JotBlog

It wasn’t an April Fools’ Day joke. Roughly a half-foot of snow fell across a wide swath of the Northern Plains and Intermountain West on Thursday as temperatures plummeted into the teens. On Friday, the snow was still chugging east, leaving behind a thick blanket of white to kick off the first full month of spring.

It was part of the same storm system that swung a vigorous cold front through much of the central United States, dropping temperatures as much as 40 degrees in only a few hours’ time. For some, it was as if the seasons switched from summer to winter in the blink of an eye.

“It was really four systems starting Tuesday night,” explained Zachary Hargrove, a meteorologist with the National Weather Service in Bismarck, N.D. “It was all from the same upper-level low churning over central Saskatchewan and central Alberta in Canada.”

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That same storm is trucking east — albeit in a tempered state. Minneapolis was under a winter weather advisory Friday, likely to see a dusting of snow or a glaze of ice accretion.

A four-in-one storm

The combination of four systems piled up anywhere from two inches to a foot of snow across the Dakotas and into the Upper Midwest. Winds gusting upward of 40 mph reduced visibility and made for treacherous travel. Recent warm temperatures resulted in snowmelt runoff into roadside cisterns and ditches. The National Weather Service warned that sliding off a roadway into one could be “disastrous.”

The barrage of weather systems stemmed from “shortwaves” rotating around a larger low pressure system banked up to the north. Much like cars driving around a rotary, the shortwaves orbit the larger low pressure system’s center, each bringing a healthy dose of snow, wind and inclement weather.

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“It was just one trough that was flowing into western North Dakota, and you had these little shortwaves rotating around the base of that trough,” Hargrove said.

The snowfall

Totals observed with that first burst were as high as eight to 10 inches in Burke County, N.D., including in the vicinity of Bowbells and Portal — just a stone’s throw from the Canadian border. Elsewhere in northwestern North Dakota, a general one to five inches fell.

“Then we had kind of a lull late Wednesday into Thursday morning, and then we had this next little wave come in,” said Hargrove. “We had some pretty good snowfall rates.”

All told, Bismarck, N.D., wound up with about a half-foot for the event. That was the theme across most of South Dakota as well, with the maximum report collected by the National Weather Service in Aberdeen around 6.5 inches.

Prolific thunder

“One part [of the storm] was more convective in nature,” said Renee Wise, a meteorologist at the National Weather Service Aberdeen office. Convection describes heat transfer through a fluid, and in atmospheric sciences is typically used in reference to towering clouds or thunderstorms. Indeed, a strip of South Dakota did experience several hundred cloud-to-ground lightning strikes, all with temperatures in the teens and lower 20s.

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“That really affected more the northeastern part of the state, especially James Valley,” said Wise, who reported the band dropped not only snow, but sleet, ice pellets known as “graupel,” freezing rain and just about everything in between.

The reason? Elevated instability. That’s what happens when the warm air needed to fuel a thunderstorm sits on top of a shallow layer of cold air. That allowed temperatures to dip well below freezing at the surface while aloft, it was warm enough to support thunder and lightning.

The staggering amount of lightning was likely due to the presence of ice pellets in the storm clouds. That can bolster the electric field enough to produce far more lightning than would otherwise occur during a simple snowstorm. In fact, the majority of cloud-to-ground lightning strikes during pure snow events are likely to be to man-made tall objects.

A staggering drop in temperatures

The bizarre band of storms was aided by a strong cold front, that stretched all the way down the Interstate 35 corridor into the Southern Plains on Friday. Temperatures dropped from 70 degrees in Oklahoma City at 6 p.m. Thursday, down into the mid-40s Friday morning. By later Friday morning, the city was hovering near 30 degrees.

We were 81 degrees in Amarillo just 9 short hours ago at 4 pm. Now at 1 am we are at 30 degrees. That is a 51 degree drop in 9 hours. #phwx #txwx #okwx

In Amarillo, Tex., the drop-off was even more dramatic — temperatures fell from 80 degrees at 6 p.m. to the 40s by 9 p.m. Thursday and even further, down to below freezing before midnight.

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On the plus side, both cities could be back near 70 degrees on Sunday as warmth again builds in from the south.

Denver hit 83 degrees on an October afternoon. Eight hours later, it was snowing.

But farther north in the Dakotas, that celebration isn’t coming quite yet. Temperatures Friday night could dip into the single digits for some, with gradual recovery before an additional chance for snow next week.

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