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 2017 Water Year Summary

Overview

The 2017 Water Year will go down as one to remember due to record precipitation, record snowpack, and significant flooding that impacted the state. Extreme wet conditions resulted in a number of disaster declarations due to snow, flooding, and landslides, along with runoff volumes not seen for decades across portions of southern Idaho.

The Water Year started off with a bang as a series of Pacific storms brought record precipitation to the region in October 2016, recharging the soil moisture and setting the stage for abundant spring runoff. Winter was highlighted by very cold temperatures across the state, and extreme snowpack across much of southern Idaho.  As temperatures warmed during late winter and early spring, low and mid elevation snowmelt combined with rain, on top of saturated and frozen soils, led to widespread flooding across the Magic Valley, the Upper Snake Plain, and extensive flooding and landslides across northern Idaho.  This was followed by significant flooding on several main stem rivers and tributaries of central and southern Idaho later in the spring.  Water managers were challenged by the extreme runoff and many regulated and unregulated river systems experienced prolonged high flows and flooding. On a positive note, reservoir systems were filled to capacity resulting in plenty of water for the growing season and above average carry-over heading into 2018.

Despite the cold winter, compared to the 30 year normal, temperatures over the entire 2017 Water Year were above normal for most of Idaho with the greatest anomalies across the southern half of the state.  Water Year precipitation was 110 to 200 percent of normal for almost the entire state. Near the end of the snow accumulation season, snowpack was around 100 percent of normal in northern Idaho while many basins in central and southern Idaho were carrying 150 to 200 percent of normal snowpack. Despite all the water the state received, hot and dry summer weather allowed for a return of moderate drought to portions of central and northern Idaho with streamflows in some basins falling below normal by summers end.

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Temperature

Despite one of the coldest winters in recent decades, temperatures experienced over the entire 2017 Water Year ranked either above or much above normal.  After a relatively warm fall, well below normal temperatures persisted across low elevations for most of the winter months which allowed the low elevation snowpack to accumulate to unprecedented levels in some of the lower valleys. For some locations in northern Idaho it was the coldest winter in over 30 years. The extreme cold led to ice jams which resulted in flooding on some rivers and streams across the state.  For the entire water year, statewide temperature anomalies generally ranged from minus 1 to plus 3 degrees (F). Last year’s anomalies ranged from plus 2 to plus 4. Above normal temperatures during late winter and spring had a significant impact on the ripening of the snowpack which led to extreme melt rates and flooding in many areas. Had it not been for below normal temperatures experienced during the early winter months, anomalies for the year as a whole would be greater.

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Precipitation

Fall rains were well above average across most of the state, excluding the southwest corner. Record setting rainfall in October was key in recharging soil moisture and setting the stage for an excellent runoff season in the spring. An active winter ensued with percentages varying between 150 and 300 percent above normal across almost the entire state. This was largely due to numerous atmospheric river events that battered the Pacific Coastline, bringing ample moisture to the region. The first and second quarters of the 2017 Water Year therefore ended on a high note. Spring and summer precipitation percentages were largely lower than in the previous months, save portions of the Central Mountains, Eastern Magic Valley and the Upper Snake Plain, which still received above average precipitation. Summertime thunderstorm activity was somewhat limited for most of Idaho. South central and southeast Idaho was the exception, with bouts of monsoonal moisture bringing normal or above normal precipitation.

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Snowpack

The 2017 Water Year snowpack was quite healthy and hit record levels across portions of southern Idaho. Winter storms brought heavy snow to southern Idaho in December and January, not only to the mountains, but to the lower valleys as well. Southern Idaho snowpack ranged from around 100 to 160 percent of median by February, while northern Idaho snowpack was lagging at around 80 percent.  Snowpack in the Big Wood, Little Wood and Big Lost Basins reached 180 to 190 percent of median by early March; daily records were seen at many SNOTEL sites in the central Idaho mountains. By April 1 (when Idaho’s overall snowpack typically peaks), basin snowpack ranged from 95 to 170 percent of median statewide, with the highest percentages in southern Idaho. Runoff from low and mid elevation snowmelt in February and March caused widespread sheet flooding in the Magic Valley and across portions of the Upper Snake Plain. Rapid snowmelt, record rainfall, and saturated soils led to extensive flooding and landslides across northern Idaho in March and early April.  Record or near record high elevation snow and extreme melt rates led to prolonged flooding on many rivers and streams across the central mountains during the spring.

