April 2011 Wyoming Spring Snowmelt Flood Potential Outlook---issued April 15, 2011

Snow water equivalents (SWEs) in early April continued above average (around 125 percent of normal) across Wyoming’s headwater watersheds. 

Current SWE trends vs Historic SWEs & Forecasted Snowmelt Flood Potentials for selected basins:

Snake River Basin:

·        SWEs were 110 to 120 percent to normal in early April.       

·        Current SWE trends are above the 30-year normal--but well below the record 1982 and 1997 SWE years.

·        LOW to MODERATE potential for headwater spring snowmelt flooding.

Upper Green Basin:

·        SWEs were 100 to 110 percent of average in early April. 

·         Current SWE trends are above the 30-year normal--but are below 1986 and 1997 SWE years---which were well above average runoff years.

·        LOW TO MODERATE potential for headwater spring snowmelt flooding across much of the basin. 

Shoshone River Basin:

·        SWEs were 100 to 110 percent of normal in early April.

·        Current SWE trends are slightly above the 30-year normal--and are below the record SWE years of 1996 and 1997.  SWE trends are above SWEs that produced record runoff in 1981 and 1991.

·        MODERATE potential for headwater spring snowmelt flooding across most of the basin---except MODERATE to HIGH flood potential across extreme upper portions of the North Fork of the Shoshone River.

Wind River Basin:

·        SWEs were 100 percent of normal in the southern Wind River Basin and 110 to 120 percent of normal across the northern Wind River Basin.

·        Current SWE trends are above the 30-yr normal in the northern portions of the basin and SWE trends are above SWE years of 1999 and 1986---but still below the all-time SWE year of 1997.  Across southern portions of the basin, SWE trends are well below the record SWE years of 1999 and 1986---but are currently below the SWE trends seen in 2010.

·        MODERATE potential for headwater spring snowmelt flooding across the northern Wind River Basin and LOW to MODERATE headwater spring snowmelt flooding across the southern Wind River Basin.

Upper North Platte River Basin:

·        SWEs were 135 to 140 percent of normal in early April.

·        Current SWE trends are at or surpassing the  record SWE years of 1982, 1986 and 1997--and well above the SWE trends of 2009 and 2010.

·        HIGH potential for headwater spring snowmelt flooding across the Upper North Platte River Basin.

Laramie River Basin:

·        SWEs were 120 to 135 percent of normal in early April.

·        Current SWE trends are near the record SWE years of 1984, 1986, and 1997--and above the SWE trends of 2009 and 2010.

·      HIGH potential for headwater spring snowmelt flooding across the Laramie River Basin.

Big Horn River Basin:

·        SWEs were 120 to 140 percent of normal (western Big Horn

      Mountains) in early April. 

·       Current SWE trends are well above the 30-yr normal—and are at or surpassing the record SWE years of 1986, 1997, and 1999. (western Big Horn Mountains) 

·      MODERATE to HIGH potential for headwater spring snowmelt flooding across the eastern Big Horn River Basin.

Powder River Basin:

·         SWEs were 105 to 115 percent of normal in early April. 

·        Current SWE trends are above the 30-yr normal—but at or near the record SWE years of 1984,1997, and 1999.  

·        MODERATE to HIGH potential for headwater spring snowmelt flooding Across Clear Creek and Rock Creek Basins. 

·        MODERATE potential for headwater spring snowmelt flooding across the Middle Fork of the Powder above Kaycee. 

Tongue River Basin:

·        SWEs were 95 to 125 percent of normal in early April.

·        Current SWE trends are above the 30-yr normal—at or near the record SWE years of 1986,1997, and 1999---but still well below the all-time record SWE year of 1984. 

·      LOW TO MODERATE potential for headwater spring snowmelt flooding across the basin.

Little Snake River Basin:

·         SWEs were 120 to 130 percent of normal in early April.

·        Low elevation (<8500’) SWE trends are above the 30-yr normal—but below record SWE years of 1997, 2008 and 2009. 

·        High elevation snow (>8500’) SWE trends are well above the 30-normal---and at or near the all-time record year of 1997.  

·        MODERATE TO HIGH potential for headwater spring snowmelt flooding across the Little Snake River Basin. 

Other factors that lead to this outlook include:  basing geometry/morphology, biological influences (i.e. beetle kill areas in southeast Wyoming), and continuing advice from outside sources and experts.

Snowmelt flood potential in Wyoming is greatly influenced by the amount of mountain snowpack.  The additional accumulation of snowpack during late April through early June is very crucial to the magnitude of runoff flooding that headwater basins across Wyoming will experience.  Bottom line is that a lot can happen between now and the onset of the annual spring runoff when it comes to the potential for flooding.

The complete Wyoming Spring 2011 Snowmelt Runoff Flood Potential Outlook graphic:



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