February 2016 Climate Stats at Boise

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February 2016 ranked among the warmest 15% and the driest 35% of February’s at the Boise airport.

Temperatures on most days were more typical of March and early April.  Highs reached 60°F or above on four days.  The 65°F on the 26th was the warmest day since the 70°F on October 25 2015.  Lows were above freezing on eleven nights, including every night from the 14th through the 21st.  Despite the unseasonably warm weather, no records were equaled or broken.

Only four days averaged below normal, including the 1st through the 3rd, due to a deep cold upper level trough, and the 23rd, when clear skies and dry air allowed the temperature to drop to 24°F in the pre-dawn hours.

Measurable precipitation fell on only five days.  The only measurable snowfall was 0.7 inch on the 4th. An average February has 2.9 inches of snow.

The pattern responsible for the relatively warm and dry February was a persistent warm upper level high pressure ridge, which established itself following the departure of the upper level trough.  There was enough flow through the ridge to allow Pacific weather systems to cross our area, but they dropped most of their rain and snow on the mountains of northern and central Idaho.

Thunderstorms accompanied a cold front which crossed the Boise area around 5 am on the 18th. Some locations even reported small hail. Early morning thunderstorms, and winter thunderstorms, are rare in
southwest Idaho.

The front was followed by winds which gusted to 41 mph at the Airport.

Another cold front crossed the Boise area around 5 pm on the 19th, bringing brief heavy rain showers and a wind gust of 47 mph.

Fog and low clouds were observed on eleven days, including dense fog on four of those days. The fog usually dissipated in the afternoons.

January 2016 Climate Stats at Boise

Jan2016The first two days of January marked the end of a cold spell which began the day before Christmas. Temperatures both days averaged nearly 20°F below normal.  The low of 4°F on the 2nd could very well be the low temperature for the 2015-2016 winter. A 1-inch snow cover enhanced overnight cooling under clear skies.  A warm ridge of high pressure aloft trapped the cold air, forming a deep inversion.  5,000 feet above the valley floor, temperatures were around 15°F warmer.

Temperatures began to moderate on the 3rd as the high pressure weakened and the inversion eroded away.  From the 4th through the end of the month temperatures averaged above normal on all but three days.

From the 7th through the 12th, westerly flow aloft split over the coast, causing weather systems to move inland north and south of our area.  There was no rain or snow at Boise during that period.

By the 13th the flow had reconsolidated, enabling precipitation to again reach Boise.  A series of weather systems brought a week of wet weather, including 2 inches of snow on the 14th and 15th.

From the 19th through the 27th, an upper level high pressure ridge dominated the Western U.S. The only measurable precipitation at Boise was a hundredth of an inch on the 23rd from a weather system weakened by the ridge.

Starting on the 28th, very moist westerly flow replaced the ridge, bringing widespread precipitation to southwest Idaho. Rain on the 28th and 29th totaled a third of an inch at the airport.

The rain ended late on the 29th after a cold front crossed the area.  Cold moist and unstable air followed the front and on the 30th, a combination of afternoon heating at the surface and cold air aloft triggered convective snow showers.

By early morning on the 31st, cold air had settled into the valley and areas of dense fog formed from moisture left over from the showers of the previous three days.  The fog lifted during the afternoon forming a low cloud cover which prevented temperatures from rising above 34°F. The average temperature that day was below normal for the first time since the 11th.

October 2015 Weather Stats at Boise

Oct2015October 2015 was the 11th consecutive month of above normal temperatures at Boise, and the 2nd warmest in 76 years of airport records.  The 90°F highs on the 9th and 10th were records for those
dates. The 90°F high on October 10 was also the latest 90°F ever recorded.  Overnight lows were also unusually warm. The 56°F on the 8th set a new record for the date, and the 59°F on the 10th tied the record set in 1942.

The late summer warmth might have lasted another week had it not been for the Walker Fire, which blanketed the valley with smoke from the 11th through the 17th.  The smoke filtered enough sunlight to prevent temperatures from warming much above the 70s.

At Boise, the average date of the first 32°F low is October 10th.  This year, at the airport, the coldest reading for the month was 37°F on the 23rd.  But that same morning, frost was seen around the Treasure Valley, some Citizen Weather Observer Program (CWOP) sites at Meridian and Eagle reported lows of 30°F and 32°F respectively.

At the Boise Airport, the latest occurrence of a freezing temperature was November 11, 1944.  Last year the first freeze happened on November 10, ranking second all-time.

The above-average temperatures were partly a reflection of a warm upper level high pressure ridge which persisted over the western U.S. through most of the month.

Showery periods raised the precipitation total to slightly above average for the month.  On the 1st and 2nd, the 17th through the 20th, and the 25th through the 28th, low pressure troughs managed to push inland through the ridge.  Showers on the 30th came from a warm front which tapped very moist air from off the Washington coast.

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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