Wednesday was the wettest day at the Baker City Airport in almost 5 years.
But that’s not the only way to gauge the significance of the day’s sogginess.
Wednesday also crammed more rainfall into its 24 hours than any of the previous 7 months managed.
Actually Wednesday didn’t need even half those hours to pull off the feat.
By 8 a.m. the day’s total of 0.41 of an inch had already surpassed the monthly total for every month since September 2019.
Wednesday’s rain total was 0.54 of an inch, a record for May 20.
It was also the dampest day at the airport since July 10, 2015. With a total of 2.03 inches of rain, that July day was the second-wettest day on record at the airport, where records date to 1943.
(The champion in that regard is Aug. 31, 1984, when a cloudburst dumped 2.29 inches of rain on the airport.)
Wednesday’s storm should help ease the local drought. The U.S. Drought Monitor Index puts most of Baker County in the “moderate” drought category.
The responsible party in this case is a slow-moving low-pressure center that tracked north across western Idaho Tuesday night and into Wednesday, said Leslie Colin, a lead forecaster at the National Weather Service’s office in Baker City.
That’s an unusual course for such a storm, said Colin, who recently celebrated 50 years working for the National Weather Service.
Spring, however, is the period when such events are most likely, Colin said.
Indeed, May is statistically the wettest month in Baker City, with an average of 1.43 inches of rain.
(June ranks second, at 1.29 inches.)
“If it’s going to happen at all, this is the season it would,” Colin said.
Wednesday’s storm was atypical in more than one way.
The weather feature that often brings rain (or snow) to Baker County is a cold front — basically, the boundary between air masses of significantly different temperatures.
When a cold front sweeps through, if it drops precipitation (there are completely dry fronts, as well), the rain or snow tends to fall heavily, but only briefly, lasting in some cases for less than an hour, Colin said.
“Once the front goes through the precipitation really drops off,” he said.
The more noticeable effects of many cold fronts are a rapid drop in temperature and a shift in wind direction from southeast to northwest.
Wednesday’s storm, by contrast, was not a cold front but a “closed low,” and compared with a cold front it dawdled along.
This meant its effects — its rain, in particular — lingered for more than 10 hours.
Rain was reported at the airport at every one-hour interval, except one, between 7 p.m. on Tuesday and 3 p.m. on Wednesday.
The storm’s track also contributed to its prolific precipitation.
Because the low’s center — the area of lowest atmospheric pressure — was east of Baker County, passing almost directly over Boise on its way north, the counterclockwise circulation around the center propelled rain bands from roughly north to south.
That pattern neutralizes the rain shadow that the Elkhorn Mountains cast over Baker Valley and much of the rest of the county, Colin said.
When storms roll in from the west, which is the more typical pattern, the Elkhorns force the moist air to rise and cool, and because the colder air is the less moisture it can hold in clouds, much of the rain or snow falls in the mountains.
(Low-pressure systems such as Wednesday’s have much the same effect, except they don’t need mountains to lift and cool the air and siphon its moisture. Air rushes toward the center of the low (air moves from areas of high pressure to low, acting like a liquid flowing downhill) and, because the ground stops its downward movement, the air is forced to rise.)
In the normal eastward tracking storm, as the air descends the east slopes of the Elkhorns it warms and can hold much of whatever moisture didn’t splash down or pile up on the peaks. This phenomenon explains why so often the Elkhorns are swathed in clouds while the Baker Valley below is dry.
But when the air flow is from north to south as it was Wednesday, Colin said, the Elkhorns can have the opposite effect, at least in a limited way, enhancing rather than retarding rainfall in Baker Valley as the air hits the wall of the Elkhorns, rises and cools.
Some of the resulting rainfall tends to spill over into the valley, Colin said.