2021 Idaho Spring Flood and Water Resources Outlook

Spring Flood Potential

Screen Shot 2021-03-23 at 10.28.21 AMThe risk for spring flooding due to snowmelt is near average across most of Idaho. The exceptions are the Big Lost Basin and Little Wood Basin, and the Medicine Lodge, Beaver, and Camas Basin in central and east-central Idaho where the risk is slightly below average due to well below normal snowpack and low soil moisture. Above normal
temperatures in December and January helped limit the snowpack in the low elevations which significantly reduces the snowmelt flood risk during late winter and early spring.

The primary factors in the development of spring flooding are the occurrence of persistent above normal temperatures, and rain on snow precipitation events. Even if mainstem rivers do not reach flood stage, smaller creeks and streams can still overflow their banks. Under the right scenario, spring flooding is possible even for areas that have low snowpack. Additionally, wildfire burn scars can have a significant impact on local flood potential during spring snowmelt.Screen Shot 2021-03-23 at 10.28.15 AM

Water Supply

National Weather Service water supply volume forecasts for the spring and summer of 2021 are near normal for northern Idaho watersheds. Meanwhile, forecasts vary considerably across central and southern Idaho with subpar snowpack and longer term hydrologic drought leading to well below normal forecasts in some areas. Forecasts for the Snake River headwaters region along the Wyoming border and the West Central Mountains are generally 75 to 85 percent of normal. Volume forecasts for the rest of the Central Mountains and southern Idaho watersheds are well below normal ranging from 25 percent to 70 percent of normal. The lowest forecasts are for the Big Wood and Little Wood Basins, and the Big Lost River Basin. Low snowpack and poor streamflows in these areas could lead to water supply concerns, especially for those relying on natural flows. These forecasts may change considerably over the next couple of months since seasonal snow accumulation and rainfall typically occur during March and April.

Temperature and Precipitation

The 2021 Water Year started out well across northern Idaho with above normal precipitation in October, while rather dry conditions prevailed across southern Idaho. Most of the state received near normal or above normal precipitation in November with the exception being near the Canadian border and portions of central Idaho where only 50 to 70 percent of normal occurred. December was a very dry month for the state as a whole. Precipitation in January was improved for some areas but dry conditions continued for the majority of Idaho. A favorable storm track in February brought normal or well above normal precipitation to the majority of the state except for the Northern Panhandle and the Central Mountains east of the Sawtooths which received less than normal precipitation. Temperatures were around normal in October and November with warmer than normal conditions prevailing in December and January. February temperatures were near normal across far southern Idaho and below normal across the remainder of the state.

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Snowpack

As of March 3, mountain snowpack was near normal for most of Idaho. The Clearwater Basin led the way with 113 percent of normal while the Little Wood and Big Lost River Basins were lagging at 66 and 69 percent of normal. Idaho snowpack as a whole typically builds through March and peaks in early April.Screen Shot 2021-03-23 at 10.28.00 AM

Reservoirs

Reservoir storage across Idaho was in good shape as of early March with major reservoir systems holding near average water with the exception of the Wood and Lost River Basins. Across southern Idaho, basin-wide total storage in the Bear River Basin was near 136 percent of average while Southside Snake Basins were near 81 percent of average. Storage in West Central Basins was near 103 percent of average while the Wood and Lost Basins were only 59 percent of average. The Upper Snake River Basin was at 120 percent of average, the Clearwater Basin at 103 percent of average and Panhandle Region was 106 percent of average. Weather patterns, irrigation demand, and flood control needs will drive reservoir operations over the next several months. Wet spring weather or extended periods of above normal temperatures resulting in rapid snowmelt and large reservoir inflows could result in significant fluctuations in reservoir discharge and downstream river levels.

Drought

Drought conditions continue to plague portions of southern Idaho. The Wood River Basins and Big Lost River Basin of south central Idaho are the focus for drought due to subpar snowpack and precipitation dating back to the winter of 2019-2020. Meanwhile, long-term dryness along the Utah and Nevada border is resulting in drought conditions as well. Weather and precipitation for the remainder of winter and this spring will determine whether or not drought conditions improve or deteriorate.

