2019 Idaho Water Year Summary

Overview:

The 2019 Water Year can be characterized as a year with a lot of variability, which is quite normal when talking about weather in Idaho.  Overall, it was a good year with respect to moisture as the majority of the state received normal or above normal precipitation. The exceptions were east-central Idaho and the panhandle region where water year precipitation was below normal. With respect to temperature, mean temperatures across the state were close to normal.

April 1 snowpack percentages were above normal for central and southern Idaho and a little below normal across northern Idaho. Northern Idaho and areas near the Nevada border received the better snow through the first half of the accumulation season while most of central and southeast Idaho lagged considerably.  In February, a series of cold and moisture storm systems brought abundant snow to the state which more than made up for the drier conditions earlier in the water year.

Springtime was rather typical with periods of warm and dry weather followed by cool and wet conditions.  Except for portions of east-central Idaho and the panhandle, spring precipitation was normal or above normal across the state. Above normal temperatures prevailed across northern Idaho during the spring while temperatures across central and southern Idaho were generally near normal or below normal. A healthy snowpack and favorable spring weather produced good spring runoff with most reservoirs topping off and adequate water supply for the state.

Summertime temperatures and precipitation were close to normal across Idaho. With the exception of the panhandle region, normal to above normal streamflows were sustained through the spring and summer months. Major irrigation projects serving Idaho ended the 2019 Water Year on a good note with normal or above normal carryover.

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Precipitation:

Well below normal precipitation occurred across most of the central mountains and far southwest Idaho during the 1st quarter (Oct.-Dec. 2018) of the 2019 Water Year. Meanwhile, the rest of the state varied from a little below to a little above normal. January and March were quite dry across the state as a whole, but sandwiched in between was an exceptionally wet February with precipitation 150 to 300 percent of normal for the majority of the state. April was another wet month, highlighted by widespread river and small stream flooding across much of west-central Idaho April 7-13 when 2 to 5 inches of rain fell on top of snowmelt. Above normal precipitation fell across southern Idaho in May while most of central and northern Idaho saw below normal precipitation. June was a rather dry month for the state followed by near normal precipitation for most of the state through the summer. The water ended on a high note with above normal precipitation in September, helping recharge soils after the moisture sapping summer months.

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Temperature:

Mean temperatures for the 2019 Water Year were near normal for most Idaho. During the 1st quarter (Oct.-Dec. 2018) of the water year mean temperatures were generally above normal for northern and central Idaho, while most of southern Idaho experienced normal to below normal temperatures. Typical monthly variability occurred during the January through June period. Cold temperatures in February and March were vital in building and maintaining a good snowpack while cool temperatures across southern Idaho in May and June resulted in a slower snowmelt and good streamflows heading into the warm and dry season.  Summertime temperatures as a whole were close to normal.Capture

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Snowpack:

Overall it was a good snow year for Idaho.  The storm track favored northern and far southwest Idaho the early part of the accumulation season with January 1 snowpack percentages ranging from 88 to 117 percent of normal. Elsewhere, snowpack lagged considerably by January 1, especially across the central mountains where percentages ranged from 44 to 73 percent of normal. Subpar snowpack was the rule through the month of January but a favorable storm track in February brought a series of cold and wet storm systems to the region. From February 1 to March 1, snowpack increased by 25 to 60 percentage points across most of central and southern Idaho while gains of 10 to 15 percent occurred across northern Idaho. By early April, when Idaho’s overall snowpack typically peaks, snowpack percentages from the Salmon River Basin south to the Nevada and Utah border ranged from 106 to 145 percent of normal while northern Idaho basins ranged from 85 to 95 percent of normal. This set the stage for an adequate water supply through the growing season.

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Streamflow:

A warm and very dry summer in 2018 left streamflows in Idaho below to much below normal in many river basins to start the 2019 Water Year, particularly across the panhandle, west-central mountains, and southern Idaho.  Streamflow percentiles varied considerably from basin through the core winter months. The snowmelt season and good spring precipitation brought robust streamflows to the state in April. Heavy rain combined with snowmelt caused main stem river flooding, flash flooding, and small stream flooding across much of central Idaho April 7-13 with record or near record crests observed on a number of waterways. Favorable spring weather resulted in a nearly ideal melt of the mountain snowpack and extended healthy streamflows through the summer for southern and central Idaho. Across the panhandle region, lower snowpack and drier spring weather resulted in below normal streamflows in many basins from late spring through summer.

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Reservoirs:

Reservoir storage carryover to start the 2019 Water Year was near normal. A healthy snowpack by early April and good spring runoff allowed major reservoir systems to either fill or come close to filling.  Flood control releases were necessary on some reservoir systems during spring runoff resulting in high flows on some rivers. As the irrigation season hit full stride reservoir systems began drawing down in mid to late June. By the end of the growing season reservoir storage was normal to above normal for almost all reservoirs across the state with good carryover for the 2020 Water Year.

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Moderate to severe drought plagued much of the panhandle region along with west central and southern Idaho through the 1st quarter (Oct.-Dec. 2018) of the 2019 Water Year. However, abundant snowfall and good spring precipitation erased drought conditions by early April. Moderate to severe drought returned to the panhandle region over the summer while central and southern Idaho remained drought free. Minimal drought across the state contributed to a rather quiet wildfire season.

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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.

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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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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/