July 2019 Climate Statistics

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The average temperature for the month was less than one degree above
normal. No temperature records were broken, but the low of 76 on the
23rd tied the record high low for that date set in 1994.

At the airport, the only triple digit high was the 102 on the 22nd.
The normal for July is 4 days of 100 or higher. The normal for
summer (June-August) is 7. Last summer had 14, with 9 in July and
the rest in August, including the 110 on August 10.

Only 0.01 inch of rain fell at the National Weather Service office in Boise,
which ranks in the driest 15 percent of Julys.

July and August are normally the driest months, averaging 0.33 inch
and 0.24 inch respectively.

As is common at this time of year, upper level troughs from the
north Pacific weakened as they encountered the strong summer high
pressure ridge over the western U.S. The ridge diverted these
systems too far north to bring measurable rain to the Boise area.

Upper-level winds were usually from the southwest, with enough
westerly component to shunt monsoon moisture east before it reached
the Treasure Valley. When it did manage to drift this far north and
west, it usually brought only cloud cover, which hindered the
heating that drives convection, so only a few sprinkles resulted.

Weak cold fronts accompanying the Pacific systems were followed by
slightly cooler and much drier air. For example, the afternoon
humidity dropped to 7 percent on the 19th and 8 percent on the 24th
after frontal passages. On the other days minimum humidities were in
the teens and 20s.myl

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June 2019 Climate Statistics

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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.

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May 2019 Climate Statistics

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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.

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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.