL OR SLIGHTLY DRIER THAN NORMAL
SOIL MOISTURE CONDITIONS.
 
   CURRENT RIVER CONDITIONS
 
IN GENERAL, FLOWS IN THE RIVERS OF MONTANA, WYOMING AND COLORADO ARE
NEAR AVERAGE.  THE NORTH AND SOUTH PLATTE RIVERS HAVE NORMAL FLOWS
WHILE THE LOWER PLATTE RIVER IS NEAR NORMAL.
 
   SPRING SNOWMELT FLOOD OUTLOOK
 
BASED ON THE CURRENT SNOWPACK IN THE MOUNTAINS OF MONTANA, MINOR
FLOODING COULD OCCUR IN THE YELLOWSTONE AND UPPER MISSOURI BASINS.
NO FLOODING IS EXPECTED IN THE MOUNTAINS OF WYOMING AND COLORADO.

THE 1999 SPRING SNOWMELT FLOOD POTENTIAL FOR SOUTHEAST WYOMING AND THE
NEBRASKA PANHANDLE IS NOT HIGH AT THIS TIME.  HOWEVER, IT IS IMPORTANT
TO NOTE THAT IT IS STILL VERY EARLY IN THE SNOW ACCUMULATION SEASON.
THEREFORE, SNOWPACK ACCUMULATION, OR LACK OF IT, OVER THE NEXT SEVERAL
MONTHS, COMBINED WITH WEATHER CONDITIONS DURING THE MELT, WILL DETERMINE
WHAT TYPE OF FLOOD POTENTIAL DEVELOPS.

THE NEXT SCHEDULED OUTLOOK WILL BE ISSUED ON FRIDAY FEBRUARY 26, IF
NEEDED.

DIETRICH