April 2018 Climate Statistics

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April was characterized by changing weather patterns. Migrating high pressure ridges and low pressure troughs at upper levels brought alternating periods of above and below normal temperatures, but overall there was a gradual warming trend, and the monthly average was slightly above normal.

Most precipitation fell during the first 16 days of the month, when half the days had measurable rain. The final 14 days were dry, except for a couple of trace events. As a whole, April was slightly drier than normal.

The first half of the month also saw three strong wind events. Boise fared somewhat better than the surrounding region, where there was local damage, as winds in the Treasure Valley weren`t quite as strong.

On the 2nd a rapidly moving upper level trough from the Gulf of Alaska crossed our area. It was accompanied by a strong jet stream, and a thunderstorm which dropped small hail on some parts of the Treasure Valley.  Some of the jet energy reached the surface as the cold front came through, producing a gust of 49 mph at the airport.

On the 7th another cold front crossed the Boise area, propelled rapidly inland by an initially strong upper level trough approaching the Washington coast.  The front triggered thunderstorms, one of which generated a gust of 55 mph from the northwest at the airport.  Over a quarter inch of rain was measured with this storm. The trough itself faded out as it progressed inland.

On the 12th a deeper, colder trough moved over our area. Two tenths of an inch of snow fell at the airport, but it melted quickly.

On the 16th an even deeper trough pushed inland, accompanied by a quarter inch of rain and a trace of snow.  That system was followed by a more settled, warmer period, with temperatures near or above normal from the 19th through the 28th. On the morning of the 18th another weather system was poised just off the northwest coast, but instead of heading for Boise, it went south to California on the 19th then east over the Colorado plateau on the 20th.

Our area was dominated by high pressure from the 20th through the 27th.  On the 27th the temperature maxed out at 90°F, two degrees shy of the record.

The early taste of summer was ended by a cold front on the 28th, followed by a slow moving upper level trough on the 29th.  The combination of cold air aloft and surface heating in the trough created instability which triggered convective showers but only traces of rain at the Boise Airport. The trough remained on the 30th and was expected to linger a few more days.

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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/