Hydrology

2022 Idaho Spring Flood and Water Resources Outlook

Spring Flood PotentialSEWApr622

The risk for spring flooding due to snowmelt is low across Idaho. Snowpack in the lower valleys has melted out and the remaining mid and high elevation snowpack is well below normal for this time of year with the exception of northern Idaho where snowpack percentages are near normal. Snowpack in most basins peaked early and is on pace to melt out several weeks earlier than normal. 

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. 

 

Water Supply 

NWSFApr622ational Weather Service water supply forecasts for April through September 2022 call for near normal runoff volumes for northern Idaho watersheds. Meanwhile, forecasts vary considerably across central and southern Idaho and are generally well below normal. Forecasts for the Snake River Headwaters near the Wyoming border and the Bear River in far southeast Idaho are generally 50 to 70 percent of normal. Volume forecasts for the Lost River Basins and Salmon River Basin are 60 to 80 percent of normal while forecasts for the Wood River Basins are 35 to 55 percent of normal. Forecasts for the Middle Snake River and the    Boise, Payette, and Weiser Basins range from 50 to 75 percent of normal. Across south central and southwest Idaho near the Nevada border runoff volumes are only expected to be 40 to 45 percent of normal. 

 

Temperature and Precipitation

In October, southern Idaho and the Panhandle Region had average to well above average precipitation while the Clearwater and Salmon River Basins in central Idaho were below average.  November precipitation was average to above average across the north while southern Idaho received well below average precipitation. Autumn was relatively warm statewide, especially in November when temperatures were 3 to 6 degrees (F) above average. December moisture varied considerably across Idaho but the majority of the state received average or above average precipitation. December temperatures were on the cool side across northern Idaho and a little above average across southern Idaho. 

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January precipitation was below average across most of the state while temperatures were near average across northern Idaho and below average across the south. Northern regions of Idaho were a mix of below average and above average precipitation in February while record dryness occurred across central and southern Idaho. February was a cold month with below average temperatures throughout the state.  March precipitation was below average with the exception of some of the Idaho Panhandle where precipitation was near average. March temperatures were average to below average across northern and eastern Idaho and a little above average elsewhere across the state.  The three month period from January through March, 2022 was one of the driest on record for many locations across central and southern Idaho. 

Snowpack

Mountain snowpack was well above normal across most of the state in early January. However, snowpack percentages trended down afterward and by the end of February had decreased by 50 to 60 percentage points across much of central and southern Idaho. As of April 6, mountain snowpack was well below normal with the exception of northern Idaho where snowpack percentages are near normal. April 6 snowpack percentages were the highest in the Spokane and Clearwater Basins at 99 and 91 percent of median, while the lowest was in the Owyhee Basin at only 48 percent of median. Idaho snowpack as a whole typically builds through March and peaks in early April.

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Reservoirs

April 1 reservoir storage in northern Idaho, far southeast Idaho, on the Henrys Fork arm of the Upper Snake, the Payette Basin, and on the main stem Snake River was generally around normal. Elsewhere across central and southern Idaho, reservoir system storage ranged from 39 to 88 percent of normal. Reservoirs on the Snake River above Heise were at a combined 57 percent of normal. Magic Reservoir in south central Idaho was only at 39 percent of normal while Owyhee Reservoir was only 63 percent of normal. Weather patterns and irrigation demand 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 continues to plague much of Idaho. Very dry conditions across much of central and southern Idaho from January through March have led to intensification of drought in some areas. Temperature and precipitation patterns for the remainder of spring and this summer will determine whether or not drought conditions improve or deteriorate. The U.S. Seasonal Drought Outlook indicates that drought is likely to persist across central and southern Idaho at least through the end of June. 

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Long Range Outlook

The outlook for April through June favors below normal temperatures across the Idaho Panhandle and above normal temperatures for most of southern Idaho, and equal chances of either below, above, or normal temperatures elsewhere. The precipitation outlook favors below normal precipitation across central and southern Idaho, and equal chances for either below, above, or normal precipitation across the Idaho Panhandle.

