May 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.