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June 2019 Climate Statistics

July 2, 2019September 19, 2019 by NWS Boise

It was the second driest June at the Boise Airport, where records go back to 1940. It was the fourth driest June at all Boise locations going back to 1864. Measurable rain fell on only two days.

The average temperature for the month was very close to normal.

June began with above normal temperatures, with highs in the 80s under an upper level high pressure ridge.

On the 7th a pacific cold front, followed by an upper level low pressure trough, brought a brief break in the warm weather. Highs were only in the lower 60s on the 7th and 8th.

By the 10th temperatures had risen above normal again as another high pressure ridge began to build over the west coast. Highs were in the 80s from the 10th through the 19th, with the exception of the 91 on the 12th. That was the first 90 degree reading since September 7.

Following the 80 degrees on the 19th, much cooler air spread south following a cold front as an upper level trough deepened over southwest Canada and the northwest U.S. Highs were only in the mid 60s on the 20th and 21st.

After a chilly 42 degree low on the 22nd, temperatures began a slow recovery as the trough exited to the east. By the 25th milder west-southwest flow aloft had returned temperatures to near normal.

On the 26th an upper level trough from the Gulf of Alaska had arrived off the Washington-Oregon coast. Coming from the cold waters of the northeast Pacific, it contained little moisture.

Ahead of the trough, southwest flow aloft helped the temperature warm to 89 on the 26th.

As the trough edged further east that day, thunderstorms developed in northeast Oregon along a weak cold front. But they never got as far as Boise, and that night the front produced no precipitation as it crossed southwest Idaho.

The trough gradually weakened and moved offshore. It continued to have little effect on our area, although on the 28th a stationary band of dense high clouds prevented temperatures from rising above the lower 70s.

May 2019 Climate Statistics

June 2, 2019October 7, 2019 by NWS Boise

It was the wettest May since 2005, and the 6th wettest on record, with a total of 3.98 inches. In addition, the period from January 1 through May 31 is the wettest on record at the Boise Airport, at 12.07 inches. It ranks in second place for the entire period of record in the Boise area (behind 1896 at 14.27 inches), and even exceeds Boise’s average annual precipitation of 11.73 inches.

No rain fell during the first two weeks of the month. During the rest of the month most days had measurable rain.

Northwest flow aloft kept temperatures below normal for the first two days of the month. A warming trend began on the 3rd as a warm upper level high pressure ridge centered offshore began to extend its influence inland. The ridge kept our area warm and dry as it moved inland over the next several days. By the 15th it was east of the Rockies, and a Pacific cold front was approaching.

Unseasonably cool and wet weather followed the front on the 16th as an upper level trough settled over the inter mountain region, where it remained for the rest of the month. The moist and unstable air provided favorable conditions for daily rounds of showers and thunderstorms, some of which produced locally heavy showers.

Record February Precipitation in 2019

March 2, 2019March 10, 2019 by NWS Boise

February 2019 precipitation ranged 200-300% of normal across southern and central Idaho, southeast Washington, and across eastern Oregon. Several places recorded their February precipitation record.

McCall set the all-time precipitation record for February at 7.57″ (normal 2.42″). McCall set the all-time snowfall record for Feb at 74.0″ (normal 22.5″), the most in Feb since 1975 when 70.0″ fell. Records began in 1906. Courtesy of COOP observer.

Boise set the all-time precipitation record for February at 3.71″ (Normal is 0.99″). Boise received 11.6″ of snow in February, the most in February since 1949 when 25.2″ fell. Records began in 1877.

Baker City, OR and Ontario, OR both broke their all-time February records for precipitation. Burns, OR came in at 4th most all-time for February.

Mores Creek Summit and Bogus Basin snow surveys reported the biggest change in snow depth and snow water equivalent in February since records began in 1942. 360-370% of normal.

September 2018 Climate Statistics

October 1, 2018October 26, 2018 by NWS Boise

With only a trace of rain, September 2018 was one of the driest Septembers on record. There were only 9 others with no measurable precipitation in 152 years of Boise area records, and only 5 others in 78 years of airport records.

Total precipitation for the four months from June through September was only 0.36 inch. June-September periods with so little precipitation are unusual, but not unique. For example:

1994 .44
1901 .39
1898 .37
2018 .36
1957 .31
2012 .31
1933 .27
1966 .27
1935 .19

Temperatures averaged only slightly above normal. September began with near normal temperatures under dry westerly flow aloft. The flow backed into the southwest on the 5th as a high-pressure ridge built over the northern Rockies. This resulted in the last 90-degree highs of the summer on the 5th, 6th, and 7th.

A cooling trend commenced on the 8th as an upper-level low-pressure trough from the Gulf of Alaska approached the British Columbia Coast. As the trough edged closer, a dry cold front passed Boise on the 10th, followed by cooler north Pacific air. Highs were only in the low 70s from the 11th through the 13th.

The trough remained parked over the west coast through the 18th, keeping temperatures a few degrees below normal. It finally moved inland on the 19th, then continued east out of our area on the 20th.

Brief warming followed on the 21st and 22nd, with highs in the low 80s, but another north Pacific trough was on the way.

Rather than stalling over the coast, this trough continued on an eastward track, pushed along by a building upper-level high-pressure ridge offshore. The trough was east of our area on the 24th. Like its predecessors, it produced no rain in the Boise area.

East of the ridge, northwest flow aloft over the Pacific Northwest states kept temperatures 5 to 10 degrees below normal, with highs of only 69 on the 24th and 25th, the first highs in the 60s since June 17. The low of 38 on the 24th was the first low in the 30s since May 2.

As the eastern portion of the ridge edged inland, warmer air raised temperatures above normal from the 27th through the 29th, with highs in the low 80s.

During that time, an upper-level low was drifting slowly eastward toward California. By the morning of the 30th, it was centered over the Oregon-northern California coast. Its only effect on Boise was a few clouds and slightly cooler temperatures.

June 2018 Climate Statistics

July 9, 2018July 22, 2018 by NWS Boise

June`s average temperature was one degree above normal, giving a ranking of 23rd warmest in the 79 years of Boise Airport records. Highs reached or exceeded 90 degrees on 6 days, which is exactly normal for June. There were no record highs or lows.

It was a month of changeable temperatures, owing to a procession of upper level troughs and ridges from the north Pacific. Highs ranged from 63°F on the 17th to 94°F on the 25th.

It was one of the drier Junes, with only a quarter inch of precipitation. Along with June 1957, it ranks 20th driest at the Boise Airport.

Measurable rain fell on only four days: the 7th, 9th, 17th, and 18th. With the exception of the 17th, rainfall totals were under a tenth of an inch.

On the 14th a trough from the Gulf of Alaska stalled over the northwest states. Cooler air aloft flowing into the trough from western Canada caused it to intensify, and it strengthened further as it entrained moisture from east of the Rockies. On the 17^th, 0.13 inch of rain fell at the airport, but much heavier amounts were reported in northern and eastern Idaho and portions of the central Idaho mountains between the 16th and 18th.

Strong winds were observed on three occasions early in the month. At 7:30 pm on the 3rd, outflow from a shower ahead of a strong cold front produced a gust of 53 mph from the south, resulting in blowing dust reducing visibilities. At 6:25 pm on the 7th, outflow from a thunderstorm gusted to 41 mph from the southeast, accompanied by brief moderate rain. And at 12:50 pm on the 9th, a gust of 43 mph from the northwest followed another strong cold front.

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.