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Long Range Outlook

The outlook for March through May favors below normal temperatures across the Idaho Panhandle and above normal temperatures along the Utah and Nevada border. Elsewhere across the state, the chances for either below, above, or normal temperatures are equal. The precipitation outlook slightly favors above normal precipitation across the Panhandle Region. Elsewhere the precipitation outlook is equal chances for either below, above, or normal precipitation.

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On-line Resources

Water Supply Volume Forecasts…
National Weather Service-Northwest River Forecast Center www.nwrfc.noaa.gov/ws/

National Weather Service-Colorado Basin River Forecast Center
www.cbrfc.noaa.gov/

USDA-Natural Resources Conservation Service
www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/

Snowpack Information…
National Weather Service-Northwest River Forecast Center
www.nwrfc.noaa.gov/snow/

National Weather Service-National Operational Hydrologic Remote Sensing Center
www.nohrsc.noaa.gov/

USDA-Natural Resources Conservation Service
www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/

Reservoir Storage…
Bureau of Reclamation Reservoir Storage
www.usbr.gov/pn/hydromet/select.html

USDA-Natural Resources Conservation Service
www.wcc.nrcs.usda.gov/basin.html

Drought Information…
U.S. Drought Portal
www.drought.gov

Peak Flow Forecasts…
Northwest River Forecast Center
www.nwrfc.noaa.gov/peak/

Colorado Basin River Forecast Center
www.cbrfc.noaa.gov/rmap/peak/peaklist.php

Temperature and Precipitation Outlook…
Climate Prediction Center
www.cpc.ncep.noaa.gov/

Idaho Spring Flood and Water Resources Outlook

The potential for spring flooding due to snowmelt is slightly elevated for portions of eastern and north central Idaho. The spring flood risk for the rest of the state is average to below average. SWE-2-12

Good soil moisture recharge from autumn rains and well above average reservoir storage has resulted in a slightly elevated threat of spring flooding along the mainstem Snake River in eastern Idaho, and along smaller tributaries above Idaho Falls. Spring flood risk is also slightly elevated in the Clearwater Basin which is currently holding one of the greatest snowpack percentages in the state. Elsewhere, the absence of low elevation snow and areas of below average mid elevation snow suggest an average or below average spring flood threat.

The primary factors in the development of spring flooding are the occurrence of persistent above normal temperatures, and rain on snow precipitation events. Even for areas that have low snowpack, spring flooding is possible under the right scenario. Additionally, wildfire burn scars can have a significant impact on local flood potential during spring snowmelt.

Precipitation and Temperature

As of early February, Water Year precipitation was near normal or above normal for the Panhandle, Spokane, Clearwater, and Salmon Basins, as well as the Upper Snake Basin near the Wyoming border. The Clearwater Basin had the greatest anomalies at 130 to 150 percent of  average. Aside from the Snake River headwaters region, Water Year  precipitation across southern Idaho stood at 60 to 80 percent of  average with south side Snake River Basins having the lowest  percentages.  Average temperatures for the Water Year have been above average for almost the entire state, especially across southern Idaho. 1

 

Snowpack

As of February 12, the highest snowpack percentages in the state were 114 and 116 percent of median in the Clearwater Basin and the Upper Snake above Palisades. Percentages were 91 to 107 percent of median for basins along the Montana border in eastern Idaho, and the Salmon, Spokane, and Panhandle Basins. Elsewhere in south central and southeast Idaho the snowpack was 57 to 81 percent of median, decreasing to 33 to 49 percent of median in the Owyhee and Bruneau Basins in southwest Idaho. Daily snowpack readings indicate record low levels for a handful of SNOTEL locations in southern Idaho.  Mountain snowpack in Idaho typically builds through March. Early April snow conditions will be pivotal to water supply conditions through the summer.  23

Reservoirs

Reservoir storage across Idaho is in good shape. Major reservoir systems across the northern half of the state were holding near average or above average storage as of February 1. Across the southern half of the state, with the exception of Brownlee at 85 percent and Mann Creek at 45 percent of average, most major projects had well above average storage which is great news considering the below average snowpack in many basins. Weather patterns, irrigation demand, and flood control needs will drive reservoir operations over the next several months. Wet spring weather or extended periods of above normal temperatures resulting in rapid snowmelt could result in significant increases in reservoir outflows and river levels.

