Severe storms, strong tornadoes to threaten central states again Tuesday

More than 100 tornadoes were reported between the Cornbelt and the Mid-Atlantic on Friday and Saturday, accompanying a violent and widespread thunderstorm outbreak that claimed at least 31 lives. Many of the same areas now must brace for another threat of dangerous storms and possible tornadoes on Tuesday.

The map showing the areas at risk of severe storms on Tuesday, drawn up by the National Weather Service Storm Prediction Center, is eerily similar to last Friday’s, although there’s some uncertainty as to how widespread the storms will become. The agency has drawn a Level 4 out of 5 risk zone over parts of eastern Iowa, northwestern Illinois and northeastern Missouri, where numerous tornadoes swarmed last week. The zone includes Cedar Rapids and Iowa City in Iowa.

A second zone with a Level 3 out of 5 risk of severe storms covers a portion of southeastern Oklahoma, northeastern Texas and southern Missouri, plus most of Arkansas - including Little Rock, which endured devastating damage from an EF3 tornado Friday. But the forecast is less certain here. While the ingredients are in place for rotating storms, inhibiting factors in the atmosphere may somewhat limit the number of storms.

Over the entire severe-weather risk area, which extends from northeast Texas to southwest Michigan and southern Wisconsin - affecting over 45 million people - damaging winds and large hail will be a threat, in addition to tornadoes. Cities in this area include Dallas, Kansas City, St. Louis, Des Moines, Chicago and Indianapolis.

On the storm’s cold side, meanwhile, a swath of heavy snow, possibly measured in feet, will combine with strong winds to deliver blizzard conditions over the Dakotas. Central and northern Minnesota could see plowable snowfall, too. The storm is already dumping very heavy snow in Utah, which has had a record season for snowfall in a number of areas.

A zone of low pressure over Utah, currently dumping snow in Salt Lake City, will gradually strengthen as it pushes east toward the Colorado Front Range on Monday evening. Then it will slip across Kansas, western Iowa and Minnesota on Tuesday while intensifying more quickly under the support of an upper-air disturbance, eventually pulling into Ontario and Quebec. Warm, moist air will stream north in advance of that counterclockwise-spinning low, with cool, dry air cascading south in its wake.

The clashing air masses will brew strong to severe thunderstorms, but how widespread remains a question. It’s possible that a “cap” of hot, dry air a mile or so above the ground inhibits surface air from rising, reducing thunderstorm chances in some areas.

For storms that can sprout through that ceiling of sorts and tower high into the atmosphere, tornado potential will climb markedly. That’s because of a pronounced dip in the jet stream, within which is nestled our potent upper-air disturbance. Winds at about 18,000 feet will be screaming at 110 mph or greater. That means any storms that poke into this layer will feel a change of wind speed and/or direction with height, known as wind shear, and develop rotation.

Meanwhile, moisture pinwheeling northwest around the low-pressure center will fall as heavy snow. Strong high pressure will build east in the low’s wake, making for a steep pressure gradient, or change of air pressure with distance, that will foster strong winds. The combination of strong winds and snow will yield blizzard conditions across the northern Plains.

Severe weather potential will be greatest in eastern Iowa, northwestern Illinois and northeastern Missouri where the cold front, warm front and dry air boundary come together at the core of the low-pressure zone. Rotating thunderstorms or supercells are expected to blossom during the mid- to late afternoon and some will be able to produce strong tornadoes in addition to damaging winds and large hail.

Farther south, in the other zone at enhanced risk - the one over Arkansas, southern Missouri and the corners of Texas and Oklahoma - dangerous storms are possible, but the previously noted cap of warm air may reduce the number of storms Tuesday afternoon. The Weather Service office in Little Rock mentioned “uncertainty” in the potential for severe weather in its Monday morning forecast discussion, noting that conditions “appear more favorable to our north.” The better chance of storminess will accompany the cold front as it blasts through during the first half of the overnight. It’s likely that a line of thunderstorms with damaging straight-line winds and a few embedded tornadic circulations will form.

More severe thunderstorms are possible Wednesday over the eastern Great Lakes and Ohio Valley but shouldn’t be quite as numerous or intense in most areas compared to Tuesday.

On Monday morning, the storm was pasting snow onto the higher elevations of Utah, Nevada and Wyoming, which are covered in winter storm warnings and winter weather advisories. Around Salt Lake City, the Weather Service is a warning of a long-duration winter storm, with totals of at least 10 to 20 inches forecast.

By Monday night into Tuesday, the snow will spread over western Colorado, northwestern Nebraska and into the western Dakotas. By the second half of Tuesday, snow will increase over northern and central Minnesota.

Blizzard warnings blanket much of northwestern Nebraska, central and western South Dakota and southwestern North Dakota.

“Travel will be very difficult to impossible,” wrote the National Weather Service in Aberdeen. “Widespread blowing snow will significantly reduce visibility … [and] very strong winds could cause extensive damage to trees and power lines.”

Peak totals could reach at least 16 to 20 inches from western South Dakota to the North Dakota-Minnesota border area.

This storm could let Bismarck, N.D., clinch a record for the snowiest cold season ever observed. Thus far, 95.4 inches of snow has fallen. The number to beat, set in 1996-97, is 101.6 inches. Eclipsing that should be no problem with the upcoming snowfall.