December 2017 Climate Stats

Dec-2017

Temperature inversions plagued the Treasure Valley most of the month, resulting in below normal temperatures at Boise on all but eight days. Although it ranked among the coldest 25 percent of Decembers since airport records began in 1940, it averaged 5 degrees warmer than December 2016.

A persistent high pressure ridge kept the Boise area dry through much of the month. Precipitation was nearly half an inch below normal, and half the December 2016 total.

The month started out fairly mild under west-southwest flow aloft ahead of an upper level low pressure trough. The trough crossed the Boise area on the 3rd, bringing the largest one-day precipitation of the month, mainly in the form of rain.

As this system exited to the east, an upper level high pressure ridge built offshore and expanded inland.

By the 5th an inversion had become established in the Treasure Valley, and temperatures remained below freezing from the 7th through the 15th.

On the 16th an upper level trough destabilized the atmosphere enough to break the inversion, and temperatures continued to warm as the upper level flow shifted into the west and southwest in advance of tet another upper level trough. The 19th and 20th were the warmest days of the month, with highs of 52 and 51.

By the 21st the trough had moved east of our area. It was followed by moist northwest flow aloft and a cold front which brought snow totaling 6 inches on the 22nd and 23rd. The accumulation of 5 inches was the first measurable snow cover since February 4.

A combination of modified arctic air, overnight clearing, fresh snow cover, and calm winds caused the airport temperature to fall to 8 degrees by sunrise on the 24th, the coldest reading since the 7 degrees on January 18.

More snow was on the way. It started at about 4 pm Christmas eve as a warm front approached from the west. By Christmas morning 3 inches of new snow had fallen.

Warming continued at higher elevations, ensuring that cold air would remain trapped in the valley for a few more days.

On the 30th a cold front passed through the area. It was felt as a warm front in the valley, as colder air aloft behind the front broke the inversion, allowing the airport temperature to climb to 43 degrees that afternoon.

On the 31st strong high pressure provided mostly clear skies.  Although a shallow inversion had formed overnight, the sun warmed the surface enough during the day to mix the air, and temperatures around Boise warmed into the 35-40 degree range.

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Precipitation across much of southern Idaho and Oregon was well below normal as a persistent upper level high pressure led to an inversion with cool conditions in the valleys and warm conditions in the mountains.

November 2017 Climate Stats

Nov-2017

November`s average of 43.8 degrees is the tenth warmest at the Boise Airport. Aside from a cool spell from the 4th through the 8th, temperatures averaged above normal on most days. Only ten nights had lows were below freezing. The low of 47 on the 22nd set a record high minimum for that date, eclipsing the old record of 46 set in 1921.

Measurable rain fell on half the days. The monthly total of 1.77 inches is nearly half an inch above normal and ranks among the wettest 25 percent of Novembers at the airport.

Typical of fall, the weather pattern was active, with passing weather systems bringing showery periods and changeable temperatures.

On the 3rd, a low pressure trough from British Columbia brought Boise the coolest weather since last winter, and over half an inch of rain from the 3rd through the 6th. Below normal temperatures persisted through the 8th. The low of 25 on the 7th was the low for the month and the coldest reading since February 25.

Warmer more seasonable weather returned on the 9th. A weak high pressure ridge over the western U.S. and a persistent low pressure trough off the northwest coast kept us under southwest flow aloft, maintaining near normal or slightly above normal temperatures through the 20th.

Weather disturbances moving inland weakened as they traversed the ridge, but they retained enough moisture for moderate amounts of rain on the 10th, and again on the 14th and 15th.

On the 21st an unseasonably warm high pressure ridge built over the Desert Southwest and northwest Mexico, creating a source of warm air for the northern intermountain region. From the 20th through the

27th, Boise’s daily average temperatures ranged from 6 to 19 degrees above normal.

Moisture and weak weather systems moving through the north portion of the ridge generated nearly daily showers as far south as northern Nevada. Amounts were mostly light, but a stronger disturbance on the 24th produced a quarter inch of rain at the Boise airport.