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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

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/

 

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 2020 Spring Flood and Water Resources Outlook

Spring Flood Potential

swe2020The risk for spring flooding due to snowmelt is near normal across most of Idaho. The exception is the Big Lost Basin and Wood River Basins where the risk is below normal due to well below normal snowpack and low soil moisture.  Relatively warm winter temperatures and a rather dry February have limited 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.

Water Supply

The water supply runoff volume forecasts for the spring and summer of 2020 are near average for northern Idaho watersheds. Meanwhile, forecasts vary considerably across central and southern Idaho.  Forecasts for the Snake River headwaters region and along the Utah and Nevada border generally indicate near normal runoff volumes. Forecasts for the Middle Snake River and Central Mountain watersheds indicate below or well below normal runoff volumes. The lowest runoff volume forecasts are for the Big Wood and Little Wood basins, ranging from 33 to 55 percent of normal. Low streamflows across portions of the Central Mountains could lead to water supply concerns for those relying on natural flows. These forecasts may change considerably since seasonal snow accumulation and rainfall typically occur during March and April.ws2020

Temperature and Precipitation

A wet September closed out the 2019 Water Year, but very dry conditions dominated through the first quarter of the 2020 Water Year. Precipitation for October through December was only 40 to 70 percent of average for most of the state with the driest conditions across the Central Mountains and southern Idaho. The January storm track brought above normal precipitation to most of Idaho with the exception of the Wood River Basins and Big Lost Basin where precipitation was below normal. In February, the Clearwater and Lemhi Basins, the Salmon River Basin, and areas along the Wyoming border had near normal or above normal precipitation while the rest of the state was below normal. Water Year to date precipitation in general is normal to below normal for northern Idaho and areas along the Wyoming, Utah, and Nevada borders. For the Snake Plain and Central Mountains, Water Year precipitation is below or well below normal. After a cold October where mean temperatures were 5 to 10 degrees below normal for most of the state, above normal temperatures have dominated for the 2020 Water Year.

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Snowpack

As of March 5, mountain snowpack was near normal for most of Idaho.  The Boise River Basin was lagging at 84 percent of normal. The Big Wood and Little Wood River Basins and Big Lost River Basin had the lowest snowpack percentages ranging from 56 to 68 percent of normal.  Idaho snowpack as a whole typically builds through March and peaks in early April.

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Reservoirs

Reservoir storage across Idaho was in good shape as of early March with major reservoir systems holding near average or well above average water. Across southern Idaho, reservoir storage in the Bear River Basin and Southside Snake River Basins was 144 to 153 percent of average. Storage in West Central Basins, Wood and Lost Basins, and the Upper Snake River Basin ranged from 110 to 168 percent of average. Meanwhile, reservoir storage in the Clearwater and Panhandle Regions was 96 to 104 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 could result in significant increases in reservoir outflows and river levels.

Drought

After a drought free start to the 2020 Water Year, dry fall conditions followed by low snowpack led to the development of drought conditions across portions of the Central Idaho Mountains. 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 above normal temperatures across the south half of Idaho while chances for either below, above, or normal temperatures are equal across the north. The precipitation outlook slightly favors above normal precipitation along the Wyoming border, otherwise the probabilities are not shifted one way or another for the rest of the state.

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MAM2020pOn-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/

2019 Idaho Spring Flood and Water Resources Outlook

Spring Flood Potentialswe38

The potential for spring flooding due to snowmelt is slightly elevated for much of the central Idaho Mountains, particularly in the Big Lost and Wood River Basins, as well as the Upper Boise, Payette, and Weiser Basins. For the remainder of the state the spring flood potential due to snowmelt is 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 may 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.

Water Supplywatersupply

The water supply forecast for the spring and summer of 2019 is near average for the majority of watersheds across Idaho. Exceptions are some above average forecasts in the central mountains and Upper Snake Region, and some below average forecasts along the southern border and in the Idaho Panhandle.

