Precipitation

September 2020 Weather Summary

September was warmer than normal. On the 4th, record highs of 102 were set at Boise and Burns. The 99 at Ontario tied the record for the date. On the 5th, the high of 100 at Boise tied the record for the date, and the 98 at Twin Falls set a new record.
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It would have been even warmer had it not been for widespread smoke and haze from wildfires.


It was a dry month, especially in southcentral and southeast Oregon, where totals were less than 5 percent of normal across a wide area.
September precipitation departure
September temperature anomaly
A very warm upper-level high pressure ridge anchored over the western region kept temperatures above normal during the first week of the month. By the 8th the ridge had migrated far enough west to allow a very cool upper-level low pressure trough to drop south from Canada.


The cold front ahead of the trough crossed our area on the 7th. Following the front, northwest winds in the 40-50 mph range were common. A gust of 54 mph from the northwest was measured near Boise.
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The trough kept temperatures below normal. Highs failed to reach 70 at a number of locations on the 8th.
Following the trough, the ridge built inland, and temperatures were above normal again on the 11th. The ridge dominated through the 17th, with highs in the 80s at many lower valley locations.
The northern part of the ridge gave way to a Pacific cold front on the 18th. Most locations received only light precipitation. Isolated thunderstorms generated outflow winds over 40 mph.
From the 20th through the 25th, the ridge was alive and well over the southwest U.S., while southwest flow aloft on its northern periphery kept temperatures in our region above normal.
Meanwhile, a more fall-like pattern was developing as westerly flow aloft strengthened across the North Pacific.
The upper-level winds carried a fast-moving weather disturbance inland on the 25th. While rainfall was generally light in the valleys, McCall set a record for the date with .54 inch.


Temperatures stayed below normal through the 27th. On the 28th the ridge, now resurrected over the west coast, brought a warming trend. It would guarantee above-normal temperatures through the 30th and beyond.

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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Interested in measuring precipitation? Join the CoCoRaHS observing network.

Join CoCoRaHS Today!cocorahs

CoCoRaHS is a practical, enjoyable and useful activity. If you have an interest in weather and would like to help your local community, as well as scientists and others interested in precipitation, then CoCoRaHS is for you. It only takes a few minutes a day and gives you the chance to participate in real hands-on science. You’ll be amazed at what you learn as you become more aware of the variable weather that impacts you, your neighbors, your state and our entire country.

Data on the web

Volunteers submit their observations using the CoCoRaHS website or apps. Observations are immediately available to the public via maps and data analysis tools, and to data users via the CoCoRaHS Web API. Data users such as scientists, resource manages, decision makers and others have come to rely on the high density, high quality measurements provided by CoCoRaHS observers.

CoCoRaHS is Educational

CoCoRaHS offers learning opportunities too. In addition to training materials, newsletters and the ‘Message of the Day’, members also enjoy opportunities to attend Webinars featuring experts in weather, climatology and other pertinent disciplines. CoCoRaHS offers classroom resources for K-12 teachers. Students get to collect and submit real scientific data – all while meeting State and National Standards in science, math, geography and more!

What is CoCoRaHS?

The Community Collaborative Rain, Hail and Snow Network, is a non-profit, community based, network of volunteers who measure and report rain, hail and snow in their backyards.

A brief History

CoCoRaHS came about as a result of a devastating flash flood that hit Fort Collins, Colorado in July 1997. A very localized storm dumped over a foot of rain in several hours while other portions of the city had only modest rainfall. The ensuing flood caught many by surprise, caused $200 million in damages, and resulted in five deaths. CoCoRaHS was born in 1998 with the intent of doing a better job of mapping and reporting intense storms. CoCoRaHS became a nationwide volunteer network in 2010 and is now international with observers helping provide critical precipitation observations, benefiting their country’s needs.

Volunteers of all ages welcome!

Individuals and family volunteers of all ages and all walks of life are the foundation of the CoCoRaHS network, Anyone can help. It only takes a few minutes to check the rain gauge and report your observations.

Training: “the Key to our success”

It is important that all CoCoRaHS precipitation reports be accurate and consistant. Training is provided on how to install gauges, properly measure precipitation and transmit reports. CoCoRaHS precipitation reports are accurate and very useful.

Why is there so much interest in rain, hail and snow?

Precipitation is essential for life. It varies greatly with topography, storm type and season. It really is true that it may pour on one side of the street and be dry on the other. A portion of a field may be pounded by hail while others nearby receive no damage. Snowfall may pile up in one neighborhood and only dust another. Rain, hail and snow are fairly easy to measure, and the data collected are very important. Meteorologists, hydrologists, engineers, builders, farmers . . . you name it, everyone seems to care about rain, hail and snow. That’s why we ask, “How much fell in your backyard?” There are limited observations across southwest Idaho and southeast Oregon, compared to the rest of the country, so we would love to have your observations. To learn more about the CoCoRaHS program and to see where your fellow observers have recorded rain amounts, visit http://www.cocorahs.org/.

Invite your neighbors, relatives and friends by sending them this “Join” link: http://www.cocorahs.org/application.aspx

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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Winter 2016 and Spring 2017 Flood Summary

This past winter and spring had its share of flooding across southeast Oregon and southwest Idaho.  Not only did spring runoff bring flooding to rivers and streams, but ice jams and snow melt caused flooding during the winter as well.  The stage was being set for an active spring flood season as far back as October 2016, when 150 to 400 percent of normal precipitation occurred across much of the region which moistened the soil profile.  The winter storm track brought well above average snowfall to most of southeast Oregon and southwest Idaho, with extreme snowfall across lower valleys.  A relatively cool and wet early spring was the final piece of the puzzle to ensure abundant spring runoff.  An indicator of how wet this past winter and spring have been, water supply forecasts for the April through September period rank in the top 10 for most of southeast Oregon and southwest Idaho, dating back to 1970.  Additionally, all major reservoir systems either have filled or are expected to fill.  Record high precipitation was seen across many areas from December 2016 through June 2017, shown in the figures below.

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The map below shows March 1 snow pack along with areas where flooding had a significant impact.

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Although the threat of snow melt flooding has diminished, summertime thunderstorms can pose a serious flood risk.  Areas of steep terrain and areas burned by wildfire are at particular risk for flash flooding due to thunderstorms.  For flood safety information, visit http://www.floodsafety.noaa.gov/.  For the latest river conditions, see http://water.weather.gov/ahps2/index.php?wfo=boi.

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

 

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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