The ridge weakened following a Pacific weather system which crossed the great basin the 27th, and temperatures at Boise returned to near normal on the 29th following a Pacific cold front.

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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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October 2017 Climate Stats

October 2017 ranks among the coldest 22 percent of Octobers at the Boise airport. Temperatures averaged 7.1 degrees below normal from the 1st through the 16th, and 1.3 degrees above normal for the rest of the month. Precipitation totaled near normal. The first freezing temperature of the season was the 31°F on the 9th, a day ahead of the average date.

There were no record highs, lows, or daily precipitation amounts.

During the first half of the month, cold upper level troughs were the dominant features. Brief periods of west or southwest flow aloft raised temperatures to near normal on the 6th, 7th, and 10th, but there were no above normal daily averages.

During the last half of the month, temperatures were near or above normal most days, thanks to west or southwest flow aloft, and strong upper level high pressure ridges on the 23rd and 24th, and again on the 27th and 28th.

This warmer pattern was briefly interrupted by a north Pacific cold front which crossed the Boise area around 4:30 am on the 20th. The frontal passage was accompanied by northwest winds gusting up to 42mph at the airport, causing visibility restrictions in blowing dust. Rain settled the dust a couple of hours later as a cold upper level trough pushed inland behind the front. Precipitation on the 20th and 21st totaled 0.37 inch at the airport. One location closer to the foothills reported around half an inch of rain.
On the 29th, the upper level ridge, which had kept Boise warmer than normal on the 27th and 28th, shifted west. This put our area under northwest flow aloft, allowing cooler air from British Columbia to spread south across the northern intermountain region.


September 2017 Climate Stats

Summer abruptly changed to fall following a cold front on the 14th, a week before the fall equinox. Temperatures averaged nearly 10 degrees above normal during the first two weeks of the month, and nearly 6°F degrees below normal for the rest of the month.  Precipitation was close to normal.  Smoke from numerous wildfires plagued the region for the first half of the month.
Highs ranged from 99°F on the 3rd to 52°F on the 22nd. Lows ranged from 69°F on the 9th to 36 on the 23rd and 24th, when frost was observed at many locations around the valley. At Boise the average date of the first 36°F degree low is September 25, and the average date of the first 32°F degree low is October 10.  The low of 69°F on the 9th set a record for the date, breaking the old record of 68°F in 1969.  The lows of 67°F on the 4th and 8th, and 66°F on the 13th, tied record high minimum temperatures set in 1998, 1967, and 2011 respectively.

The first half of the month was dominated by a strong and very warm high pressure ridge over the Western U.S. and southwest Canada.  At its strongest, during the first week of September, the ridge extended as far north as Northern B.C. and Alberta.

By the 12th, westerly flow aloft across southern Canada had eroded the north portion of the ridge, and temperatures in our area cooled to near normal by the 14th, ahead of an upper level trough which was deepening and drifting south from B.C. As the trough continued to deepen, it pushed a cold front across eastern Oregon and southwest Idaho during the evening of the 14th.  The trough began a stretch of unseasonably cool fall-like weather which was to last through the 25th, interrupted briefly on the 17th by warmer air ahead of a second even colder trough which originated west of Alaska.

That system brought showers, gusty winds, and the coolest temperatures of the month.  As it exited to the east, temperatures moderated to near normal by the 26th.

From the 27th through the 29th an upper level ridge provided clear skies and slightly above-normal temperatures. But during the night of the 29th, another Pacific cold front crossed the area, followed by more cool, showery, and breezy weather to end the month.


July 2017 Climate Stats

Jul-2017

July was hot. The average temperature of 81.6 degrees ranked the second warmest month for all years back to 1877.  Only July 2007, which averaged 83.1 degrees, was hotter. On the other end of the scale, July 1993 averaged a rather chilly 65.0 degrees.

It was a dry month. Only 0.01 inch was measured at the airport. Dry summer months are not unusual. Since 1877, 33 July’s were as dry or drier.