Temperature and Precipitation

Wet weather at the beginning of the Water Year (Oct. 2018) was beneficial in recharging the soil moisture after a very dry and warm summer. However, fall precipitation as a whole was below normal across the state and subpar precipitation continued through most of January. February precipitation was exceptional across west-central and southern Idaho with much of the area receiving 200 to 300 percent of normal. February temperatures were well below normal across the state, especially in northern Idaho. Precipitation for the 2019 Water Year through February has been normal or above normal for most of central and southern Idaho, and a little below normal for most of the Panhandle Region. Mean temperatures for the 2019 Water Year have been near normal.

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Snowpack

As of March 8, snowpack from the Salmon River Basin to the southern border was above normal. The highest percentages were in the Big Lost Basin, Wood River Basins, Payette Basin, and Weiser Basin, ranging from 133 to 152 percent of median. Northern Idaho snowpack varied from 89 to 100 percent of median for the Northern Panhandle, Spokane, and Clearwater Basins. Idaho snowpack as a whole typically builds through March and peaks in early April.

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Reservoirs

Reservoir storage across Idaho is in good shape as of early March with major reservoir systems holding near average or above average water. Across southern Idaho, reservoir storage in the Bear River Basin was near 140 percent of average and Southside Snake River Basin storage was near 95 percent of average. The Upper Snake Basin and Wood River and Lost River Basins was near 125 percent of average. Storage in West Central Basins was near 98 percent of average, and the Clearwater and Panhandle Regions were near 97 and 104 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 could result in significant increases in reservoir outflows and river levels.

Drought

Moderate to severe drought plagued portions of Idaho through the fall and early winter, especially across southern Idaho and near the Utah and Nevada border. Drought conditions were eased by well above normal precipitation and a large increase in mountains snowpack during February. The latest U.S. Drought Monitor shows lingering drought across far southwest Idaho and along the Utah border. Weather and precipitation for the remainder of winter and this spring will determine whether or not drought conditions continue to improve or deteriorate.

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Long Range Outlook

The outlook for March through May favors above normal temperatures across Idaho. The precipitation outlook slightly favors wetter than normal conditions over far southeast Idaho, but does not shift the probabilities one way or another for the rest of the state.

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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/

For a comprehensive report on each river basin in Idaho, see the Natural Resources Conservation Service report:   Idaho Water Supply Outlook Report March 1, 2019

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 https://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/

Current snow pack – February 26, 2019

Many of you are wondering about the current snow pack. Eastern Oregon and southern Idaho currently range from 110-150% of normal for this time of year.

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Bogus Basin, just outside of Boise, is unofficially approaching record snow depth for this time of year. Mores Creek Summit is also near record. Shown are snow water equivalent and snow depths at the end of Feb since the 1940s with today’s values.

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McCall: 55 inch snow depth, is the most snow on the ground in February since 2008. Not close to the record of 75″ snow depth set in February 1952.

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2018 Idaho Water Year Summary

Overview:

The 2018 Water Year was pretty good for Idaho overall. The hydrologic system was still benefiting from the extreme moisture received the previous year which provided excellent reservoir carryover and above normal streamflow through the fall and winter.  

Precipitation patterns favored northern Idaho and portions of the Upper Snake Basin in eastern Idaho, resulting in normal or above normal precipitation for these regions. By winter’s end, basin snowpack percentages for these areas were around 120 percent of median. Large snowpacks in adjacent areas of British Columbia and western Montana helped set the stage for spring flooding across the Panhandle region.  In contrast, precipitation was subpar for much of southern Idaho with some locations receiving only 50 to 70 percent of normal. The Owyhee Basin in far southwest Idaho had the lowest snowpack percentage in the state at 45 percent of median.
Dry and very warm conditions through the summer brought a return of drought conditions to the Panhandle region while drought conditions expanded near the Nevada and Utah border.  Streamflows fell below normal in a number of basins from late spring through summer as very dry weather persisted.

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Temperature:

Mean temperatures for the 2018 Water Year were above normal for almost the entire state. The southern half of Idaho, particularly the higher elevations, experienced the greatest anomalies with much of the area in the top 10 percentile.

