Winter 2019-2020 in Review

Winter 2019-2020 was much warmer than normal, with the greatest departures in the western portion of our County Warning Area (CWA). The average temperature for December through February at Ontario, Oregon was 7.4 degrees above normal, making it their third warmest winter on record. 

Snowfall was generally below normal, mainly because of the unusually warm temperatures, but below-normal precipitation in some areas also played a part.

Most of our forecast area had near normal precipitation for the season. Camas, Twin Falls, Gooding, and Jerome Counties, and the Owyhee Mountains, were drier than normal. Southern portions of Harney, Malheur, Owyhee, and Ada Counties were wetter than normal, as was Baker County, with the notable exception of the Baker Valley, which was significantly drier than normal.

One of the large-scale controlling mechanisms for winter weather in the Northern Hemisphere is the Arctic Oscillation (AO). It is characterized by a counterclockwise circulation of winds around the Arctic, centered at about 55 degrees north latitude. This winter it was in its positive phase, meaning that the winds were strong and consolidated.  This tends to hinder arctic air from moving south into the lower 48 states.  Most of this winter the AO was strongly positive, especially in January and February.  In fact it set all-time records twice in February.  

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New Daily Records

Dec 21 – McCall – record high temperature of 43 (old record 42 in 1906)
Dec 22 – McCall – record high temperature of 48 (old record 46 in 1940)
Jan 26 – Boise – record rainfall of .74 inch (old record .40 inch in 1970)
Feb 5 – Boise – record snowfall 2.6 inches (old record 2.1 inches in 1948)
Feb 5 – Baker City – record rainfall of .25 inch (old record .16 inch in 2010)

December

Temperatures were relatively mild, especially in the Treasure Valley. Ontario had its sixth warmest December on record. Baker City had its eleventh warmest. 

Aside from portions of Harney County, most of our CWA was drier than normal. Factors contributing to the precipitation deficit included a split in the jet stream, leaving our area between northern and southern branches. Most of the Pacific moisture moved south over the southwest states instead of east over our area. During periods of west or southwest flow over our CWA, some valleys were in the precipitation shadows of mountain ranges. This effect was especially pronounced at Baker City and Mountain Home, downwind from the Elkhorn and Owyhee Mountains. However,  precipitation at McCall was enhanced by lifting over the higher terrain to their east. 

Despite widespread below-normal precipitation, there were several storms which produced significant snowfalls. 

On the 1st and 2nd, 5 to 10 inches of snow fell in southeast Oregon and the mountains of southwest Idaho. Amounts were lighter in the Treasure and Magic Valleys, where only 1 to 3 inch amounts were reported.  

On the 8th, heavy snow fell in the mountains of Baker County Oregon and the west central Idaho and Boise Mountains. The greatest reported amount was 13 inches at Atlanta Summit and Deadwood. 

On the 12th, Bogus Basin received a 9-inch snowfall. Tamarack ski area got 14 inches, and Brundage accumulated 12 inches. The heaviest amount was 15 inches at Mores Creek Summit. 

From the 13th through the 16th there were relatively light amounts of snow across most of the area. Exceptions included 7 inches near Frenchglen in Harney County, 12 inches east of Three Creek in southern Twin Falls County, and 8 inches near Murphy Hot Springs in Owyhee County, all on the 14th. 

On the 19th, 6 inches was measured at Council in Adams County and at Placerville in Boise County, while 9 inches fell at Bear Saddle Snotel in Washington County.

On the 20th, 8 inches fell east of Meadows in Adams County and at Halfway in Baker County. Tamarack in Valley County picked up 11 inches. 

December 2019 precipitation

December 2019 temperature

January

Temperatures were above normal across nearly all of our CWA, and much above normal in the Treasure and Magic Valleys and southern Malheur County. Boise was +6.2° F above normal for the month – making it the fifth warmest January on record.

Strong westerly flow aloft maintained a progressive pattern which brought Pacific weather systems across our area at frequent intervals. This hindered the formation of long-lasting temperature inversions and discouraged arctic air from pushing south.

A persistent ridge of high pressure off the California coast steered storm systems, including a series of atmospheric rivers, into the Pacific Northwest. Atmospheric rivers are long narrow streams of air carrying large amounts of moisture. They are responsible for most of the water vapor transport outside the tropics. 

Most of our region was wetter or much wetter than normal. The mountains of central and northern Idaho and northeast Oregon saw a significant improvement in snowpack. 

Snow fell nearly every day in the mountains.

A storm on New Year’s Day dropped heavy snow on the mountains of Baker County and central Idaho. Storm totals included 14 inches at Tamarack and McCall, 15 inches at Brundage, and 5 inches at Bogus Basin. 

Strong winds were reported at many locations that day, including gusts of 67 mph  east of Van in northern Harney County, and 65 mph south of Rogerson in Twin Falls County. Elsewhere, gusts of 45 to 55 mph were common.

From the 9th and 14th, snowfall was heavy in the mountains. Even in the lower valleys moderate amounts fell. 

A storm total of 18 inches was reported at Centerville in Boise County.  In Valley County storm totals included 23 inches near Donnelly, 17 inches at Yellow Pine, and 9 inches at Cascade. In Baker County Halfway’s total was 17 inches. Featherville in Elmore County accumulated a total of 36 inches. 

Total snowfall at Boise for the six days was 8 inches, more than half the total this winter. The most on the ground was 4 inches on the morning of the 15th. 

A couple of places reported strong winds on the 14th, including a gust of 63 mph at Rome in Malheur County and 58 mph at Twin Falls.

January 2020 precipitation

January 2020 temperature

February

February was colder than January at several locations. Normally  February is around 5 degrees warmer than January. Still, Februrary was warmer than normal across most of our area. 

On the 5th and 6th a warm front, which had entrained tropical moisture from east of Hawaii, brought heavy snow to the mounains and moderate amounts of rain and snow to the valleys. 

In Baker County, 7 to 8 inches of snow fell near Baker City, and 12 inches was measured at Sumpter.  In Valley County 17 inches fell near McCall and at Yellow Pine. Near Donelly 9 inches was reported. In Adams County New Meadows got over 25 inches. The storm total at Brundage was 40 inches. 

A storm on the 16th dropped 4 to 5 inches of snow on Boise County. McCall received only 3 inches. Rain fell in the lower valleys. By far the greatest reported amount was 0.53 inch at Boise, which set a new record for the date.

During the afternoon and evening of the 23rd, gusty northwest winds followed a strong cold front. Some the the stronger gusts reported included 67 mph at Baker City, 77 mph at Wagontire in Harney County, 67 mph at Midvale Hill in Washington County, 53 mph at Burns in Harney County, 56 mph at Black Canyon in Payette County, and 65 mph near Hammett in Elmore County.

February 2020 precipitation

February 2020 temperature

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
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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.