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April 2016 Climate Stats at Boise

May 3, 2016 by NWS Boise

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 4^th and 5^th and the 14^th and 15^th, briefly lowering temperatures to a few degrees below normal. Northwest winds accompanied the cooler air, gusting to 36 mph on the 4^th and 37 mph on the 15^th. Only light amounts of rain fell.

A strong and wet upper level trough plowed into the ridge on the 22^nd and passed directly over Boise on the 23^rd, 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 23^rd, 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 25^th, Northwest winds gusted to 45 mph at the airport. The 28^th through the 30^th were not quite that windy, but gusts exceeded 30 mph each day.

March 2016 Climate Stats at Boise

April 3, 2016August 29, 2016 by NWS Boise

Mar2016 March 2016 was the 7^th 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 1^st through the 13^th 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 14^th, keeping temperatures below normal through the 18^th. The low of 28°F on the 18^th was the first freezing temperature since February 25^th.

On the 19^th 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 21^st. The 70°F on the 20^th, The First Day of Spring, was the high for the month and the first 70°F since October 25^th.

As the trough drew closer on the evening of the 21^st, 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 25^th, 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 28^th. 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 29^th 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 30^th and 31^st in response to a high pressure ridge which extended from the California coast all the way to Alaska.

February 2016 Climate Stats at Boise

March 3, 2016August 29, 2016 by NWS Boise

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

February 2, 2016August 29, 2016 by NWS Boise

The 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 3^rd 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 7^th through the 12^th, 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 14^th and 15^th.

From the 19^th through the 27^th, 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 23^rd from a weather system weakened by the ridge.

Starting on the 28^th, very moist westerly flow replaced the ridge, bringing widespread precipitation to southwest Idaho. Rain on the 28^th and 29^th totaled a third of an inch at the airport.

The rain ended late on the 29^th after a cold front crossed the area. Cold moist and unstable air followed the front and on the 30^th, a combination of afternoon heating at the surface and cold air aloft triggered convective snow showers.

By early morning on the 31^st, 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 11^th.

October 2015 Weather Stats at Boise

November 2, 2015August 29, 2016 by NWS Boise

October 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.