A semi-permanent warm upper level ridge, which normally forms over the Colorado Plateau during late spring and early summer, kept most of the Northwest U. S. dry through the month. Weather systems from the Gulf of Alaska were blocked from penetrating very far south of the Canadian border, and were forced to move east over southern Canada.

On three occasions the ridge expanded north. From the 4th through the 9th, the 13th through the 15th, and the 29th through the 31st, Boise was directly under the ridge, resulting in triple digit highs.

Monsoon moisture drifting north from Nevada produced locally wet late afternoon and evening thunderstorms. On several days, thunderstorms formed over Owyhee County, but they weakened as they moved over the Owyhee Mountains. Any rain that reached the ground usually missed the National Weather Service rain gauges.

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June 2017 Climate Stats

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June was unsettled, with alternating cool and warm periods more characteristic of spring than summer.

Overall, June was slightly warmer than normal, but daily averages fluctuated from 15 degrees above normal on the 7th to 12 degrees below normal just a few days later on the 11th. No records were equaled or broken.

Precipitation was slightly more than twice normal, although it was a mostly dry month. The anomaly occurred on the 11th and 12th, when 1.15 inches of rainfall was measured at the airport. The total for the other 28 days was only a quarter inch.

The first week was warm and dry, dominated by an upper level high pressure ridge extending from the Colorado Plateau to the northern Plains.  The high for the month of 97°F on the 7th was repeated on the 25th.

On the 9th, a cold upper level low pressure area from the Gulf of Alaska began to invade the Pacific Northwest states.  By the 12th, it was centered over the Idaho-Nevada border.  The well developed counterclockwise circulation pulled in copious moisture which originated in the Gulf of Mexico and crossed the Rockies from eastern Montana.  This resulted in the heavy rain which fell mainly overnight between the 11th and 12th.

Despite the subtropical origin of this moisture, cold north Pacific air kept Boise’s temperature well below normal.

Following this system, the temperature rebounded to normal on the 15th.  And on the 19th and 20th the upper level ridge, having expanded north from the Desert Southwest, pushed the temperature up to 96°F both days.

Sojourns of westerly or northwesterly flow aloft kept temperatures close to normal from the 22nd through the 24th, and from the 28th through the 30th.  A brief incursion of the upper level ridge brought more hot weather on the 25th, with a high of 97°F for the second time during the month, and a high of 96°F on the 26th.

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May 2017 Climate Stats

May-2017

Although only half of the normal monthly precipitation fell at the Boise Airport, and measurable rain was recorded on only 7 days.  Nevertheless, strong thunderstorms did occur in our region on the 5th, 12th, and 16th.  Most of the severe weather missed Boise, but high winds, large hail, and brief heavy downpours hit a number of locations in eastern Oregon and southwest Idaho.

Temperatures averaged close to normal. The high of only 49°F on the 17th was the only record.  It was the coolest maximum temperature ever recorded on that date, breaking the old record of 51°F set in 1896. The last freezing temperature of the season was 31 on the 13th. The average date of the last 32 degree reading is may 8.

The first official 90 degree high of the season occurred on the 30th. The last time 90 degrees was reached was September 29, 2016.

Typical of spring, changing weather patterns brought a succession of warm and cool periods.  Upper level high pressure ridges with above normal temperatures were displaced about every 6 days by cold fronts followed by upper level troughs with unseasonably chilly north Pacific air, gusty northwest winds, and showers.

The most noteworthy of these weather systems arrived from the Gulf of Alaska on the 16th, passing directly over the Boise area that afternoon. It was ushered in by a thunderstorm which formed over the lower Treasure Valley and dropped half an inch of rain on the Boise Airport. Small hail was seen in some parts of town.

During the late afternoon of the 30th, a dry thunderstorm rolled off the Owyhee Mountains and arrived at the airport just before 7 pm, with gusty winds and blowing dust.  A peak gust of 45 mph was measured at 6:58 pm MDT.

