Tornadoes—possibly intense—and widespread severe hailstorms are expected to sweep through parts of the Midwest on Saturday afternoon and evening. In its Day 1 outlook issued at 10:30 am EDT Saturday, the NOAA/NWS Storm Prediction Center placed much of northern and central Illinois under a moderate risk, the second highest of SPC’s five risk categories. A somewhat lesser but still significant severe threat encompasses most of the Corn Belt, extending southward through the Mississipp Valley (see the two figures below).
Extensive cloud cover is the only factor limiting the potential strength of Saturday's volatile setup in and near northern Illinois, according to SPC. Because of this "confidence remains too low to delineate a potentially concentrated area of greatest risk, which would otherwise support high risk upgrade. As the situation evolves, a possible upgrade remains possible for the [3 pm CDT] outlook update, given the larger-scale pattern/environment which continues to appear favorable for several significant tornadoes."
Saturday's storms will be moving at extraordinary speeds—perhaps 60 mph or more—which adds to the warning challenge as well as the potential for tornado damage.
A potent upper-level trough and jet streak (core region of the jet stream) will be ripping into the mid-Mississippi Valley on Saturday afternoon just as a very warm, moist air mass surges into the air, setting the stage for fast-moving, potentially tornadic supercell thunderstorms.
Severe hailstorms ripped northeast from Oklahoma into Missouri through the afternoon and evening on Friday. By Saturday morning, SPC had logged more than 100 reports of severe hail. Hailstones 3 inches in diameter pummeled the Jefferson City, Missouri, area. Another storm to the south dropped a grapefruit-sized hailstone (4.5") at Houston, Missouri.
Friday night's storms were “elevated”, meaning they drew on a feed of warm, moist air flowing atop cooler air near the surface. Such storms rarely produce tornadoes, since the air at the surface is more stable than aloft, but elevated storms often lead to widespread large hail and heavy rain.
Saturday's biggest threat is likely to be concentrated in time and space during a few hours late Saturday afternoon and evening in and around far eastern Iowa and northern and central Illinois. This area will be near and behind a warm front pulling through the Midwest and ahead of a cold front pushing eastward from the Plains. Along with an unusually juicy air mass for late March—temperatures may top 70°F, with dew points in the mid- to upper 60s—the area will be overtopped by a ferocious jet stream, with southwesterly winds at around 35,000 feet roaring at speeds of 120 to 160 mph by Saturday night.
The high instability and strong upper winds will also fuel large hail, including the potential for so-called giant hailstones, running at least two inches (hen-egg size) in diameter. More reports of baseball-sized-or-larger hail are certainly possible.
As the tornado threat diminishes later on Saturday night, intense thunderstorms with large hail and the potential for at least isolated tornadoes will continue to race eastward into Indiana and Ohio and build southward toward Missouri and Arkansas, spreading east into Kentucky, Tennessee and northern Mississippi. Even southern Wisconsin and far southern Michigan could see severe hail.
It’s not too early for severe weather in the Midwest
Early spring puts an accent mark on the mercurial nature of Midwestern weather. Both snow and severe weather are possible this time of year, and while tornadoes tend to favor the South and Southern Plains in late winter and early spring, they’ve been known to strike much further north. The infamous Tri-State Tornado ripped across southern Missouri, Illinois, and Indiana on March 18, 1925; it remains the deadliest and longest-track tornado in U.S. history, with 695 fatalities and a continuous path estimated at 174 miles.
Late March is on the early side for tornadoes in northern Illinois and Indiana, but not by much. The EF4 Rochelle-Fairdale tornado raked an area less than 100 miles west of Chicago on April 9, 2015. The notorious Palm Sunday outbreak of April 11, 1965, produced deadly tornadoes as far north as southern Wisconsin and Michigan, including long-track F4 tornadoes north of Grand Rapids and Lansing.
The most eye-opening analog occurred exactly 100 years ago Saturday: an outbreak on Palm Sunday 1920—March 28—that killed more than 380 people. This disastrous outbreak produced eight F4 tornadoes, one of which plowed across the northern part of Chicagoland, including Melrose Park and Wilmette. It's one of six F4/EF4 or F5/EF5 tornadoes on record in the Chicago metro area.
The coronavirus and shelters: All bets are off
People who may need to use a community shelter in a tornado threat this spring—including residents of northern and central Illinois for Saturday—would be well advised to check on the status of that shelter well in advance, given the turbulent status of coronavirus precautions. Some local officials are maintaining a business-as-usual approach to shelters, while others are keeping shelters closed, as summarized by Matthew Cappucci in a Capital Weather Gang article on Thursday.
It’s generally up to local authorities (county or municipal) to make the call on whether a shelter opens or not. The Illinois Emergency Management Agency and NWS released a joint statement on Friday: "At this time, IEMA and the NWS are recommending that your first priority should be to protect yourself from a potential tornado. However, the decision to open a community shelter will ultimately be at the discretion of local officials. Before you make the decision to go to a community shelter, you should first check with local officials to ensure they will be open. This should be done ahead of any thunderstorm, well before any warnings are issued. If you rely on community shelters, now is a good time to explore other options that might keep you safe from a tornado while also limiting your potential exposure to COVID-19."
“State and local emergency managers are responsible for public shelters and we defer any questions to them on this topic,” the NWS said in a statement published by CapWxGang. “The role of the NOAA [National Oceanic and Atmospheric Administration] National Weather Service is to provide forecasts, warnings, and decision support.”
Gilbert Sebenste, a longtime meteorologist and severe storm expert in northern Illinois, raised an important point for people in Saturday’s threat area: “One thing is for sure—you need to decide this today, or before the tornado watch flies. These storms will be moving at 60 miles an hour. You will not have time to decide in the moment.”
The shelter question extends well beyond tornadic threats. As reported by weather.com’s Jan Wesner Childs on Friday, a team at the Union of Concerned Scientists has compared coronavirus projection models from Columbia University with NOAA's most recent spring flood forecast and came to some startling conclusions about which communities are most likely to be hit with both the global pandemic and spring flooding between now and May 31.
"For communities already strained and tense, waiting and hoping, grieving and fearful, NOAA's flooding forecast paints a grave picture in which they must — somehow, some way — meet the intertwined challenges that severe flooding and a pandemic present them," Kristy Dahl, senior climate scientist for the UCS, said in a blog post Wednesday.