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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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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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.


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.


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.



Water Supply Forecasts


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.


Water Supply Volume Forecasts…

National Weather Service-Northwest River Forecast Center
National Weather Service-Colorado Basin River Forecast Center
USDA-Natural Resources Conservation Service

Snowpack Information…

National Weather Service-Northwest River Forecast Center
National Weather Service-National Operational Hydrologic Remote Sensing Center
USDA-Natural Resources Conservation Service

Reservoir Storage…

Bureau of Reclamation Reservoir Storage
USDA-Natural Resources Conservation Service

Drought Information…

U.S. Drought Portal
U.S. Drought Monitor
National Drought Mitigation Center

Peak Flow Forecasts…

Northwest River Forecast Center
Colorado Basin River Forecast Center

Temperature and Precipitation Outlook…

Climate Prediction Center

Annual Timing of Severe Weather in the NWS Boise forecast area.

As we move into spring, we also enter severe weather season in the NWS Boise forecast area. Research conducted locally on severe weather reports in our area from 1955 to 2005 yields important information about the onset of each of the major severe weather threats in our area: tornadoes; hail; wind; and flooding. NOTE: Hail is defined as 1″ or greater, wind is defined as 58+mph.

This first image shows time as a circle, with Jan 1 at the top and June 30 at the bottom, and time progressing clockwise. The four different colored lines indicate the relative frequency with which separate types of severe weather occur. The numbers on the concentric circles indicate the total number of occurrences of a given type of severe weather in a moving 19-day window (to smooth the raw data a bit and make it more presentable). As you can see below, wind (red) and hail (purple) reports ramp up rapidly in April, with flooding (green) increasing in May. Tornadoes (blue), while minimal in number, show a narrow peak in late April and a longer peak in from late May into early July. The hail (purple) maximum is in late June. Wind has two significant peaks in activity, with one in late June and another from late July to early August. The late June peak is probably associated with stronger late- Spring weather systems, while the second peak is associated with hot (relatively) dry days when evaporation of rain produces downbursts.


The second image contains the same data presently differently. The height of the peak for each day is the total number of reports (within the moving 19-day window) of all four phenomena. This is a good way to view the period with the highest overall threat of severe weather. Here we can see that the period of maximum severe weather shows the same basic two peaks as the wind (red) reports. This makes sense because wind is our most common form of severe weather. Clearly, June is our “busiest” month, with late July into August a close second.


One type of “severe” weather not touched on in this study is fire weather. Experience indicates that the storms that lead to the second wind maximum often start fires via lightning and then may lead to rapid spread due to the high winds.

Finally, here’s a map showing the location of severe weather reports from roughly 1955 to 2010. The brownish dots are wind, the green are hail, and the purple are tornadoes (Note: this map was originally created for a different study and therefore the colors do not match the graphs above).