On the 31st, isolated afternoon showers and early evening thunderstorms developed west of Boise and intensified as they moved north ahead of an approaching Pacific cold front. They had little effect on the upper Treasure Valley, but the front ended the five day stretch of summer-like weather.

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April 2017 Climate Stats

Apr-2017

April was characterized by changeable weather typical of spring.  Mild periods were interrupted by days with below normal temperatures. The only record was the unseasonably warm low of 52 on the 6th, which broke the old record of 51 set in 1898.

The unsettled weather was the result of a steady progression of upper level ridges and troughs across the inter mountain region.

Unseasonably cold sea surface temperatures and cold air aloft resided off the northwest coast for the entire month, contributing to several cool spells and frosty mornings in Boise.

It was the 16th wettest April since 1878, and the 9th wettest April since 1940, when record keeping began at the Boise airport.

The 0.51 inch of precipitation on the 8th eclipsed the old record for the date of .40 inch set in 1881. This event was caused by an increasingly moist and unstable southwest flow ahead of an upper level offshore trough. The scenario was aided by a relatively weak disturbance moving through the flow ahead of the main trough.

A tenth of an inch of snow fell on the 8th, but there was no accumulation.

It was a breezy month. Nearly half the days had gusts which reached or exceeded 30 mph.  The main event was the 53 mph gust from the southwest on the morning of the 7th. Convective showers were forming in moist and unstable
air ahead of a strong cold front. Southwest winds exceeded 40 mph as low as 5000 feet above the surface, and a downdraft from a shower approaching the airport added to this wind as it descended to ground level.

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March 2017 Climate Stats

Mar-2017

March 2017 was warm and wet, with a major storm to finish out the month.  Without that storm, march would actually have been slightly drier than average.  But with 2.86 inches, it turned out to be the second wettest march at the Boise airport, and the seventh wettest march going back to 1878 in the Boise area.  The 1.40 inches on the 30th tied January 16, 1896 as the tenth wettest day in the Boise area. At the airport it was the seventh wettest day.

There were showers each day from the 3rd through the 11th, and there was measurable precipitation on nearly half the days from the 12th through the 31st.

The average temperature of 48.9 degrees tied 1992 as the second warmest march at the Boise airport, and the fifth warmest march going back to 1878 in the Boise area.

There were no record high temperatures, but the lows of 52 on the 18th and 48 on the 20th broke the previous daily records for warmest lows. The 49 on the 19th tied the record high low for that date.  There were no freezing temperatures from the 8th through the 27th.

During the first 28 days of the month there were no significant storms. Predominantly westerly or southwesterly flow aloft and eastward migrating high pressure ridges kept temperatures mild.  Minor weather systems moving inland from the Pacific provided light precipitation.

On the morning of the 29th, an ordinary looking upper level trough off the British Columbia coast was approaching the northwest U.S. By evening it had noticeably deepened as it neared the Washington coast.  A third of an inch of rain fell at the airport that evening, way out ahead of the main storm.

The rain stopped before midnight, only to resume just before 7 am MDT on the 30th as the cold front approached. As the front passed Boise around 8:30 am, the rain became heavy, and west winds increased to around 25 mph. The rain changed to snow just before 10 am, but only a trace fell before the snow ended shortly after 11 am.

Just over one inch of precipitation fell in 6 hours on the morning of the 30th.  Interestingly, the amount of moisture available for precipitation, measured by radiosonde during the two hours before the rain started, was a mere half inch. So how could nearly three times that amount fall on Boise?

As the upper level trough rapidly intensified directly over southwest Idaho, more moisture was pulled in from outlying areas and lifted over the treasure valley.  At the same time, the strong cold front provided additional lift to turn that moisture into rain and snow. Also, a closed circulation developed aloft, slowing the storm movement. This allowed more time for precipitation to accumulate.

As the system strengthened at upper levels, surface low pressure to our east deepened, causing northwest winds to increase through the afternoon. A peak gust of 52 mph was measured at the airport at 4:06 pm.

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