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Reservoirs

Wet fall weather and a large winter snowpack made for a challenging spring for reservoir operations.  Not only was the snowpack well above normal, but extreme snowmelt rates occurred leading to very large inflow peaks on some of the reservoir systems. Operators for large federal reservoirs and smaller privately owned reservoirs were taxed in keeping up inflows and flood control space. Some of the smaller privately owned reservoirs in southern Idaho were pushed to the brink on their reservoir capacity. Flood flows occurred below a number of reservoirs in southern Idaho, with 101 consecutive days of flood flows below Lucky Peak Dam on the Lower Boise River. Owyhee Reservoir was filled to capacity for the first time since 2011. By late spring, flood control operations ceased, reservoirs were full, and an abundant supply of water was available for the growing season along with above average carry-over in most systems for the start of the 2018 Water Year. Runoff volumes ranked near the top of historical records across most of southern Idaho, with record volumes at some locations fed by central mountain snowmelt.

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Streamflow

Record October precipitation pushed monthly streamflows above average across much of central and northern Idaho in the fall of 2016. Cold temperatures and low regulated flows, typical of winter, held streamflows at or below normal across most of Idaho in December and January.  A dramatic increase in streamflows occurred in February and March as warming temperatures initiated low and mid elevation snowmelt which was accompanied by periods of rain. High elevation snowmelt sustained the high streamflows through the spring and even through the summer across portions of central and southern Idaho.  Hot and dry weather allowed some basins to recede below normal during the summer months, particularly across northern Idaho.

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Drought

Wet weather during the fall and winter eliminated the abnormally dry conditions and pockets of moderate drought that were present at the beginning of the 2017 Water Year. However, above normal summer-time temperatures and below normal precipitation allowed areas of drought to redevelop across portions of central and northern Idaho by summers end.

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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/

Idaho 2016 Water Year Summary

The 2016 Water Year was another year marked by above average temperatures across Idaho, although anomalies were not quite as warm as the previous year. Compared to the 30 year normal, temperatures were several degrees (F) above average for most of Idaho with the greatest anomalies focused across the west half of the state. Normal or above normal precipitation occurred across the majority of the state, with areas of below normal precipitation primarily in northeast and southwest Idaho. April 1 snowpack was generally average to above average. However, warmer than normal temperatures once again brought an early melt of the snowpack, and some areas in northern Idaho even saw a record early melt off. The early snowmelt shifted the runoff timing and brought above average streamflows to many basins during late winter and early spring. As runoff passed through the system early, many basins saw streamflows recede to below normal by late May and June, particularly across northern Idaho. Reservoir storage was in good shape overall to start the growing season, but high irrigation demand left some reservoirs in the Upper Snake region with below average carry-over for next year. Long term drought impacts lingered across the state but eased over the course of the water year.

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Temperature

The upward trend in temperatures experienced over the past several years continued during the 2016 Water Year. With the exception of a few pockets in south central and eastern Idaho, the entire state experienced warmer than normal temperatures. Anomalies generally ranged from plus 2 to 4 degrees (F), with northern, west central and southwestern portions of the state experiencing the greatest anomalies. Above normal temperatures were noted at all elevations, having a significant impact on the ripening and early melting of low and mid elevation snowpack. Had it not been for below normal temperatures experienced during a portion of the summer (particularly in July), anomalies for the year as a whole would be greater.
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Precipitation

Fall rains varied considerably across Idaho with areas of well below normal to much above normal precipitation early in the water year. A favorable shift in the storm track occurred in December which brought abundant low elevation rain and mountain snow. This ended the first quarter of the 2016 Water Year on a high note, with most of southwest and portions of south central Idaho having received 125 to 200 percent of normal precipitation. January precipitation varied considerably, with some basins receiving well above normal precipitation while others fell well short. Dry conditions plagued much of the state in February, especially across the central and south. However, another favorable shift in the weather pattern brought excellent precipitation to the region in March and the second quarter of the water year ended favorably for the majority of the state. Spring precipitation was disappointing and except for locations near the Utah and Nevada border, most of the state experienced well below normal precipitation. Summertime thunderstorm activity was somewhat limited for most of Idaho. South central and southeast Idaho was the exception, with much of the area receiving normal or above normal precipitation.

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Snowpack

Snowpack during the 2016 Water Year was much improved compared to the previous year. Around April 1 (when Idaho’s overall snowpack typically peaks), basin snowpack generally ranged from 95 to 130 percent of median, with the highest percentages across south side Snake River basins. However, warm and dry conditions through the month of April were not kind to the snowpack. Snow melted at a record high rate in April and by the end of the month basin percentages had fallen below or well below normal, with the exception of some south side Snake River basins. Snowpack melted out 2 to 4 weeks ahead of normal at most SNOTEL locations and some areas in northern Idaho even saw a record early melt off.

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Streamflow

Above normal temperatures led to early runoff of Idaho’s snowpack. This produced above normal streamflows for much of the state during late winter and early spring. Streamflow averages in mid to late spring receded below normal in a number of basins, especially across northern Idaho. Meanwhile, highly regulated river systems across southern Idaho saw mostly average streamflows in mid to late spring. The warm and dry months of summer left many basins with below normal streamflows by year’s end. A few locations in southern Idaho hit record low 7-day average streamflows in late June and early July.

