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April 2018 Climate Statistics

May 7, 2018May 7, 2018 by NWS Boise

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 2^nd 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 7^th 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 12^th 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 16^th 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 19^th through the 28^th. On the morning of the 18^th another weather system was poised just off the northwest coast, but instead of heading for Boise, it went south to California on the 19^th then east over the Colorado plateau on the 20^th.

Our area was dominated by high pressure from the 20th through the 27^th. On the 27^th 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 28^th, followed by a slow moving upper level trough on the 29^th. 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.

March 2018 Climate Statistics

April 2, 2018April 24, 2018 by NWS Boise

The changeable weather of March 2018 was typical of late winter and early spring. The progression of low pressure troughs and high pressure ridges brought alternating periods of warm and cold temperatures, yielding a close-to-normal average. There were no record highs or lows.

Episodes of active weather and moderate precipitation occurred as each trough and its accompanying cold front entered our area.

The 0.54 inch rainfall on the 14^th tied the record for the date set in 1927. The 1.89 inches for the month was 0.50 inch above normal and ranks in the wettest 20 percent of Marchs at the airport, and the wettest 25 percent at all Boise locations.

Snowfall for the month was 2.6 inches, mostly falling on the morning of the 25^th. The monthly normal is 1.3 inches.

The first four days of March were dominated by an upper level trough which brought cold temperatures but little precipitation. The 23°F on the 4^th was the low for the month.

Temperatures rose above normal on the 8^th as an upper level ridge began to build over the region.

The ridge reached its maximum amplitude on the 12^th, allowing temperatures to rise into the low 60s. As the ridge began to exit, southerly flow ahead of the next upstream trough pushed the temperature to 69°F on the 13^th, the high for the month.

The trough sent a strong cold front inland, and convective showers ahead of the front were preceded by northwest winds gusting over 30 mph. When the showers reached the Boise airport just after 4 am, moderate rain began and persisted for about an hour before tapering off and ending. The trough drifted slowly inland on the 16^th and 17^th, preventing temperatures from rising above the upper 40s on those days.

The last weather maker of the month was another strong upper level trough which stalled off the coast of British Columbia on the 22^nd. This trough extended far enough south to tap subtropical moisture.

Like its predecessor, moderate rain preceded the cold front on the afternoon of the 22nd. The front passed the airport at about 7:30 pm, accompanied by the first thunderstorm of the spring season. The strongest convection and heaviest rain stayed west and north of Boise.

As the trough moved inland and weakened on the 25th, it produced the last measurable precipitation of the month, falling as 2.1 inches of snow. The cold north pacific air was responsible for a high of only 45°F that day, the chilliest maximum since the start of spring.

Following this system, west-northwest flow aloft returned temperatures to near normal from the 28^th through the 31^st.

February 2018 Climate Stats

March 2, 2018April 24, 2018 by NWS Boise

February 2018 can be described as three seasons in one month.

It began with early spring. The 1^st through the 9^th had the same average temperature as the last half of a normal march. The 59°F on the 2nd tied the record for the date set in 1881. The 61°F on the 3^rd eclipsed the old record of 60°F in 1953.

Temperatures from the 10^th through the 18^th were near normal for late winter.

It was back to mid-winter from the 19^th through the 28^th, which averaged colder than a normal December.

The average temperature for the entire month was deceptively close to normal.

February is usually the driest winter month at Boise. This February had only 56 percent of normal precipitation, despite heavier than average snowfall. Measurable precipitation fell on only 4 days.

Snowfall totaled 7.4 inches, over twice the normal 2.8 inches. There was at least patchy snow cover each day during the last week of the month. The 4.9 inch snowfall on the 22nd set a record for the date. The old record was 4 inches in 1912. The greatest snow depth at the Boise Airport was 5 inches on the 22nd.

A warm upper level high pressure ridge off the coast was responsible for the spring-like weather from the 1^st through the 9^th. It also kept most of the precipitation associated with Pacific weather systems east of our area.

The ridge gradually shifted west, allowing northwest flow aloft to transport cooler but not unseasonably cold air into the Intermountain region from the 10^th through the 18^th. One weather system embedded in the flow was strong enough to bring Boise a quarter inch of precipitation on the 14th, including half an inch of snow.