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Fall and winter temperatures were normal or slightly below normal for the northern half of Idaho. Meanwhile, fall temperatures across the southern half of Idaho had a warm tendency. Springtime brought above normal temperatures to the entire state which spurred a strong snowmelt runoff for northern Idaho and portions of eastern Idaho. Summertime temperatures were generally around normal across the northern half of the state while southern Idaho experienced above normal temperatures.

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The storm track favored northern and central Idaho during the first quarter of the 2018 Water year resulting in normal to above normal fall precipitation across the northern half of the state.  Most of southern Idaho wasn’t as fortunate and started the water year off rather dry. Precipitation patterns continued to favor northern and central Idaho along with much of eastern Idaho during the winter and early spring. This served to build a healthy snowpack in these areas and set the stage for a good runoff season.  Across southwest Idaho winter precipitation lagged considerably which raised concerns for drought and agricultural water supply. By the end of June, hot and dry weather typical of summer settled in across the state and very little precipitation was received through the remainder of the water year.

Precipitation:prec1

 

Although a small percentage of annual precipitation occurs during the summer, it was an exceptionally dry period across the state. Almost all of Idaho was in the bottom 10 percentile for July through September precipitation and many areas experienced near-record dryness. For example, Lewiston experienced its 6th driest July through September on record.

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Snowpack:

By late winter a robust snowpack accumulated across northern Idaho, portions of the Upper Snake Basin in eastern Idaho, and adjacent areas of western Montana and British Columbia.  In contrast, much of south-central and southwest Idaho missed out on a lot of the winter storms resulting in a subpar snowpack, especially near the Utah and Nevada border. By early April, when Idaho’s overall snowpack typically peaks, basin percentages were around 120 percent of median from the Panhandle region south across the Clearwater Basin, and across portions of the Upper Snake Basin. In south-central and southwest Idaho, snowpack percentages dropped off to around 70 percent of median or less. The Owyhee Basin had the lowest early April percentage at 47 percent of median.

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Streamflow:

2017 set the stage for the 2018 runoff season. With the exception of a couple basins, streamflows for the 2018 Water Year started strong and were normal to much above normal through the fall, winter, and spring. Abundant water in the hydrologic system kept water flowing in some streams that are typically dry during the fall and winter, such as lower reaches of the Big Lost River in eastern Idaho.  Rapid melting of the large snowpack and full reservoir systems pushed spring snowmelt flows to much above normal for portions of the Panhandle, Central Mountains, and upper Snake River regions. As a result, spring flooding occurred across portions of the state, especially the Panhandle region. Despite the strong spring runoff for much of Idaho, streamflows fell below normal in many basins from late spring through summer.

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Reservoirs:

Reservoir storage got a head start due to excellent carryover from the previous water year and most major reservoir systems were able to fill.  Large snowpack in British Columbia, western Montana, and northern Idaho resulted in near-record or record high runoff prompting large flood control releases on some river systems. Even in parts of southwest Idaho where lower snowpack resided, large reservoir releases were necessary to accommodate high snowmelt inflows. As the irrigation season hit full stride reservoir systems were drawn down near a normal pace and ended the water year with near average carryover for the next water year.

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Drought:

Abundant fall precipitation erased drought conditions during early stages of the 2018 Water Year. However, above normal temperatures, subpar snowpack, and limited spring rains across the south allowed drought to creep back into the picture by late spring over southwest Idaho.  As has been the trend in recent years, above normal temperatures and below normal precipitation dominated through the summer. This allowed drought to expand across southern Idaho into the West Central Mountains as well as the Panhandle Region.

