Above: The Storm Prediction Center's severe weather outlook for Saturday, March 28, 2020, issued at midday Friday, March 27. (NOAA/NWS/SPC).
Tornadoes—possibly intense—may erupt in and near northern Illinois on Saturday, as the ingredients are on track to coalesce for a regional severe weather outbreak.
Update (3 am EDT Saturday): In its Day 1 outlook issued at 2 am EDT Saturday, the NOAA/NWS Storm Prediction Center placed the area under a moderate risk, the second highest of SPC’s five risk categories. "An upgrade to high risk will be considered in later day 1 updates," SPC said.
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.
Update: Severe hailstorms ripped northeast from Oklahoma into Missouri through the afternoon and evening on Friday. By early Saturday morning, SPC had logged more than 90 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.
As of Friday afternoon, SPC had a slight risk outlined through early Saturday morning from eastern Kansas to most of Indiana.
The set-up for Saturday
Saturday could be a volatile day for severe weather, albeit in a compact area. The biggest threat is likely to be concentrated in time and space during a few hours late Saturday afternoon and evening in and around 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.
SPC warned in its Day 2 outlook issued midday Friday that a “significant severe weather event” is possible on Saturday. 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 resulting shear will be strongly supportive of strong updraft rotation—and thus attendant risk for tornadoes, a few of which could be significant,” SPC said.
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. I wouldn’t be surprised to see at least a few reports of baseball-sized hail. As the tornado threat diminishes later on Saturday night, intense thunderstorms with large hail may continue to race eastward into Indiana and Ohio and extend southward into Kentucky and Tennessee. Even southern Wisconsin and Michigan could see severe hail.
In a detailed Facebook Live weather briefing on Friday, Victor Gensini (Northern Illinois University) noted how very warm air just above the surface—drawing off record heat that topped 100°F on Thursday in Texas and Oklahoma—will enhance the potential for instability while serving as a "cap" to inhibit storm formation in the highest threat area until mid-afternoon Saturday.
"My confidence is increasing that we're looking at a significant tornado day," Gensini said.
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 state of Alabama issued a statement on Sunday encouraging residents not to skip seeking shelter in a life-or-death situation because of coronavirus fears: “The decision to seek shelter in a community storm shelter is certainly made more difficult by the consideration for COVID-19, and each individual will need to make an educated decision on where and when to shelter from a tornado. At this time, the Alabama Department of Public Health (ADPH) is recommending that your first priority should be to protect yourself from a potential tornado. If a warning is issued for your area, you are more likely to be affected by the tornado than the virus.”
The state of Illinois has a stay-at-home order in place. The state’s FAQ page on that order that makes no mention of what to do during severe weather. Update: 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.