Drought

After record setting precipitation and snowpack last year, abnormally dry conditions have returned to portions of west central and southern Idaho. Weather and precipitation for the remainder of winter and this spring will determine whether or not conditions improve or deteriorate for areas experiencing short term dryness. Good reservoir carryover will help ease drought concerns for those served by major storage projects.

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Long Range Outlook

The outlook through the end of February favors below normal temperatures and normal or below normal precipitation for the state. The outlook for March, April, and May favors below normal temperatures for the Panhandle, and above normal temperatures across far southern Idaho. The precipitation outlook for March, April, and May slightly favors wetter than normal conditions for the Panhandle, but does not shift the probabilities one way or another for the rest of the state.

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Water Supply Forecast

National Weather Service April through September water supply volume forecasts vary from 90 to 135 percent of average for most of the central Idaho mountains and north across the Clearwater and Panhandle regions. In southern Idaho, forecasts for the Big Lost Basin, the mainstem Snake River and tributaries above American Falls range from 80 to 125 percent of average, with the exception of Willow Creek near Ririe with a forecast of 38 percent of average. Forecasts for the rest of southern Idaho range from 39 to 73 percent of average with the lowest percentages in southwest Idaho. These forecasts may change considerably over the next couple of months since seasonal snow accumulation and rainfall typically occur during February, March, and April.

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Online Resources

Water Supply Volume Forecasts…
National Weather Service-Northwest River Forecast Center www.nwrfc.noaa.gov/ws/

National Weather Service-Colorado Basin River Forecast Center
www.cbrfc.noaa.gov/

USDA-Natural Resources Conservation Service
www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/

Snowpack Information…

National Weather Service-Northwest River Forecast Center
www.nwrfc.noaa.gov/snow/

National Weather Service-National Operational Hydrologic Remote Sensing Center
www.nohrsc.noaa.gov/

USDA-Natural Resources Conservation Service
www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/

Reservoir Storage…
Bureau of Reclamation Reservoir Storage
www.usbr.gov/pn/hydromet/select.html

USDA-Natural Resources Conservation Service
www.wcc.nrcs.usda.gov/basin.html

Drought Information…
U.S. Drought Portal
www.drought.gov

U.S. Drought Monitor
www.droughtmonitor.unl.edu/

National Drought Mitigation Center
www.drought.unl.edu/

Peak Flow Forecasts…
Northwest River Forecast Center
www.nwrfc.noaa.gov/peak/

Colorado Basin River Forecast Center
www.cbrfc.noaa.gov/rmap/peak/peaklist.php

Temperature and Precipitation Outlook…
Climate Prediction Center
www.cpc.ncep.noaa.gov/

Idaho Spring Flood and Water Resources Outlook – Mar 1, 2017

The potential for spring flooding due to snowmelt in 2017 is well above average across most of southern Idaho. Meanwhile, the spring flood potential is about average for northern Idaho. One thing to remember is that swe31mountain snowpack in Idaho generally peaks in early April, leaving several weeks to add to our snowpack and the flood potential.

The storm track through the winter has been very favorable for southern Idaho, resulting in an exceptional snowpack across the southern half of the state. Relatively warm weather accompanied by rain in February caused much of the snow in the lower valleys of southern Idaho to melt. However, substantial low elevation snow remains across portions of south-central and eastern Idaho. Additionally, well above average mid and high elevation snow exists across southern Idaho with a number of SNOTEL sites measuring record or near record snowpack. Across the northern half of Idaho, snowpack is near average.

