Spring 2018 Flood and Water Resources Outlook
Mountain snowpack across Wyoming is 100 to 115% of average
by National Weather Service
March 1, 2018
This Spring and early Summer 2018 flood outlook includes the Wind, Big Horn, Green, Powder, Sweetwater, Shoshone, and Upper Yellowstone River Basins. A small portion of the Snake and the Lower North Platte Basins is also used to make this runoff forecast. The outlook is from today through the end of June.
Many basins east of the continental divide receive up to 55 percent of their water year total for precipitation during the months of March through May. Accordingly, mountain snowpack can drastically change from late winter to late spring for basins east of the continental divide.
Wyoming generally does not see any significant mountain snowmelt runoff until early to middle May, with mountain snowmelt runoff peaking around early June. Therefore, at this time, it is way too early to make any long range prediction on & the magnitude of flooding due to mountain snowmelt runoff.
This outlook is based on various diverse hydrological factors such as snow water equivalents (SWEs) in the mountain snowpack, basin morphology (i.e. how basins respond to snowmelt), amount of biological factors (beetle kill, blight), low elevation snow depths, and likely temperature and precipitation trends into late spring/early summer.
Current water precipitation is averaging 90 to 100 percent of normal across Wyoming.
Mountain snowpack across Wyoming is 100 to 115 percent of average.
Wyoming carryover reservoir storages are 115 to 125 percent of average for March.
Above normal snowmelt streamflow volumes are expected across major drainages from central through northwestern Wyoming.
Mountain snowpack and associated snow water equivalents (SWEs) across central through northwestern Wyoming were generally above to much above average by the end of February; while SWEs across basins in southern Wyoming were generally below average. SWEs at the peak snowmelt runoff elevations (8,500’ – 10.000’) were the highest across the Shoshone and Upper Yellowstone Basins at 160 to 170 percent of median. The Lower Green and Little Snake Drainages had SWEs at 75 to near 85 percent of median at the peak snowmelt runoff elevations.
This outlook is based on various diverse hydrological factors such as snow water equivalents (SWEs) in the mountain snowpack, basin morphology (i.e. how basins respond to snowmelt runoff), antecedent soil moisture, biological factors (bark beetle kill/spruce blight), low elevation snow depths, and likely temperature and precipitation trends during late spring/early summer.
Moderate to High potential for flooding associated with snowmelt runoff is expected across extreme lower portions of the Big Wind Watershed and over upper sections of the South Fork of the Shoshone Watershed.
Moderate potential for snowmelt runoff flooding is forecasted across the middle to lower portions of the North/South Forks of the Shoshone Basin, middle to lower sections of the Little and Big Wind Watersheds, and along headwater creeks and streams along the west side of the Big Horn Mountains.
All other of headwater basins across Wyoming can expect a generally Low potential for flooding due to springtime snowmelt runoff.
The next outlook will be issued in late March.
Links for more information:
Other hydrological information for Wyoming can be found at the NOAA hydrology website:
Monthly Wyoming Hydrologic Summary and Graphics:
(updated monthly around the 15th of every month)
Wyoming Drought Information Page:
(updated at least once a month)
Wyoming Graphical Water Supply Outlook:
(updated by the 10th of every month—January-June)
Wyoming Average Precipitation by Basin:
Wyoming Spring Snowmelt Runoff Flood Potential Graphic:
(updated around the 25th of the month---February-May)
Current and Forecast Wyoming Streamflows and/or River Stages: