Spring has come to much of the United States, but you won’t know it by this week’s weather in the northern Rockies and northern plains.
Stormy April weather is not uncommon there. Spring snowstorms are a normal occurrence from Colorado and Wyoming eastward through the Dakotas to Minnesota.
So Zephyr is not a complete surprise as the polar vortex takes one last plunge southward before retreating for the summer.
Spring Snow In The Upper Midwest And Rocky Mountains
Arctic air in Canada pays no heed to the calendar. April forays into the northern Rockies and upper midwest are a common event, and temperatures in Denver can go from the 70s to the 30s overnight. April is the snowiest month on average in Casper, Wyoming, and April of 2013 was the snowiest single month EVER in Duluth, Minnesota.
When a season-finale cold blast arrives from Canada, warm and humid air from the Gulf of Mexico has pushed far enough north to give spring storms more moisture to work with — and thus the potential for greater snow accumulation.
Zephyr’s Snowy Weather Side
Zephyr will not be so much a low pressure snow-producer as an upslope event. The heavy snow will fall on the east-facing slopes of the Rocky Mountains in such places as Denver and Casper.
Cold air spills southward on the east side of the mountains, then is overrun by the warm, moist flow from the southeast. The lifting caused by the warm air riding up the mountain slope wrings out the moisture.
The precipitation will begin as rain at moderate elevations. Denver and Casper at about one mile elevation will receive about 6 inches of snow, with amounts held down by the initial period of rain. Cheyenne at 5700 feet will get up to 8 inches. The real snow bonanza will fall in the mountains west of Denver, where the precipitation will fall entirely as snow, and up to 18 inches is expected.
As Zephyr moves east, a narrow band of snow will stretch through Nebraska, Iowa, Wisconsin, and U.P. Michigan. After that, Zephyr will produce very heavy snow in eastern Canada, where the winter storm season extends into May.
Zephyr’s Severe Weather Side
Snowstorms at this time of year generally have a severe-weather sector. With warm and humid air from the Gulf of Mexico covering most of the eastern United Sates, the arrival of Zephyr’s cold front will produce the lifting required for the atmosphere to become unstable. Thunder and hail will occur along and ahead of the front, and there is a significant chance of tornadoes.
Affected areas include tornado alley (Texas, Oklahoma, Kansas, Arkansas, Missouri) today; the northern Gulf states northward through Tennessee and Kentucky on Monday; and coastal Carolina and Georgia, along with northern Florida on Tuesday.
What’s After Zephyr?
With the alphabet exhausted, the winter storm season will come to an end, not so much grammatically as meteorologically. There are unmistakable signs that the polar vortex is headed back where it belongs, and spring will finally cover the lower 48 in the wake of Zephyr. It is true that these signs have appeared periodically this winter but the calmer weather never materialized. This time all forecast models say the same thing: the strong jet stream will relinquish its grip on the plains and midwest. Seasonable to somewhat above average temperatures will prevail for at least a few weeks. But there will still be plenty of weather to talk about.
Attention turns now to the increasing threat of severe weather, including tornadoes, in the south, plains and midwest; in California and the southwest, the drought goes on and the dry season approaches; fire danger increases nationwide with the rising angle of the sun; and hurricane season begins June 1.