An extensive upper-level low pressure system over the northeast Pacific was the dominant feature through the 17th. It was responsible for steering weather systems into western North America. Between systems, southwest flow east of the low brought warmer air.
The first couple of weather systems, which came through on the 3rd and the 6th, brought only light precipitation. Strong west winds followed a cold front on the morning of the 6th. A gust of 48 mph was measured at the Boise Airport.
The first significant precipitation of the month came on the 14th when 0.36 inch of rain fell at the airport as a low pressure trough crossed the area.
On the 16th a much larger and colder trough from Siberia was nearing the west coast. Before moving inland on the 19th, it had deepened far enough down the coast to tap subtropical moisture southwest of California. On the 20th it was centered directly over Idaho. That day 0.80 inch of rain fell at the Boise Airport, breaking the previous record for the date of 0.56 inch in 1985. Other parts of town were drenched by even heavier rain, including 1.31 inches at Boise State University.
Here are the rainfall totals over the past 72 hours. New daily rainfall records were set yesterday, including 0.80" at Boise (beat 0.56" set in 1985) and 0.54" at Baker City (beat 0.49" set in 1960). More showers are expected later today. #IDwx #ORwx pic.twitter.com/nzH335KJzj
— NWS Boise (@NWSBoise) May 21, 2020
With a high of only 51 degrees, the 20th was the coolest day of the month. The record coolest high temperature for the date is 48 in 1902. Below-normal temperatures would continue through the 24th.
— NWS Boise (@NWSBoise) May 20, 2020
An upper-level high pressure ridge building over the coast on the 24th started a warming trend which was briefly interrupted by a weak Pacific cold front on the 26th.
Starting on the 27th, temperatures rose each day as the ridge strengthened. From the 28th through the 30th, highs were in the 90s for the first time since September. On the 30th the high of 98 tied the record for the date set in 1986. And the low of 69 broke the previous record of 65 set in 2015.
A low pressure system that had been spinning over the subtropical Pacific west of Baja finally moved inland over California on the 30th. As it progressed northward toward eastern Oregon and southwest Idaho, it was preceded by hot dry windy weather and scattered thunderstorms. Precipitation was mostly light. Only a trace of rain dampened the airport, but a few spots closer to the foothills got heavy downpours.
Near record high temperatures expected on Friday and Saturday. A strong cold front expected on Saturday with very windy conditions expected. Here is the weather pattern. Rainbow colors represent warm and cool while gray/blue/green areas represent precipitation. #idwx #orwx pic.twitter.com/NGw3msxjCX
— NWS Boise (@NWSBoise) May 28, 2020
The peak gust at the airport was 48 mph from the east-southeast at 5:43 pm MDT. Winds were likely stronger elsewhere around the valley, as there were reports of trees damaged or blown over and power outages. There was a lot of tree damage across the region, especially in the mountains with this system where wind gusts up to 74 mph were reported. There were also some great photos of the damaged trees:
Keith Balmer sent this picture in on the east side of Gooding, ID. This tree came down when strong winds blew through Southern Idaho today due to a cold front. Looks like this tree has to have at least 12 feet in diameter! #IDwx @RyanDennisKMVT @GKHottle @NWSBoise @spann pic.twitter.com/MAfQYylPbg
— Eric Brill (@EBrillWx) May 31, 2020
By that evening the first round of showers had moved north and east of Boise, and at 9:15 pm MDT a cold front passed the Boise Airport from the west.
There was more weather to come later that night, as the upper-level trough behind the cold front was still on its way. The arrival of the trough was announced by a thunderstorm just before 3 am MDT on the 31st. Hail one-half inch in diameter fell at the airport, but there was only 0.09 inch of total precipitation.
Thunderstorms between midnight and 6:00 am are very rare in our part of the country. At Boise the chance is only 2 percent in May and 4 percent in June.
Here are the rest of the climate graphics for southwest Idaho and southeast Oregon.