September 2016 Climate Stats at Boise

Sept2016
September’s average temperature was slightly below normal due to cool upper-level troughs which dominated the inter-mountain region for most of the month.

Highs were only the 60s on six days.  The high of 63°F on the 14th was the coolest day since June 15.

A very warm upper-level ridge built over the region on the 25th and 26th, bringing Boise the highest temperatures of the month.  The 93°F on the 28th was only two degrees shy of the record for that date.

As the ridge moved east on the 29th and 30th, southwest flow aloft on its western flank, combined with southeast surface winds, continued the warm weather.

Precipitation was one third of normal. For most of the month, Boise was on the western margin of moist air moving up from the south, so upper-level troughs interacting with this moisture dumped most of their rain on south-central and southeast Idaho.

Locations east of Boise received substantial monthly precipitation totals, ranging from just under an inch at Mountain Home to over two inches at Twin Falls and Pocatello, much of that from the 21st through the 23rd.

August 2016 Climate Stats at Boise

AUG2016

August 2016 was dry and seasonably warm, with the average temperature within one degree of normal. There were no record high or lows set.  Only traces of rain fell at the airport. That’s not unusual, because August is generally the driest month at Boise.  Historically at Boise, 25% of Augusts had no measurable rainfall and 50% had no more than a 0.10″.

Weather patterns were typical of this time of year. Pacific systems were relatively weak, mainly moving inland north of our area with precipitation confined to northern and central Idaho and adjacent sections of eastern Washington, northern Oregon, and western Montana.  But even those areas were drier than normal, and little if any rain fell on southwest Idaho.

Incursions of monsoon moisture were usually deflected to the east ahead of trailing pacific cold fronts, so measurable rain from that source came no closer than the Nevada border and eastern Idaho.

At the Boise Airport, where records go back to 1940, June-August 2016 ranked among the driest 15% of summers.

July 2016 Weather Stats at Boise

JUL2016

July 2016 was the first month with below normal temperatures since November 2015. Although July’s average temperature and total precipitation were close to normal in Boise, it was an unusual month.  The average temperature of 75.4°F was the lowest July average since 2001.  The low temperature of 47°F on July 11th was the coolest July temperature in Boise since 2000.  The high temperature of 69°F on July 10th was the coolest July daily high temperature since 2001.  The hottest day of July 2016, 101°F on the 27th, was the lowest monthly July maximum since 1997. There were no record daily high or low temperatures set in July 2016.  A daily precipitation record was set on July 10th, when the total rainfall of the month fell (0.27in) in one day. Even heavier amounts were measured elsewhere in the Boise area, including 0.76in at Eagle and 0.61in at Meridian and over an inch was measured in surrounding areas.

June 2016 Weather Stats at Boise

June2016

June 2016 was the fourth warmest at the Boise Airport, and seventh warmest all-time, but still around 4 degrees cooler than June 2015, the warmest on record.  Temperatures reached 90°F on 13 days in June 2016 (June average is 5 days).  It was a dry month, ranking among the driest 20 percent of Junes on record.  New record daily highs were set on the 5th (97°F) and the 8th (101°F). Record high daily minimum temperatures were set on the 6th (68°F) and the 8th (69°F).

May 2016 Climate Stats at Boise

May2016May was unsettled, with showery periods interspersed with warm dry weather.  June-like temperatures alternated with cool blustery days more typical of March. There were no record highs or lows.

During the first week, temperatures averaged as much as 15 degrees above normal, thanks to an upper level ridge and southwest flow aloft.  Highs reached 81°F on the 3rd and 85°F on the 4th.  On the 5th and 6th, a low pressure system which soaked up plenty of moisture off the southern California coast, moved inland over the Desert Southwest. Some of this moisture reached southern Idaho, which was under an unusual easterly flow on the northern periphery of the low. Showers and thunderstorms crossed the Boise area on the 6th, dumping heavy rain on a few spots. One observer in southwest Boise measured 1.36 inches of rain in 20 minutes, resulting in local flooding across the Boise Metro area. Another observer in the foothills north of Boise reported 0.64 inches in 30 minutes!

The low was kicked east out of the intermountain region on the 7th and 8th by a cold upper level trough which moved over western Canada from the Gulf of Alaska. Cooler drier air associated with the trough drifted into the Treasure Valley and kept temperatures a few degrees below normal from the 9th through the 11th.

Temperatures soon rebounded, with highs from 80°F to 85°F from the 12th through the 14th under a temporary upper level ridge.  A weak trough followed the ridge, with cooler air and showers on the 15th.

