Precipitation was near normal, and temperatures averaged slightly warmer than normal.
A cold upper level low pressure trough over western Canada and the Northwest states during the first three days of the month held temperatures below normal.
Southwest flow aloft ahead of a low pressure trough off the Pacific Northwest coast started a warming trend on the 4th. As the trough moved south along the California coast on the 5th and 6th, showers spread inland, but the Boise area received only light precipitation. The air was unstable enough for thunderstorms both days.
With our area between the low pressure system over the southwest states and the jet stream to our north and east, weak high pressure kept temperatures above normal from the 5th through the 11th.
A cool down was on the way as the flow over western Canada shifted into the north on the east flank of a high pressure ridge off the west coast of North America.
A cold front from British Columbia passed Boise during the evening of the 11th, accompanied by a wind gust of 45 mph from the north. Temperatures the following day were 10 to 15 degrees cooler, and temperatures stayed below normal through the 14th.
Boise fared better than eastern Idaho, and much better than states to the east, where a late season arctic outbreak brought snow and winter temperatures.
After a brief warmup on the 15th, another cold front from British Columbia crossed the Boise area that evening. Compared to the previous front, the cool down was minor. A convergence zone in the Snake River Basin near Mountain Home created show showers on the morning of the 16th.
A warming trend began on the 17th as the high pressure ridge began to move inland from the coast.
Increasing westerly flow aloft gradually weakened the ridge, and on the 22nd and 23rd a disturbance embedded in the flow brought the largest amount of precipitation for the month. Most of it fell on the 23rd with scattered convective showers, some of which produced brief heavy rain and small hail.
On the 25th and 27th a couple of weak disturbances dropped most of their precipitation over north Idaho, with only light showers at Boise.
On the 28th a strong high pressure ridge developed over the western U.S. as it amplified and drifted east, it provided the season’s warmest weather so far.
On the 29th strong southwest flow aloft on the west flank of the ridge, down valley flow at the surface, and mostly sunny skies helped boost the temperature at Boise to 88 degrees, one degree shy of the record set in 1992. Severe thunderstorms containing 1″ hail were reported in Baker City.
On the 30th the southwest flow carried an energetic weather disturbance across southeast Oregon and southwest Idaho. Late that morning a line of thunderstorms formed over Harney County. The storms intensified rapidly as they approached Idaho. They crossed the border at about 1:45 pm, and strong outflow ahead of the storms arrived at the Boise airport at 2:30 pm. Five minutes later a gust of 61 mph was measured just ahead of the rain, which was heavy at times. Golf-ball size hail (1.75″) was reported at Oreana, ID (40 miles southwest of Boise).
The winds were likely stronger in some parts of town, as there were reports of power outages, downed trees, and other damage. Large trees across the Treasure Valley toward the Boise Mountains and Magic Valley were knocked down as a result of this storm. Numerous large trees were knocked down from Prairie to Pine to Featherville with this storm.
Here are the rest of the climate graphics for southwest Idaho and southeast Oregon. McCall was cooler than average due to a deep snow pack on the ground within the valley, which kept their temperatures lower than other areas.