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Reservoirs

Early snowmelt and shifted runoff timing led to higher fill rates early in the season. Most large federal reservoirs along with most smaller non-federal reservoirs either filled or came close to filling and storage was generally in good shape to start the growing season. However, warm and dry spring and summer weather led to declining reservoir inflows and high demand for irrigation water which left large federal reservoirs on the Upper Snake River System with below average carry-over storage at the end of the water year.2016wateryear9
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Drought

Long-term drought continued to impact Idaho for much of the year. Drought impacts were eased thanks to near normal snowpack and adequate water supply across most of the state.2016wateryear11

 

Drought Level since 2000 across Idaho and Oregon

Drought levels across Idaho and Oregon have improved since the recent drought from 2014-2015.  Here a look at the amount of area affected by drought in Idaho and Oregon. The graphs depict the state percentage of area affected by drought. Data is courtesy of the United States Drought Monitor.

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Idaho 2015 Water Year Summary

The 2015 Water Year ended with significant temperature and precipitation anomalies across Idaho. Compared to the 30 year normal, temperatures were several degrees (°F) above average for the majority of the state. Below normal precipitation occurred across the most of Idaho, but most notable was the lack of snow during the winter and early spring. Warm temperatures combined with low snowpack set the stage for early runoff, with many areas losing their snow 4 to 6 weeks early. April 1st snow water equivalent (SWE) rankings were within the driest 5 percent for the majority of SNOTEL sites. The early runoff caused streamflows to peak well ahead of normal in most basins, and flows receded to levels typical of late summer as early as June and July. Record low streamflows were experienced at many stream gauges over the course of the summer.

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Temperature

Idaho was part of the much talked about record or near record warmth that dominated the western states during the 2015 Water Year. Average temperatures were well above normal throughout the state, particularly during the core winter months. Most of central and southern Idaho experienced positive temperature anomalies of 4 to 8 degrees (°F) during the January through March period, with pockets of southern Idaho averaging as much as 10 degrees above normal. Winter temperature anomalies weren’t quite as large (generally 3 to 6 degrees above normal) across northern Idaho, but still had a major impact on the snowpack. Daily temperature records were set at many SNOTEL sites over the course of the winter, for both daytime maximum temperatures, and nighttime high minimum temperatures. The relatively warm weather also brought an early start to the spring snowmelt and runoff.

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Precipitation

Precipitation favored northern and central Idaho, and portions of southwest Idaho during the first quarter of the 2015 Water Year. The first significant snowfall of the season impacted the Boise area the second week of November, dumping 5 to 9 inches of snow across the Treasure Valley. Dry conditions dominated most of the state from mid winter into early spring, except across the Panhandle Region where normal to above normal precipitation was the rule. Very dry conditions prevailed across central and northern Idaho during the spring, while normal to above normal precipitation occurred across southeast Idaho. The Idaho Panhandle continued to suffer from dry conditions through the summer months while most of central and southern Idaho received near normal or above normal precipitation.

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Snowpack

Warm temperatures and rain combined to take it’s toll on Idaho’s snowpack during the winter and spring. A number of storm systems brought significant precipitation to the state, but high snow levels resulted in more rain instead of snow, particularly at mid and low elevations. In fact, low elevation snowpack was absent or just a fraction of normal across much of the state through the winter. By February, the snowpack was already ripe and ready to melt in some areas. Overall snowpack across Idaho typically peaks the beginning of April, but April 1 of 2015 was marked by snow water equivalent (SWE) percentile rankings in the driest 5 percent, and many SNOTEL sites were at new record low SWE. Snowpack melted 4 to 6 weeks ahead of normal at many SNOTEL locations.

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Streamflow

Above normal temperatures led to early runoff of Idaho’s snowpack. This produced well above normal streamflows for much of the state during late winter and early spring. Peak flows occurred much earlier than normal, and were lower than normal as the snowpack gradually melted. Low flows normally seen in late summer and early fall were occurring by June and July in many areas. Daily and monthly record low flows occurred at several USGS streamgages from late spring through summer.

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Reservoirs

Snowmelt and runoff timing led to higher fill rates early in the season. Not all reservoir systems were able to fill though, especially the smaller reservoirs in southern Idaho. Warm and dry weather led to declining reservoir inflows and higher demand for irrigation water earlier in the year. Most large federal reservoirs across southern Idaho filled or came close to filling, but strong irrigation demand throughout the warm season left below average carry-over supplies.

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Drought

Long-term drought continued to plague southern Idaho through the 2015 Water Year. Drought conditions expanded across central and northern Idaho, spurred on by persistent above normal temperatures, below normal precipitation, and poor snowpack.

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