A ridge building inland from the northwest coast brought a warming trend from the 16th through the 18th. It was abruptly ended by another Gulf of Alaska trough, which arrived over the Pacific Northwest on the 19th and covered most of the Intermountain West by the 21st.

Unseasonably cool, moist and unstable air associated with the trough generated scattered showers and isolated thunderstorms each day from the 19th through the 22nd.  The high of 55°F at Boise on the 20th was the coolest day since April 14.

Although the trough remained over the western U.S., temperatures moderated to near normal by the 23rd due mainly to long days and widespread sunshine.

The next trough in the series had less effect on our area. Initially centered over southeast Alaska and the British Columbia coast, it sent a dry cold front across the Boise area on the 27th, lowering high temperatures by around 5 degrees.

The trough moved east along the Canadian border over the Memorial Day Weekend, with little effect on Boise other than breezy northwest winds

A warming trend began on the 31st as an upper level ridge built offshore.

April 2016 Climate Stats at Boise

Apr2016

April 2016 was the warmest April ever recorded at the airport and it tied for 3rd warmest since temperature recording began in the Boise area in 1864.  It was the 5th consecutive warmer than normal month.

A warm upper level high pressure ridge, which dominated the Inter-mountain West during the first three weeks of the month, was responsible for the record warmth.  Relatively weak Pacific weather systems interrupted the ridge a couple of times, on the 4th and 5th and the 14th and 15th, briefly lowering temperatures to a few degrees below normal.  Northwest winds accompanied the cooler air, gusting to 36 mph on the 4th and 37 mph on the 15th.  Only light amounts of rain fell.

A strong and wet upper level trough plowed into the ridge on the 22nd and passed directly over Boise on the 23rd, dumping nearly half an inch of rain at the airport, and up to twice that amount at a few Treasure Valley locations west of Boise.

This system ushered in a pattern change, with westerly flow aloft splitting over the Northwest coast, and a cool upper level trough expanding over the Western United States, lowering temperatures in our area to near normal.  There was no measurable rain at Boise after the 23rd, as most precipitation was either deflected south to California and the Desert Southwest, or confined north and east of Boise.

Following a cold front on the 25th, Northwest winds gusted to 45 mph at the airport.  The 28th through  the 30th were not quite that windy, but gusts exceeded 30 mph each day.

March 2016 Climate Stats at Boise

Mar2016March 2016 was the 7th warmest at the airport, where records go back to 1940.  It was a month of changeable and sometimes extreme weather typical of early spring.

From the 1st through the 13th the temperature averaged 8 degrees above normal. This was primarily due to persistent relatively warm southwest flow aloft.  Disturbances embedded in the flow weakened as they moved inland, but they carried enough moisture to bring showers to Boise nearly every day.

The pattern changed at mid-month. Northwest flow aloft followed the passage of an upper level trough on the 14th, keeping temperatures below normal through the 18th.  The low of 28°F on the 18th was the first freezing temperature since February 25th.

On the 19th an upper level ridge made a brief sojourn over the Intermountain region and temperatures were again above normal.

After the ridge departed to the east, southwest flow aloft ahead of a Gulf of Alaska trough kept temperatures above normal through the 21st. The 70°F on the 20th, The First Day of Spring, was the high for the month and the first 70°F since October 25th.

As the trough drew closer on the evening of the 21st, strong thunderstorms developed along the cold front, dumping over a third of an inch of rain at the Boise Airport.  Two to three times that fell at other locations around town, along with hail up to grape size which covered the ground in some places.

That trough paved the way for two more Gulf of Alaska troughs.

A second trough arrived on the 25th, ushered in by chilly northwest winds gusting to 43 mph at the airport.  In contrast to the trough, four days earlier, there was only a trace of rain.  The winds subsided that evening and skies cleared, allowing the temperature to fall to 27°F after a week of above-freezing lows.

The third and deepest trough arrived on the 28th. The coldest air was already over Boise early that morning, reflected by a low of 28°F.  By evening the upper level low center and coldest air had settled over the Great Basin and California.

By the morning of the 29th a deep surface low had formed, centered near the Utah-Colorado border.  By afternoon it had developed into a major spring snowstorm for the Magic Valley and southeast Idaho.

Boise, being too far north and west to see any precipitation, experienced only dry northwest winds, with gusts in the 20 to 30 mph range at the airport.  In contrast to the cold wet weather farther east, the temperature at the airport reached 60°F that afternoon.

Under mostly clear skies temperatures continued to warm on the 30th and 31st in response to a high pressure ridge which extended from the California coast all the way to Alaska.

February 2016 Climate Stats at Boise

Feb2016

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.