The primary factors in the development of spring flooding are the occurrence of persistent above normal temperatures, and rain on snow precipitation events. Even for areas that have low snowpack, spring flooding is possible under the right scenario. Additionally, burn scars can have a significant impact on local flood potential during spring snowmelt.

Precipitation and Temperature

Water Year to date precipitation was above normal for almost all of Idaho. Percentages were highest in the Panhandle, Central Mountains, south-central and southeast regions, where 150 to 300 percent of average precipitation occurred. Lowest percentages in the state were across west-central and southwest Idaho at 100 to 130 percent of normal. Average temperatures have been average to below average across northern, and most of central and southwest Idaho. Across southeast Idaho, the average temperatures have generally been a little above average for the Water Year.

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Snowpack

As of March 1, snowpack was above median across southern Idaho with record or near record snowpack across much of south-central and extreme southeast Idaho. Percentages ranged from 157 to 192 percent of median in the Wood and Lost River Basins, Snake Basin above Palisades, Bear River, Raft River, Blackfoot, Willow, and Portneuf Basins. Elsewhere south of the Salmon River, basin percentages were generally 110 to 140 percent of median. Across the Clearwater, Spokane, and Panhandle Regions, snowpack ranged from 87 to 99 percent of median. Mountain snowpack in Idaho typically builds through March, and early April snow conditions will be pivotal to water supply conditions through the summer.

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Reservoirs

Basin-wide reservoir summaries as of March 1 indicate average to above average storage across most regions of Idaho. Large inflows on the Owyhee System in February boosted reservoir levels to 100,000 acre-feet above average. This was a welcome site after multiple years of drought and below average reservoir levels on the Owyhee System. Weather patterns and irrigation demand will drive reservoir operations over the next several months. With the exceptionally large snowpack across much of the south, above average reservoir outflows and high river levels are a good bet on rivers of southern Idaho this spring.

Drought

Abundant autumn rain and a good winter precipitation have erased drought conditions across the state according to the latest U.S. Drought Monitor. Weather and precipitation through this spring will determine whether or not conditions continue to improve before heading into the warm and dry season. The U.S. Seasonal Drought Outlook suggests that drought conditions are not likely to return to Idaho through the spring.

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Long Range Outlook

The outlook for March, April and May indicates equal chances of above normal, normal, or below normal temperatures across Idaho.  Probabilities slightly favor above normal precipitation during the period.

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Water Supply Forecasts

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National Weather Service April through September water supply volume forecasts vary from 115 to 225 percent of normal for the southern half of Idaho. Across the northern half of Idaho, percentages are generally 100 to 115 percent of average for the April through September period. These forecasts may change considerably over the next couple of months due to seasonal snow accumulation and rainfall that occur in March and April.

Resources

Water Supply Volume Forecasts…

National Weather Service-Northwest River Forecast Center www.nwrfc.noaa.gov/ws/
National Weather Service-Colorado Basin River Forecast Center www.cbrfc.noaa.gov/
USDA-Natural Resources Conservation Service www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/

Snowpack Information…

National Weather Service-Northwest River Forecast Center www.nwrfc.noaa.gov/snow/
National Weather Service-National Operational Hydrologic Remote Sensing Center www.nohrsc.noaa.gov/
USDA-Natural Resources Conservation Service www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/

Reservoir Storage…

Bureau of Reclamation Reservoir Storage www.usbr.gov/pn/hydromet/select.html
USDA-Natural Resources Conservation Service www.wcc.nrcs.usda.gov/basin.html

Drought Information…

U.S. Drought Portal www.drought.gov
U.S. Drought Monitor www.droughtmonitor.unl.edu/
National Drought Mitigation Center www.drought.unl.edu/

Peak Flow Forecasts…

Northwest River Forecast Center www.nwrfc.noaa.gov/peak/
Colorado Basin River Forecast Center www.cbrfc.noaa.gov/rmap/peak/peaklist.php

Temperature and Precipitation Outlook…

Climate Prediction Center www.cpc.ncep.noaa.gov/