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2018 Idaho Spring Flood and Water Resources Outlook

M1The potential for spring flooding due to snowmelt is elevated for portions of eastern Idaho and northern Idaho. The spring flood risk for the rest of the state is average or below average. There remains an elevated risk of spring flooding across portions of the Upper Snake Basin due to above average mountain snowpack. This includes the mainstem Snake River above American Falls Reservoir and the Henrys Fork near Rexburg. An elevated flood risk also exists across the Panhandle Region where some of the highest snowpack percentages in the state reside. Elsewhere across Idaho, early May snowpack conditions suggest a low probability of spring flooding due to snowmelt. 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 where drought conditions exist, or that have low snowpack, spring flooding is possible under the right conditions. Additionally, burn scars can have a significant impact on local flood potential during spring snowmelt and rain events.M2

Water Supply

National Weather Service April through September water supply volume forecasts for northern and eastern Idaho, and the mainstem Snake River across southern Idaho range from 115 to 150 percent of average. Elsewhere, water supply forecasts are 85 to 110 percent of average for most of the Central Mountains and only 30 to 70 percent of average for south central and southwest Idaho. The lowest forecast percentages are in far southwest Idaho in the Bruneau and Owyhee River Basins at less than 40 percent of average.M3

Temperature and Precipitation

As of May 1, temperatures for the 2018 Water Year have been average or slightly below average across the northern half of the state while most of southern Idaho has experienced a little above average temperatures. Water Year precipitation was above average for the Panhandle, Spokane, and Clearwater Basins. The Clearwater Basin had the greatest anomalies in the state at 130 to 150 percent of average. The Salmon Basin and Upper Snake Basin near the Wyoming border were generally 100 to 130 percent of average. Elsewhere in southern and southwest Idaho the Water Year precipitation was mostly in the 70 to 90 percent of average range with pockets of around 60 percent.

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Snowpack

As of May 2, the highest snowpack percentages in the state ranged from 138 to 145 percent of median in the Clearwater, Spokane and Northern Panhandle Region. Not far behind were basins in eastern Idaho such as the Little Lost and Birch Basins, Henrys Fork, Teton, and Snake Basin above Palisades at 120 to 130 percent. The Payette, Boise, Salmon, Wood and Lost Basins ranged from 74 to 109 percent of median. Southside Snake River Basins along the Nevada border were a mixed bag ranging from a low of 17 percent of median in the Owyhee Basin to a high of 86 percent of median in the Raft River Basin. Northern Idaho basins and those near the continental divide reached their peak snowpack in mid to late April. The low elevation snow is gone and melting of the high elevations will increase over the next few weeks as temperatures warm.

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Reservoirs

Reservoir storage across Idaho is in good shape. As of May 1, storage in major reservoir systems throughout Idaho was 100 percent of average or greater, except where systems were heavily drafted to make space for anticipated snowmelt runoff. Weather patterns and irrigation demand will continue to drive operations through late spring as reservoirs are topped off. Wet spring weather or extended periods of warmth resulting in rapid snowmelt and large reservoir inflows could result in significant fluctuations in reservoir discharge and downstream river levels.

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Drought

Idaho is currently free from any official drought classification according to the U.S. Drought Monitor. However, below average precipitation for the Water Year and poor snowpack has put much of southern Idaho in the abnormally dry category. Weather and precipitation for the remainder of spring will determine whether or not conditions improve or deteriorate for areas experiencing the dryness. Good reservoir storage will ease drought concerns for those served by major storage projects.

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Long Range and Seasonal Outlooks

The outlook for May favors above normal temperatures across the state. The May precipitation outlook favors above normal precipitation for southeast Idaho and below normal precipitation for the northwest half of the state. The seasonal outlook for June through August favors above normal temperatures and below 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
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/

 

Spring Snowpack and Flood Outlook – April Update

The potential for spring flooding due to snowmelt in 2018 is well above average across most of northern and eastern Idaho and Montana. Meanwhile, the spring flood potential is slightly below average for southwest Idaho and southeast Oregon.

The storm track over much of the winter brought above normal precipitation values to much of the region. However, it was a tale of two cities on the northern end (cooler) and the southern end of the storm track. Cooler conditions on the northern portion of the storm track led to above normal snow values across Washington, northern Idaho, and western Montana, while the southern end of the storm track brought warmer conditions to Oregon, southwest Idaho, Nevada and Utah, leading to a below normal snow pack. The following graphic illustrates the amount of snow pack compared to normal for mid April.

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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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