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.