Final 2011 Wyoming Spring Snowmelt Flood Potential Outlook---issued May 10

Snow water equivalents (SWEs) in early May continued to be above to well above average (around 130 – 230 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 increased to 150 to 170 percent of normal by early May.     

·       Current SWE trends are well above the 30-year normal--but have increased to at or slightly above the record 1982 and 1997 SWE years.

·        MODERATE  to HIGH potential for headwater spring snowmelt flooding.

Upper Green Basin:

·        SWEs increased to 130 to 150 percent of average by early May.

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

·       Generally a LOW to MODERATE potential for headwater spring snowmelt flooding across much of the basin.

·        MOD to HIGH potential for spring snowmelt flooding near the Green River near LaBarge

Shoshone River Basin:

·        SWEs increased to 120 to 130 percent of normal by early May.

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

·        MODERATE potential for headwater spring snowmelt flooding across South Fork of the Shoshone River---MODERATE to HIGH flood potential across the North Fork of the Shoshone River.

Wind River Basin:

·       SWEs in early May were 100 to 110 percent of normal in the southern Wind River Basin and 140 to 160 percent of normal across the northern Wind River Basin.

·       Current SWE trends are well above the 30-yr normal in the northern portions of the basin and SWE trends are well above SWE years of 1999 and 1986---but also slightly above 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 still trending below the SWE trends seen in 2010.

·        MODERATE to HIGH 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 increased to 160 to 185 percent of normal by early May.

·        Current SWE trends are still at or surpassing the  record SWE years of 1982, 1986 and 1997--and still 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 increased to 145 to 165 percent of normal by early May.

·       Current SWE trends have remained near the record SWE years of 1984, 1986, and 1997--and still well 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 increased to 140 to 165 percent of normal (western Big Horn Mountains) by early May.

 ·      Current SWE trends are well above the 30-yr normal—and have surpassed 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 increased to 120 150 percent of normal by early May.

·        Current SWE trends are above the 30-yr normal—but are 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 increased to 125 to 135 percent of normal by early May.

·       Current SWE trends are well 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.

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

Little Snake River Basin:

·         SWEs increased to 160 to 175 percent of normal by early May.

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

 ·        High elevation snow (>8500’) SWE trends are well above the 30-normal

---and have continued to be 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.

Upper Bear River Basin:

·        SWEs increased to 160 to 210 percent of normal by early May.

·       Current SWE trends are well above the 30-yr normal—and above the record years of 1983,1984, and 1986. 

·      MODERATE to HIGH potential for headwater spring snowmelt flooding across the basin.

   

Other factors that lead to this outlook include:  basin 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 through early June is very crucial to the magnitude of runoff flooding that headwater basins across Wyoming will experience. 

This will be the LAST Spring Snowmelt Runoff Flood Potential Outlook for 2011.

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



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