The ridge continued to drift west as a very cold upper level trough deepened over western Canada. Strong northerly flow on the west side of an upper level low over Hudson Bay pushed the trough south of the border. The cold front ahead of the trough passed Boise at about 9 pm on the 17^th. Following the front, the wind increased from the northwest, and a peak gust of 43 mph was recorded at the airport just after midnight.

Temperatures initially were not unseasonably cold, but by the 19^th modified arctic air had begun to filter into the Treasure Valley, while the true arctic front was stalled in the central Idaho mountains.

Although the coldest air stayed well to our north and east, clear skies, light winds, and very dry air allowed the temperature to drop into the single digits in the Boise area by sunrise on the 20^th. The low of 9°F at the airport was the coldest since the 8 degrees on December 24, 2017.

The cold pattern persisted through the end of the month, with a deep upper level trough anchored over the western U.S. and western Canada. Temperatures remained unseasonably cold, and weather disturbances moving into the trough from Alaska brought periods of snow.

January 2018 Climate Stats

February 13, 2018February 13, 2018 by NWS Boise

January 2018 was the fourth warmest on record at the Boise Airport. For all Boise locations it tied January 1914 for sixth place. The monthly average of 37.8 degrees was 6.5 degrees above normal, and it even exceeded February’s normal of 36.5 degrees. Only the first four days of the month were below normal.

It is noteworthy that, prior to this year, the top five warmest Januarys at the airport occurred during El Niño winters, and three of those were strong El Niños. January 2018 didn’t fit the pattern, as a weak La Niña was in effect.

There were two record highs, each occurring ahead of strong cold fronts. The 58°F on the 18th and the 59°F on the 30^th broke the previous daily records of 57°F in 1998 and 56°F in 1992.

The unusually mild temperatures resulted from a combination of a progressive pattern and a persistent warm upper level high pressure ridge over the Western U.S.

Westerly flow aloft brought mild moist Pacific weather systems inland across the northern intermountain region, temporarily displacing the ridge, which always managed to rebuild. The active pattern hindered the formation of temperatures inversions and blocked invasions of cold air from western Canada.

Precipitation totaled 1.36 inches, close to the January normal of 1.39 inches. Most of it fell during two episodes.

On the 9th a cold front produced nearly half an inch of rain at the Boise Airport. During the afternoon of the 19th another cold front generated nearly a quarter inch of rain. The upper level low pressure trough which followed the front brought an additional two tenths of an inch of rain.

On the 30th the last cold front of the month passed the airport at 1:40 pm MST, with a wind gust of 34 mph from the northwest, but no precipitation.

The only measurable snowfall was 0.1 inch on the 25th. The total for January 2017 was 21.5 inches. Normal is 5.1 inches. There have been Januarys with less snow. Only traces fell in 1934, 1961, and 2003.

McCall, Idaho recorded its 4th warmest January on record in 2018. Daily average minimum temperatures in January 2018 averaged 21.4°F, 3rd warmest on record.

Burns, Oregon recorded their 6th warmest January on record in 2018.

Baker City, Oregon had their 3rd warmest January on record in 2018. Average maximum temperatures reached 42.9°F, tied for the warmest on record in January since records began in 1928.

Jerome, Idaho had their 3rd warmest January on record since 1916. Two record highs were set in the month of January. Average minimum temperatures tied for the second warmest on record in January at 28.5°F.

Ontario, Oregon had their 4th warmest January on record. A daily record high temperature of 54°F was recorded on January 12.

With the upper level high pressure ridge over the Great Basin,the storm track was directed to the north across British Columbia, Washington, northern Idaho, and western Montana, leading to warmer than normal temperatures.

December 2017 Climate Stats

January 4, 2018 by NWS Boise

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 7^th 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.

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

December 4, 2017January 4, 2018 by NWS Boise

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 3^rd, 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.

October 2017 Climate Stats

November 1, 2017November 17, 2017 by NWS Boise

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

October 1, 2017November 17, 2017 by NWS Boise

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

August 7, 2017November 17, 2017 by NWS Boise

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.

June 2017 Climate Stats

July 3, 2017 by NWS Boise

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.